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The following was translated from
the German by Janet Christie and posted here by Derek Sikes.
Refer to the original document for literature cited, tables
and figures, which are not included here. This reproduction
is not to be used for any purpose other than private study,
scholarship, or research.
Ecological Investigation
of Necrophorus F. by Erna Pukowski Zeitschrift fur
Morphologie und Oekologie INTRODUCTION
In certain cases the Necrophorus
can indeed be found also during the day in natural
conditions. Every animal which has found carrion at night
stays there during the day. There they hide on the underside
or slide inside in some which contain cavities within, where
they are able to devote themselves to undisturbed feeding.
Often on windy but sunny spring or late autumn days, thus
shortly before or after the winter hibernation of the
animals, one sees flying Necrophorus. Probably they are
driven to it by the hunger, and that at all times of the
night it is too cold to search for food. In May the species N. vespillo, N.
vespilloides, N. humator, and N. germanicus, in August the
species N. investigator, N. fossor are already in
reproduction. At this time the beetles begin with that
activity to which their name gravedigger points: they bury
small carrion in the soil in order to receive it as food for
their descendants. Their gonads have developed immensely to
a mature state, and both sexes happen now on the search for
carrion that corresponds to the demand of their broodcare
instincts, and describes at the same time the meeting-place
for males and females. When a body is found and a pair are
together then the preconditions for reproduction are
fulfilled. If only a male arrives at the
carrion, one can make the following highly remarkable
observation: the males which initially set about the digging
work, break it off after a short time in an effort to reach
a high point in the immediate vicinity of digging. The male
climbs,a blade of grass, a stone or even a small heap of
soil, for want of a high lying point. In all observed cases
this is commonly a very surprising pose which the male assumes. The animal
orients itself so that the head is as low as possible but
the abdomen stands as high as possible in space. The head is
thereby bowed so low that the mandibles often rest on base,
grass stalk, stone or soil. The whole body, which normally
shows in every Necrophorus in front of as well as behind a
distinct slope from the ventral aspect, becomes extended in
a slope from behind causing a steep angle. The body weight
thereby rests on fore and middle tarsi, while the hindlegs
are only lightly placed or even held freely suspended. The
hind end of the body is so strongly extended that the
abdomen appears longer and more slender and the last segment
which normally lies hidden in the body appears. In this
position the male persists, refraining from short breaks
without clearly visible alteration for more than an hour.
Only the tip of the abdomen betrays slight swinging or
rotating movement; the last segment stretches away from the
others occasionally revealing the intersegmental membrane. I
would like to describe this strange operation as the
"Ventilation" of gravediggers. Although not all 200 experimental
sites were in operation simultaneously (for the meat was
eaten frequently by other animals or carried off by the
gravediggers), I was still able to examine in the course of
Summer 1930, 254 cases in the described way. An experiment
was only counted then, when the carrion was found already in
a subterranean hole, which Fabre called the crypt, there in
the presence of this crypt for sole proof for the
termination of digging work is recognized. The examination
gave the following results. We see how well a female fared in
the field of other females! How do the males behave among
one another? The two worked during the whole duration of the
experiment on the same carcass without mutually disturbing
each other; the mole is sunk further into the soil through
their common vehicles. Finally even a third male arrived.
Suddenly, a loud chirping becomes audible and in a wild
chase 2 males appear under the carrion, the third (white)
close behind them. This comes (as in the females) at last
usually to a fight, from which the white male comes off
victorious and turns back to this work, while the fugitives
appear to keep away from the mole now. After a further hour
in which nothing more in particular happens and the mole is
rapidly fully covered with soil, the observations were
discontinued. On the following morning by digging it up, I
found on the carrion the white pair alone, whose isolation
had taken place in the described way. The fight takes another start, if
one of the escaping animals, which ought to be withdrawn or
a young beetle appeases his hunger on the carrion intended
for the brood. In both cases the persecutor tries to seize
with his forelegs from behind the last or second last pair
of his opponents and these pulled close with a jerk. Here
the animals soon lie against one another on their sides. In
contrast to the above situation they have an equal tendency
to be righted as Fig. 6b illustrates. The rest of its course
follows as described above. A recurring effect of fighting is
the speedy escape of the weaker animal which as result of
receiving attraction a clear change of behavior can be
observed. The digging instinct which rules the beetle up to
his defeat, becomes displaced by the flight instinct. In by
far the majority of cases the escaping beetles collected in
the nearest vicinity in the earth without venturing a new
attack during the following hours. On the contrary one sees
them either clearly avoiding the body or in their search for
a hiding place not giving it more consideration than an
indifferent hindrance. Still, the duration of this happening
appears to be dependent on the intensity of the sustained
defeat. Nevertheless, the following
observation allows one to assume an association between male
and female: the distinctly audible chirping that the male as
well as the female produces with the help of shell
organs--most irregularly--arouses an impression now and
then, that as if the partners mutually answer one another.
In most cases the chirping proceeds in the way that the
verse of the animal which is composed of several shrill
noises of the same kind start the verse of the partner;
schematically it can be represented in the following way at
which the number of the quoted shrill noises added is of no
importance: 2. Relations of
the beetles to their offspring. they are attached through one of
the outer sides of the wood border and held by the right
angled aluminum strip underneath the glass plate on the
ground is a prop. Both long walls are held so tightly only
by the metal strip that they try to slide slightly here and
there as in a rabbet. This is implicitly required in order
to be able to change the glass plates during the observation
without strong shock. That often proved to be necessary
against the work of the beetle held together dirt through
small soil particles or carrion. The vessel top was closed
with wire gauze. number of hindrances which it
rescues from the ground in the shape of roots or small
stones, it merely fluctuates the working hours. The beetles
push out more and more soil from under the carrion and
thereby deepen the already started hole without broadening
it at the same time. The part of the body which is dug under
begins to sink under its own weight (Fig. 8a). c .Rounding of the body and
completion of the crypt. outside of the carrion ball happens
in older larvae is not explained merely as a reaction to the
outgoing attracting smell, but from mutual attractive
contact playing a part. The thigmotactic attraction of the
larvae prevent in a suitable way any dispersion of the
animals. Actually it is extremely rare to observe a larva
leaving the nest prematurely. Only from time to time one can
surprise the larvae as they hang their hind ends far out
from the crater edge, Fig. 19. This is the attitude which
the older larvae maintain for defecation. The emerging
pellet of excrement falls off on the outside of the ball of
carrion containing a mixture of food and excrement and with
that prevents a reduction in the nutritive value of the
carrion and at the same time strong soiling of the larvae is
avoided. A larval nest which belong to N.
investigator becomes laid free through cautious removal of
the crypt roof. The female which in the first moment was
escaping, soon appears again, hurries round about the
carrion, clambers up on it and tastes with the antennae the
edge of the crater. Then it suddenly leaves the ball of
carrion, climbs the crypt wall sidewaysand advances into the
adjoining soil. Already the grave digger is half hidden in
it, then she pauses and with stronger and from backwards
directed movements of the hind legs hurls back loose earth.
At the same time also the upper half of the beetle falls
back into the lying soil. Then the female pauses, turns back
to the crater and tastes alternately with the antennae the
freshly raised material and the still partially uncovered
crater. Then still once again begins the described spectacle
at another place. Now is also plain to been seen, that the
working animal through raising and lowering her thorax
shakes the earth mass lying over her and through that,
brings it sliding off. Soon the ball of carrion is
completely covered with earth. The female ceases working and
disappears under the spilled material. Once more opening the
brood hole at the end of 4 hours shows that the crypt is
again completely made: the ball of carrion lies in a cavity
with smooth walls as before the disturbance. The typically expedient behaviour
of the grave diggers remains throughout in the frame of
demands which in the wild are put on the animal. That the
Necrophorus sank carrion only a few cm in the earth, may
have occurred frequently enough that from the natural
reasons, perhaps through the step of a grazing animal or a
heavy downpour, the crypt fell together and the brood often
lay there abandoned from damaging weather and hostile
persecution. 2.The stagnant air in the crypt
over-ladened with waste products, for example CO2 and H2O,
would essentially entirely change through the inflow of
fresh outside air. The causal attraction would in this
instance be primarily chemical. Four of the mentioned Necrophorus
species, namely N. germanicus, N. humator, N. vespillo and
N. vespilloides pupate after a pause for rest of 12-17 days
and complete their development still in the same years. N.
fossor and N. investigator are different. There the newly
emerged adults _ of these species appear in the surroundings
of Frankfurt all at the end of June at the earliest, so the
young generation also run through the larva and development
mostly first in September. After that the animal enters a
standstill in development: the prepupal stage overwinters in
the pupal cradle. Xambeu (1892) had already stated this for
N. fossor and Main (1927) also noticed that at least one
species overwinters as a larva. That the break in
development is not a direct result of environmental factors
for example low temperature, but view as fixed genotypical
development rhythm, is shown in the following experiment.
The wandering larvae of both these species were put at the
beginning of October 1930 in their rearing cage in a
greenhouse in which the temperature of 19_C , prevaded.
Although under the same experimental conditions other ~
Necrophorus species had already developed up to the pupa,
the larvae of N. fossor and N. investigator remained
standing at the pre-pupal stage and died off gradually
during the winter. Control animals taken from the same nest
put in the smallest_flower pots, filled with earth, shut
above with wire gauze, buried a ~ w cm deep in the Botanical
Gardens of the University of Frankfurt and the~_oil-there
covered with a layer of leaves. In April of the following
year (1931) these animals were found in strong health in the
prepupal stage, pupated at the end of May and emerged at the
conclusion of the pupal rest to adult. SUMMARY 15. At the end of a 7 day period of
development the larvae leave the carrion and predominantly
withdraw from the brood room in a horizontal plane.
-
der Tiere 27(3): 518-586. 1933.
RESULTS
A. General Life of the
Adult
1. Occurrence -- frequency
2. Environment
3. Temporal occurence
4. Nutrition
B. Reproductive
Biology
1. Relationship of beetles to one another
a. Attraction of
females
b. The isolation of pairs from a large number of
individuals end the resulting conclusion
c. Ecology of the fight
d. Fight with strange species or races of
gravedigger
e. Course of the fight
f. Relations between partners of the pairs
2. Relations of the beetles to their
offspringa. Brood care
b. Burying of a body
c. Rounding of the body and completion of the
crypt
d. Egglaying
e. Moisture and digestion of the body
f. Male participation in brood care
g. The brood at the time of larval development
h. Feeding of the larvae
i. Ecology of feeding
j. Defense of the larvae and their food
k. Maintenance of the crypt
l. Defense of males at the nursing
m. Development of full-grown larvae to young beetles
INTRODUCTION
The striking instinct of gravediggers (Necrophorus
fabricuis) to bury small animals was first reported by
Gleditsch (1752) with the help of experiments, in which
development of beetles with different species of small
animal corpses at their disposal was observed. Gleditsch
succeeded in putting the burywork of the gravediggers in
relation to their reproduction. On the suggestion of these
chemists Melim from Bremer (1755) who independent of the
mentioned report, followed the burial of a mole by the
gravedigger and recognized in them the remarkable broodcare,
which also occupied A. I. Rosel von Rosenhof (1761) with the
remarkable life of the gravedigger. We find his result put
down in the celebrated "In sektenbelustigung" equipped with
coloured pictures of adults, larvae and pupae.
From the statement of Gleditsch (1752) that a gravedigger
had brought a stick to undermining to collapse was fed on
the body of a toad, Lacordaire concluded about the
intelligence of this beetle. First the French researcher J.
H. Fabre destroyed the fable of the intelligence of the
gravedigger while he showed with the help of numerous
carefully carried-out experiments that the actions of this
animal are fixed within the setting of instinct, but not as
was first interpreted a reasoned thought.
If the experiments of Fabre lead to greater clarity so too a
new problem escaped his observations that researchers had
perceived as such, but not been able to solve. This new
problem lies in the fact that on the buried carrion there
always remains a single pair of gravediggers, highly
suitably, even if more beetles worked at it. The question of
how this appearance is to be explained was discussed
recently in terms of animal psychology and sociology (Renter
1913, Schroder 1929) however until now no satisfactory
solution was found. On the suggestion of my highly revered
lecturer, Herr Geheimen Regierungsratses (private
administrator adviser) Prof. Dr. zur Strarsen, I put the
solution of named problem to the test.
Fulfillment of the task required most precise knowledge of
the whole digging event within which the isolation of pairs
from a larger number of individuals must take place; this
could only be achieved through extensive experiments
preferably under natural conditions. For this reason I have
investigated in depth the general ecology of the genus
Necrophorus.
The experiments were carried out on the following species:
Necrophorus germanicus L., humator B1., vesplllo L.,
vespilloides Herbst, investigator Zett, fossor Er.
(interruptus Steph.) The species germanicus, was sent to me
mostly from Danzig; I baited the other animals around
Frankfurt a.M.
RESULTS
A. General Life of the Adult
1. Occurrence - frequency.
Although around Frankfurt not all species of the genus
Necrophorus occur, humator, vespillo, vespilloides, fossor
and investigator still occur with certainty. The first three
species are found in large quantity, and investigator and
fossor in smaller quantity. N. germanicus is scarce and N.
vestigator even more so.
In order to catch the gravediggers, I arranged measures of
bait in the following state: a pail, whose base was
perforated for the draining of rainwater filled to 10-15 cm
with earth. Inside a slightly decaying large piece of meat
was laid and the entire pail was put so that it is not
exposed to direct sunbeams, yet still breezy enough for the
smell of the carrion to flow about unhindered. Beetles
flying by are easily able to attain the body; but there they
are caught for the narrow limit of space left them is too
little for flying away and they are not able to clamber up
the smooth and steep walls of the pail. Besides few make
attempts to fly for there they find rich booty and the soil
layer offers them the possibility of shelter. Every 10-14
days the traps were baited with inew meat, and were able to
be used continuously in this way.
In order to give a rough impression of the frequency of the
gravediggers, I have included 3 tables which give again the
total yields of the baits. The catch was made after the meat
had layed out for 3-4 days in warm weather. On baits M.S.,
M.E. and M.W. named beetle species in the following numbers
were found. From this set up it is obvious that the
gravediggers are by no means as rare as one accepted from
chance finding.
2. Environment
The searching of numerous baits gave me the opportunity to
study the occurrence of frequent species (humator, vespillo,
vespilloides) around Frankfurt a.M. with regard to their
environment and to confirm the winning conception with a
systematically employed series of experiments. The traps
were put out in different areas of the nearer and farther
surroundings of Frankfurt, and the caught animals registered
with time, numbers and species. Data in Table 1 are from a
large series of arbitrary catches. The baits are labelled
with the numbers I-VI. All catches given here were made at
the same time of year, namely mid Mav 1931. M (bait) I and M
II were in Tannus near Eppstein. Distant mixed woodland
(Pinus and Fagus) is the near and far environment. M III and
M IV were in meadows at Ginnheim and Rodelheim in Niddatal
which stretches to the foot of Tannus. M V stood in the
middle of a large, very dry confer wood area in Frankfurter
Stadtwald. M VI lay in mixed woodland (Pinus, Picea, Fagus)
in Walldorf and close to the woodedge not far from extensive
meadowland. So M VI joined the environmental conditions of
previously chosen places.
From Table I in mixed woodland from Eppstein at M I and II
altogether 41 individuals of humator and 36 of vespilloides
were found, but not a single example of vespillo. Forty-four
examples of N. vespillo were found from meadows at Ginnheim
and Rodelheim in M III and M IV. However M III and M IV show
uniform vespillo- material. The yield in pine stock brings a
new species picture. Here in M V the domination of N.
vespilloides really comes to light. Only at M VI are the
three N. species commonly found.
This proves that the findings of the single catching points
at the same time of year concerning the species distribution
are by no means all equal to each other. Rather it looks as
though clearly there is a factor governing the occurrence in
appearance: The habitat of N. vespillo is meadowland, while
humator and vespilloides are indigenous to woodland. M VI
supports as its surroundings are mixed woodland as well as
meadow, the importance of control experiments.
Correspondingly examples are found of humator and
vespilloides which come from the adjoining meadow. Those in
the mid-part of the table containing beetle numbers perhaps
appear scarce. But the unquoted records of the same year
(1931) as well as the records for 1929, 1930, and 1932,
extend the total to 2357 examples, and corroborate the
reported results, eliminating the probability that these
patterns are due to chance alone.
Naturally, the membership of a species to a clearly defined
environment from the records must be all the clearer, all
any time a habitat and untroubled from strange intruders
opposed to them. For the species vespillo it was possible to
employ a trial experiment in a handy absolute territory. It
was carried out in Freistaat Danzig at a place where there
is no woodland for many kilometres in circumference. The
experiment caught 64 examples of N. vespillo and one of N.
vestigator. Trapping in Freistaat Danzig, on different
ground confirmed the Frankfurt results. It was possible in
Frankfurt in spite of little expansion of affiliated areas,
to construct a correlation of species and environment
showing a considerable environmental loyalty of the species.
This applies particularly to N. humator and N. vespilloides,
which in general were found not more than about 100 m
distance from woodedge; while vespillo advanced a kilometer
into the woods.
The results from M V in Table I indicate that N.
vespilloides is found predominantly in stands of confer
wood. Hence its occurrence in mixed wood was due to the
presence of scattered pine stands while N. humator occurred
particularly in deciduous woods. Although this hypothesis is
supported by numerous results, isolated results still occur
which directly contradict this. A moment on the ecology of
Nicrophorus however helps to eliminate the apparent
contradiction. Because the gravedigger buries small carrion
as food for the descendants, they are dependent upon the
condition of the ground which offers unequal resistance
depending upon the material covering the ground. The conifer
woods, which appear as pine woods in a large part of the
Frankfurt area, stand here on predominantly very dry, sandy
ground. The ground is almost free from rooted plants,
covered with an even rough humus layer of needle litter
which is interrupted here and there by small moss lawns
(Hypnum, Polystichum species). Elsewhere the ground is
damp,deciduous leaf woods, which in west Germany is mostly
dark brown to black crumbling humus with a far higher
moisture content than the sandy ground of pinewoods. Its
covering with leaf-litter and loosely twisted roots of
ground plants, is about average in resistance to mechanical
destruction falling between the loose leaf litter of
pinewood and the dense layer of roots of meadows. The ground
of valley meadows (and only these are common here) is heavy
and moist to wet. It bears an extremely dense root layer of
its predominantly Gramineae covering.
The described soils differ so strongly from one another in
their condition that it was understandable, when every N.
species preferred for digging capability and behavior an
adequate environment. There is also clay-sandy deciduous
wood soil and moist to marshy pine wood soil. At
such;places. the picture appears reversed exceptionally. In
wet pinewood N.humator rules but in dry deciduous wood N.
vespilloldes does thus accounting for the above
contradiction. The exception offers the best proof that the
ground is the primary habitat characteristic for
the-gravedigger.
Extreme differences from the rules are rare in the Frankfurt
area. More frequently one finds that the relative population
between N.vespilloides and N.humator in mixed woodland
shifts in favor of one or other sides only somewhat.
according to whether the experiment was put in a dry, sandy
or a more damp, humus mixed wood. For example the yield from
M I and M II in mixed woodland of the Tannus in Table I
shows the proportion 41:36 where N. humator forms the larger
quantity. In mixed wood of sandy urban woods likewise both
species are found, but in totally changed proportion
(Results M.S. S.520), 7 N. humator and 69 N. vespilloides.
The sample numbers of N. vespillo from adjacent meadowland
were approached in the woods.
3. Temporal occurrence
The baits were visited during the warmer times of the year
for Necrophorus species. The more or less early time
appearance of beetles in the spring as with their
disappearance in autumn is naturally dependent on favorable
or unfavorable weather. In 1930, the spring was warm, and on
4 April the first gravedigger, N. humator, although the bait
was on the edge of a small, damp deciduous wood in the
middle of a further stretch of meadow equally favorable to
N. vespillo. N. vespillo appeared for the first time that
year 16 days later. Due to cold springs in 1931 and 1932 I
cannot say what the average date of appearance is. In both
cases the animals did not appear until May and they were
caught at the same time. While N. humator, N. vespillo and
N. vespilloides appear at approximately the same time and
also disappear to hibernate at the same time. N.
investigator and N. fossor (interruptus) make a striking
exception. These species were not found until the end of
June or beginning of July during all three summers of the
study (1930, 1931, 1932). The late appearance of these
species is connected with their individual cycles which are
delayed for months in relation to the 3 other species, N.
humator, N. vespillo, N. vespilloides. On particularly warm
November days one finds here and there flying Necrophorus
but as a rule, all species begin their hibernation around
the beginning to middle of Octoher.
Although gravediggers were recorded as common animals, one
rarely finds them without the aid of bait. This is based on
their nocturnal life style. The gravedigger comes out of the
ground in which he has been secreted for the day, around
sunset. If one observes this happening, one sees as with a
small shake of the ground two appearing antennae. Apparently
the beetle examines the environment through his smelling
organs. After that the animal leaves the sheltered province
and climbs any blade of grass. In spite of his size and
weight he brings this about with dexterity. If the grass
bends to the ground under his weight, another blade is
climbed. Apparently, the beetle operates by instinct, to
raise himself somewhat above ground level. In this way he
manages to contact initially the light scents from possible
food and secondly he can lighten himself for take-off. There
he remains sitting preening himself with antennae moving,
ready to begin his flight in search of food.
The gravediggers are pronounced dusk and night animals when
living in a well maintained terrarium of 50-60 beetles and
only rarely does one appear in the daytime. If the
environmental conditions are unfavorable for the beetles,
e.g. strong sunlight, high temperature, extreme drought,
narrowness of cage and lack of food, they are driven to the
surface during the daytime. They then undertake rapid
running around and continual attempts to escape in which
they fly round wildly. These are however clear reactions to
abnormal conditions.
Their life in the dark of night or the soil makes the
gravedigger correspondingly sensitive to bright light, yet
the degree of sensitivity varies with the individual. Most
times they retire into the soil with sudden lighting with
white light individual answers to a strong light attraction
done with Thanabos. Others again are not disturbed in their
occupation. In strongly softened light one can observe
almost all individuals undisturbed. For this reason this
sort of illumination was employed chiefly for
observation.
4. Nutrition
In quick flights, thereby frequent and uninterrupted in
varying direction, the gravediggers wander through woods and
meadow till the wind carries to them the scent of carrion.
At once the flight stops, flying round the source of the
smell in narrow circles and finally land on it or in their
near surroundings. In the last case one sees the beetle
hurrying to his find with swinging antennae.
Although the Necrophorus lets himself be enticed to the
scent of carrion, they are not exclusively carrion eaters,
but predominantly predators. Already more than lOO years ago
an English researcher, Bell, observed how gravediggers
seized and consumed the fly larvae from a carcass. The
description of this event, which was first published in
1873, has remained unnoticed and still long afterwards
Necrophorus is reported as a typical carrion eater. Later
Ch. U. Clark (1895) and W. T. Davis 1915 often have called
attention to the predatory life style. Most recently F.
Steele (1927) showed this in a perfect series of experiments
on the North American gravediggers describing the facts of
eating charmingly and vividly.
My experiments showing the preference for diptera larvae by
European species is of value. It was especially pointed out
that also N. vespilloides is no exception to the pattern of
nutritional habits. The fact that this species was sometimes
found in rotten woodland fungi, gave occasion to the
statement (Chenu and Desmarest 1851), that N. vespilloides
did not feed from carrion but only from fungi. But that is
not true. Although these gravediggers were enticed through
the smell to decomposing fungi, they were still only hunting
for the living diptera larvae in the fungus. I saw a hungry
male of this species consume 17 maggots in 35 minutes after
which the meal was interrupted.
The predatory nature of the genus Necrophorus is clearest
expressed in the species N. germanicus. In the night, this
large, extremely agile animal hunts for Geotrupes unless fly
maggots are within reach. Klungelhoffer (1843) and Schmidt
(1883) always observed a fierce struggle between N.
germanicus and Geotrupes (mutator) stercorarius and both
concluded from that the predatory life style of N.
germanicus. This conjecture I was able to confirm. I went
along the following train of thought: If those observed
cases from the above metioned authors are not chance
records, particulary N. germanicus, rather instinctive
hunting for Geotrupes it must be expected that it reacts to
the smell of horse dung the typical habitat of their
prey--as N. vespilloides reacts to the smell of fung. In
order to prove this, the following experiment was set up.:
In a round terrarium (lOOx94x58 cm) in which several N.
germanicus were held, I brought some horse dung. The
Necrophorus, which until this moment were wandering round
apparently aimlessly in the cage, stopped with antennae
beating keenly or others immediately began their direction
of marching and hurried to the dung heap. This is possibly
explained by the water or bacterial content in the fresh
dung attracting the Necrophorus to approach. That however
contradicts the behavior of the beetle when they have
approached the source of the smell. None of them bite into
the heap with their mandibles, rather all run as if
searching it over and turn around now and again a large
piece of dung with head and thorax. One cannot keep the
false impression as these animals search the dung for its
occupants. Single individuals withdraw, the majority however
remain at the site and bury themselves superficially in the
dung. One particularly lively female hid for example under a
lump of dung. Only the head with perceiving antennae, was
still to be seen. At this point I would like to take the
opportunity to mention an observation of V. Lengearcken, who
found a N. germanicus under a ball of horse dung lying in
the wild. Animal and dung ball were already sunk somewhat
into the ground. Undoubtedly it had to do with the same
event which I was able to observe in the terrarium. There
also I saw here and there how the dung was sunk somewhat
into the earth through the agitation beneath the beetle.
So the beetles remain to a certain extent in ambush, only
alternating now and then hastily leaving his place. This
behaviour of the beetle then follows without there being any
Geotrupes in the cage or in the vicinity! After a fresh dung
pat is searched through for the Geotrupes by N. germanicus,
they wait for the moment which enticed hy the smell of dung,
happens with all probablity in a short time.
Now I placed about 60 Geotrupes into the terrarium. All
belonged to the species G. sylvaticus, which I was able to
collect most easily in the surroundings of Frankfurt. Nearly
all the dung beetles groped about for a long time at the
nearest way of the dung heap. Scarcely had the first reached
their goal when those still inanimate predators became
lively, appearing in masses. Everywhere the large predatory
gravediggers claimed their prey which to begin with chirped
loudly, but desperately, to defend themselves. Also the
previously mentioned female left its place under the
dungball and hurried smartly towards the near prey by the
shortest route. N. germanicus attacked its prey not from
ambush as one would presume from its initial behavior but
hurried openly to his prey and seized it with legs and
mandibles. Every caution appeared totally useless as the
observer did not receive the impression that Geotrupes
sylvaticus was able to recognize his enemy as such. One sees
on the contrary the dung beetles marching on the Necrophorus
and even in direct proximity no impression was
noticeable.
That what took place now between the attacker and his prey
can by no means be described as a struggle. For both animals
are too unequal in strength and quickness. N. germanicus
seized the weaker, G. sylvaticus with both legs and thereby
threw itself on the back or the side. This pose was taken
willingly as the eating of large fly maggots, so that more
than half of the gravediggers can be found after a feed with
these larvae, the prey held in front of them between first
and second pairs of legs. Small fly larvae are nevertheless
held free in the mandibles.
Through the extremely tight hold of predators, Geotrupes
sylvaticus becomes pressed against the ventral side of its
opponent and can be eaten conveniently. One hears loudly
between the weakening chirping of the dung beetles cracking
of the chitin armor under the strong mandibles of the
gravediggers. At some point the outer skeleton becomes
broken. In the majority of observed cases the prey was slit
open on the ventral side of the area between pro- and
meso-thorax. In other cases the predator succeeded in
destroying the strongly chitinized (thorax) neck plate. Once
the body is open, the organs are pulled out and eaten. In
the course of half an hour I saw one N. germanicus male
seize and consume five Geotrupes in the described way. The
others who were no less lucky in their hunting so that the
remainder of the Geotrupes lay around everywhere in the
terrarium. The N. germanicus seizes the dung beetles so
quickly when hungry, but they are indifferent to them in the
satiated state, hurrying by without any attempted attacks on
the Geotrupes.
The experiment was performed several times with the same
results: N. germanicus habitually hunts for Geotrupes! To be
sure, fly maggots and Geotrupes were offered to them at the
same time and as a rule the fly maggots were preferred.
In spite of their preference for living prey Necrophorus
show only a slight inclination for cannabalism. Healthy
individuals did not cannabalize others when very hungry and
under confined living conditions. An exception to this may
arise in fresh, recently colored young beetles which at
times consume uncolored individuals--usually brothers and
sisters. This is seen only in rearing under artificial
conditions,indicating environmental conditions as causative
factors. While in nature the beetle immediately goes out in
search of food after leaving the pupal stage, in the
terrarium he is held back by enormously limited room. From
deficiency in other food the animal seizes conspecifics
which are convenient prey for him. This was easy to prevent;
shortly before the hatching of the beetles some meat was put
in the cage. Still in nature, cannabalism can occasionally
be observed, when sick or hadly injured individuals are
attacked and consumed.
Besides the predatory nutritional ways all here-mentioned,
Necrophorus species are also carrion eaters. As Steele
(1927) already mentioned for the North American species, he
showed the little decayed food of the strongly decomposed.
Largely independent of the type of meat, the Necrophorus
accept every kind of carrion that is offered to them. Meat
or internal organs of vertebrates such as sheep, pigs, beef,
horse or even tiger and turtle soup whose bodies were placed
at my disposal by chance served the gravediggers for food
just as the bodies of small vertebrates or even many
invertebrates like earthworms, slugs, and larger insects.
When one puts a hungry animal on a piece of meat, it tests
the food here and there with antennae and mouth appendages.
In a somewhat damp and soaking place one sees it stick to
and drive in its mandibles. The choice of place to eat
corresponds to completely the preference of the gravedigger,
on small vertebrate bodies firstly the body opening, eyes or
possibly wounds as this also Steele (1927) was able to
establish for those species which he had examined. Yet
without having a visible ingredient cut off, the jaw becomes
open again, in order to immediately grip to new dense area
behind the place again, a match that is repeated so long
with between the mandibles rises a weak border of ordered
meat. One sees the mouth appendages working this border
again and again. After a long time, often after an hour, the
beetle turns to another place, without that, that border
would have completely disappeared.
If one now disturbs the animal in some way, it vomits
immediately a part of the food taken up, in which from
microscopic examination are traced no formed ingredients.
Therefore the meat is not devoured in small pieces,
particularly rather milled through and pressed out by
continual work of the mouth appendages to then take up the
food in liquid form. The tough and hard tissue stays behind,
as it also happens in the-consumption of a diptera larva
whose cuticle stays behind--when also in young larvae till
crushed to an unrecognisable state. Besides the mechanical
work to gain liquid food, it would be possible that the
material becomes liquified through the effect of a deposited
extraintestinal gut secretions. Although the vomiting of a
digestive secretion was not able to be explained up till
now, certain observations and considerations make the
surmised events probable: A fresh piece of insect from which
a Necrophorus had eaten showed for a long time at the eating
place distinct alteration. Brown color and jelly-like
blurred contrasted the place clearly with almost unaltered
surroundings. Further speaks the admitted factor, that
disturbed gravediggers vomit intestine contents,
respectively intestinal secretions for the operation of a
preoral digestion during meals; to us this appearance is
faced only in beetles with extraintestinal digestion.
Further evidence for this species the food in-take by the
nearest relations of the gravediggers, the Silphinae, is
wide, as Heymons and V. Lengercken pointed out
(1926-1932).
B. Reproductive Biology
1. Relationship of beetles to one another.
a) Attraction of females
What ecological meaning comes from the "ventilation" of male
gravediggers? I was able to observe it 13 times, the results
of which are summarized below.
1) The "ventilation" was regularly performed in the
immediate vicinity of a carcass.
2) Ventilating animals were without exception males with
ripe, plump full testes.
3) Never was a female observed in the vicinity.
4) Only on warm summer evenings (June, July, August) after
sunset was ventilation able to be observed.
These facts suggest "ventilation" of male gravediggers
involves the females. Presumably flowing from either the tip
of the abdomen or the intersegmental membranes are sex or
species specific scent to signal passing females and to
guide their approach. This assumption is supported in that
at times one sees both sexes coming to the carrion's scent.
When the carrion is buried superficially by the male,
thereby checking the enticing smells, the sex scent of the
males was able to lure a female; a peculiar profit results
for the prey is protected from any other carrion eaters, and
remains reserved in all probability for the species.
The declared explanation also found experimental
confirmation in degrees: after a series of failed
experiments I succeeded finally in two cases in bringing to
puberty males isolated on a carcass to "ventilate." That
this experiment frequently failed, I believe goes back to
the artificial environmental conditions. Out of the obvious
assumption that the above mentioned points 1-4 are taken
from, the size of the cage (my successful exp. were carried
out in a cage of the dimensions of lOOx94x58 cm) and above
all good ventilation are important in success.
So the term "ventilation" in the gravedigger ought to be
authorized. Still it ought to be pointed out that a
Necrophorus is not designed for a such extreme placement of
the abdomen in ventilation. Although the movement of the
hind body is not equal to that of Hymenoptera--a still
striking apparent convergence finds the ventilation of
gravediggers remaining like the termites. From Tollin (1862)
a male termite that has found no partner through swarming
who scarcely had begun digging work, stretched the abdomen
in the air and stuck to it for many hours each day.
b. The isolation of pairs from a large number of individuals
and the resulting conclusion.
When a male and female of the same species meet on a
suitable body for broodcare (or the body has been found by
an already paired female (vgl. S 539) the carrion is buried
and serves the larvae as food, till they are full grown and
move further into the soil for the pupation. At this time
the store is consumed until from insignificant amount
remaining, has sufficed even for the brood's parents. But
what happens if the same quantity of food has to suffice for
the brood of several females? Without doubt the development
of all larvae would be greatly endangered through shortage
of food. This danger is likely -for it happens often enough,
that through the smell of the carrion several females are
attracted with mature~ ovaries. How is the threatened danger
averted?
J. H. Fabre (1899), as I have already mentioned in the
introduction, made the striking statement that he never
found several females on buried carcasses. Always he
mentions only one pair there, even if beforehand more than
two individuals had been working at the digging at the same
time. My first task was to examine this declaration of Fabre
for its accuracy. Fabre, known as a sophisticated observer,
remains to be considered for this observational experiment
on Necrophorus which was carried out in an aviary, therefore
under artificial conditions, with only a limited number of
beetles, namely 14 individuals. The question which I had to
answer ran accordingly: is only one single pair really found
on buried carrion in all cases, including under natural
conditions?
All my experiments were made correspondingly in natural
conditions--thus they were not successful. Through laying
out suitable pieces of meat to attract the grave diggers,
they were allowed to bury the prey undisturbed and after a
known time the number of beetles remaining on the carrion
was checked by digging it up. Digging up is too crude a
method. Through the warning of shaking the ground, many
beetles hide further into the soil and cannot be found and
cautious digging is very time- consuming.
In order to avoid this disadvantage, 200 flower-pots (12 cm
high, 14 cm diem. at top) were distributed in woods and
meadows in the surroundings of Frankfurt. They were sunk in
the soil to the upper edge and filled with dug material till
full, after the base of the vessel was covered with a fine
mesh wire net, to prevent the escape of beetles from beneath
(Abb. 3). A small piece of meat about the size of a mouse
was laid on the soil in the flower pot. On a small adjacent
piece of wood a number was marked under which the results
were entered in the records. The experimental site was
visited several days in succession and observations made on
them continuously registered. After the meat was sunk into
the ground, through the work of grave diggers. I waited 2-3
days, then dug the flower pots from the ground and over
turned them on a firm base. Its contents lay exposed in this
experimental procedure and could be searched through without
trouble, which is also of importance for later mentioned
observations. Escape of beetles was excluded through this
arrangement.
In 197 of the mentioned cases a pair was found on the
carrion or in the near vicinity within the flower-pot; in 53
cases, I was able only to find a female. In 4 cases I found
in the near vicinity of the crypt 2 females (which by
further investigation proved to be sexually mature). My
experiment then proved perfectly that really only 1 female
remains behind on the buried carrion and it has in the great
majority of cases the male with it. The number of exceptions
is so small that they may be neglected. Also it ought to be
emphasized that the results were not being influenced
substantially perhaps by the lack of competitors: in most
cases more distant individuals were found in a small
distance outside the flower pot in the soil.
If the initially asked question is answered in the
affirmative, we are faced secondly with the problem of how
the separation of a single pair is brought about from a
large number of available candidates. This looks as if it
involves animal psychology. But the apparently puzzling
occurrence is clarified through broader consideration. The
grave diggers dispute their right to possession of the
carrion by fighting one another, and it is predominantly
individuals of the same sex which fight.
The description of a typical start to an experiment comments
on the cause of events in detail. In a spacious terrarium, 4
pairs of the species N. vespillo were introduced. Every
female was marked with a color mark on the pronotum. All
males received at the same time in the same color (white,
light blue, light green and yellow) a mark on the elytra so
one was able to recognize again at a glance the sex of the
animal and at the same time the individual. The pairs were
first confined to this experiment after they, separated from
the others, had proved their sexual maturity through desire
to dig.
On the day when the animals were committed resting onto the
soil, I layed a dead mole on the soil surface in the middle
of the cage. At sunset the first female (bright blue)
appeared and hurried to the mole, climbed and clambered
around on it, in order to disappear. Here & there the
shaking of the dead body showed that the female was digging.
Meanwhile the light green and yellow females also turned up.
While the first ran here and there the corner of the cage,
the yellow male marched smelling to the carrion. Arriving
there it soon slipped under the body. While for several
minutes nothing happened, till both animals, the bright blue
female and the yellow male by chance at the same time came
forth from under the mole, each at a different place. Now
the female scent appears to come from the new comer (male).
The female pauses visibly and runs directly to the male with
quick steps, hesitates, reaches with the antennae the
abdominal tip, that aggravates him, as the male has quickly
hurried over by the approach of the female. Scarcely has the
female succeeded in testing the hind end of male with
tasting tips of the antennae, than it turns away. Then the
male begins to follow the female. With a few, hasty skips it
stops, a little shake with the antenna and with the loud
chirping of both animals, mating takes place. After that the
pair turn toward their work again.
In the corner of the terrarium, the second female appears
level with the soil. It bears a white color on the neck
plate and is already well known to me because of its
strength and savageness. This beetle also goes to the
carrion immediately and meets there with the female. Both go
immediately one after another and both appear to recognize
the smell of the hind end of the body of the sex of
opponents. The movement of the grave digger at their work
particularly on warm summer evenings was described already,
and quickly mentioned, yet the aggressive savageness
surprises so, with those in the situation the two females
rushed at one another. The fight begins.
But, scarcely begun, it is already finished. The bright blue
female, which at first had taken possession of the mole,
left the place of the fight in a rush to escape. It searched
the walls of cage and clambered at them, and as this did not
succeed, it buried itself in the furthest corner. It was to
be seen no more for the rest of the evening. The strong
female with the white mark now crept under the mole where
meanwhile the male was pursuing his work. The victorious
female thus thrust aside the loser, and took possession of
the prey itself. Soon it emerged again a few cm distance
from the body. Close examination showed that it had created
a passage, which began under the mole and ran to the surface
here. One is reminded of passages which lead from the inside
of old fortresses to freedom. In this security the female
remains sitting till hidden to head and forelegs in the
soil, and throws itself from here out onto every grave
digger which wants to approach the carrion. First it is the
light blue female that comes that way. Although after
itself, the female has information about the sex of the
approaching animal, it creeps under the carrion, and turns
up there shortly again in the mouth of its subterranean
passage. The newly arrived male occupies himself meanwhile
with the digging work and has soon disappeared in the soil.
An approaching female (yellow) is treated less peacefully.
Scarcely had it shown its identity through its scent as
such, than it was attacked savagely, and was driven away
after a strong fight. The bright green female did not come
off any differently. From this the white maintained its
place. During the watch the guarding animal stayed directly
in the soil. I sat the yellow, already once expelled female,
with forceps on the mole. Then it had plenty of time to
work, and was mated with the bright blue male. By accident
the white female came into its vicinity, the fight was
repeated, then again the yellow animal left. Now it was no
longer possible, to bring the defeated female onto the mole
and for it to remain there. I tried it so often, but these
animals were good at escaping from these places. The strong
female even set back the yellow, and then also attacked it,
when I held it tightly with the forceps. Both layed into one
another so quickly, that I was able to lift both fighters
together without them separating. During these maneuvers a
newly arrived male (white) succeeded in getting under the
body unnoticed by the white female.
If the described events are valid as typical, then they also
show certain peculiarities. For example, the sort and manner
as the victorious female creates an ambush in the form of a
subterranean passage, do not describe the rule, although
this was also observed in other females now and again.
Frequently the female briefly undertook trips radiating out
from the carrion but always returning to the body. I would
like to interpret this behavior of the female as a species
watch (guard) service, during this the close vicinity of
possessions at risk is searched for strange females and
prevents an unnoticed intruder going by. Further the
experiment demonstrates that the second female was superior
to all remaining in strength and drive to attack. If this
were not the case, then the carrion could be conquered still
by the third or even fourth female and so several times
change owners.
The experimental description has shown in which way the
fight between grave diggers of the same sex brings about the
peculiarity of a single pair from a large number of
individuals. It is true that the described mechanism is not
absolutely clear, but still works with greater certainty
except with a very small number of exceptions (4 out of 254
cases) in which two females were found on one and themselves
buried the carrion.
But how is it, that 53 times only females were found. So far
it is not a matter of cases in which a female mated
elsewhere had layed out the burrow; this is the result of
one female changing instinct after completing the passage
digging. The female that puts up with a digging male,
however, attacks after completion of the crypt of the
partner and drives him away. Most of the females enter the
change in behavior shortly after egg laying, yet can before;
after this time the male can be expelled from the brood
chamber. Under these circumstances, different sexes of the
same species of grave diggers alone fight. I was never able
to observe, under the same conditions, fighting between
different sexes of individuals in N. vespilloides.
The fighting instinct of males and females has a certain
distinction. The female behaves during the time of passage
digging in a vigilant, and limiting manner and neglects the
digging work more or less. The male however turns
exclusively to the work. First if they by chance meet at
their work, the fighting instinct is aroused. Then their
fight does not differ from that of the female.
The circumstance that temporarily several males are able to
be found digging at once on the same carcass, is naturally
very conspicuous in the biology and has given occasion to
far-reaching, theoretical remarks. One remark by Fabre says
that the researcher saw, on average, more males to use the
help of pairs, often also the effect of social instinct. "Un
couple etait-il dans l'embarras, avertus par le fumet, des
aides surviennent, servants des dames, qui se glissent sons
la piece, la travaillent de l'echine et de la patte,
l'enterrent, puis s'en vont en laisent a leurs joies les
maitres de caens". Quite similarly Renkev (1913) interpreted
this appearance as "the first germ of an altruistic
instinct", while Schroder (1929) in his even wanted to
recognize a " particular case of the use of strange foreign
expediency".
Alverdes (1925) expressed the presumption the opposite way,
that the assembly of several grave diggers at carrion
suitable for broodcare showed only "association", in a
sociological respect, whose origin lead back to
environmental factors, not the existence of social
instincts. This explanation neglects the crucial point of
the problem, namely the question of what causes the surplus
individuals to leave the carrion. So far, however, only the
carrion work of the males from my results confirms Alverdes'
prediction.
For inspite of the common work which in truth is only based
on the chance adding up of single results, in fact no
relationship exists between the digging males. Every male
works for himself alone and for himself personally. His
advantage lies in the chance to reproduce successfully and
this possibility is never abandoned without a fight in favor
of a competitor. Every male looses much more than every
female, first then his place and if it is forced to do that
by a mate of the same sex, which is superior to him in
strength and skill. Through this therefore, the prediction
of Renkers' (1913), as also the explanation of Schroder's
1929, is without foundation.
c. Ecology of the fight.
The main value of the fight between gravediggers of the same
sex is frequently that a single pair remains behind at the
carrion and their descendants find sufficient food. The
existence of the instinct to fight is already sufficiently
motivated through this.
But it appears to me that this fighting still raises a
further use: An effective choice within the species. This
opposes us more clearly in the fights of females. Any female
which is in possession of a body has to prove, as it were,
before a new approaching female by way of a fight its
strength, skill and the normal functioning of certain
instincts. Then the carrion passes into possession of the
superior animal and so that the possibility of egg laying on
the piece of carrion in question remains only for the better
equipped female, and obviously encourages the propagation of
the stronger and healthier female. While one is allowed to
accept that this stronger selection puts the reproduction of
weaker or even genotypically retarded females in
question.
The translation of these results in the fighting of males
with one another however meets with difficulties. While with
the driving away of female animals also these germ-cells
become distant, when any female first proceeds to egg-laying
after the burying of the carrion and these long winded
preparations, the numerous males are chased away often first
because of the small desire to fight off male animals after
they have already mated with healthy females of the same
carrion. In such cases their violent removal becomes
meaningless as a means of eliminating inferior quality. A
cause of selection, through fighting, therefore can be found
in the males only in one case, in which the chased male did
not succeed in mating. The possible drawback in this for the
species made up for through the gain would result from the
adding up the work performance of present male at the same
time.
This remark "natural breeding choice", which is recognized
in the fighting between gravediggers of the same sex appears
suitable from consideration to permit a classification in
the narrower conception of "sexual breeding choice".
Although those fights, at least by the females, determine
the permission of participation of individuals in
reproduction, nevertheless the lack of connection of
fighting animals to the representatives of the different
sexes excludes the sexual breeding choice. The fighting of
gravediggers is for possession of the carrion, not for
possession of females, and males respectively. Any
Necrophorus fights only in the direct vicinity of a suitable
body for the broodcare. At meetings of the sexes under other
conditions, for example of large carcasses, mating pairs are
frequently found without observations of hostile attitudes
or fights between individuals of the same sex.
d. Fight with strange species or races of gravedigger
Gravediggers are sometimes also ready to defend the stores
of food committed to the brood against representatives of
strange Necrophorus species. Naturally it gives in easily
for in districts with mixed environmental characters several
species are attracted to a carcass. Yet fights do not always
result from this meeting; the weaker species usually hastily
retreats before interacting with a larger species. This is
established particularly frequently in N. vespillo and N.
humator. A N. vespillo female leaves the prey it has
fiercely defended against females of its own species, when
approached by N. humator of either sex. Even strong N.
vespillo females, which in size are scarcely smaller than
weak example of N. humator, retreat, leaving the carrion to
a N. humator. Occasionally particularly aggressive
individuals of N. vespillo fight against an enemy of a
strange species with variable results, as I observed in
eight cases. The fight developed then as between the same
species, however it went ahead independent of the sex of the
two participants. More males and females attack
representatives of the weaker species, for example N.
vespillo or N. vespilloides, without any hesitation provided
that these do not avoid the attack through hurried
escape.
A remarkable observation is inserted here. In the
surroundings of Frankfurt the species, N. vespilloides shows
considerable variation concerning elytral markings. The
black crossband which normally goes through the elytra
unbroken, slightly behind the middle and often extends so
far that the distally lying red yellow crossband, is reduced
to two almost round spots, is displaced and in several
examples is so strongly reduced that only a row of small
black dots or a long line remains between which both red
yellow marks come together. There the existing various
different kinds of elytral marking show all transitions,
missing any possibility which separates stronglv differing
forms with the help of named characteristics as hereditary
race of the species N. vespilloides
The investigation offered me the opportunity to establish
whether really only one pair from a large number remained
behind on a buried carcass. I observed that females with
strong elytral markings either always paired alone or with a
male differing from the norm by the same degree. This
observation is based on twelve recorded cases which were all
positive. I never observed a negative case, and I can rely
on several more positive but not recorded observations. If
one considers the relative scarcity of this extreme
observation, it appears that it could not be due to chance;
one explanation is that the abnormally marked females fight
or drive away the normal males and only leave males similar
to themselves unmolested. This supposition assumes a
physiological difference of those females from the normal
which can only be based on hereditary mutation. Then would
mention the possibility for it that the weaker differences
of all degree is considered genotypical without being
isolated physiologically from the stem type. The extremely
different form had the value of a beginning species.
e. Course of the fight
After establishing which individuals fight each other we now
move towards the kind of fight. The typical fight of a
gravedigger happens with such speed that it is impossible at
the first observation to grasp in this confusion fast moving
legs of individual actions. Repeated observations of the
incident supported by instantaneous photographs finally
allowed recognition of the real stages of the fight. How
quickly the movements were carried out comes from special
difficulty of the photographic technique. At first, when the
exposure time was reduced to under 1/100 sec the picture
received sharp contours. Most were therefore taken with an
explosive of 1/250 sec. I operated with "Agfa instant
flashes." It offers the advantage that at the moment of the
brightest illumination which lasts for the duration of 1/250
sec., the adjusted shutter of the camera was released
electronically. The duration of the fight amounted to the
quickness of movement corresponding to a few seconds. Only
rarely for example with very fatigued animals did it take a
longer time.
The beetles usually simultaneously came upon the lip of the
abdomen of others scent, began the fight in opposite
positions. Next the opponent throws himself at the side and
seizes the opposite side of the head and thorax. In this
attitude one sees how each of the fighters works the abdomen
of the adversary with its mandibles. The closed jaws slide
repeatedly from distal to proximal part and bite, where they
push off the hindrance as at times on the segmental
boundary. To this point the fight still remains undecided.
Soon the firm hold of the beetles appears to loosen. Each
individual tries to move his adversary with his legs, for
this the longitudinal axis is put across its own. If one of
the fighters succeeds in this, the fight is soon decided.
The beetle which at first squeezes his opponent with his
legs, now has an easy match. He presses his victim together
many times with such strength that one hears the chitin
cracking. The animal which is stuck under the firm grip of
the legs in a living vice is unable to offer resistance
while the limbs are shackled to him. Fig. 5 shows this phase
of the fight with which its end is reached. Again free, the
defeated gravedigger takes to flight immediately.
Finally still the result of the fight remains to be
considered from the individual concerned. The weapons of the
gravediggers which are employed in the fight are above all
the legs, in particular the last pair, the mandibles being
of secondary importance. Although the fight advances on
strength and wildness one hears at times loud cracking of
the chitin. I have never seen a beetle emerging hurt from
it. Conspicuously indeed the comparatively high percentage
of mutilated individuals remains, which one comes across at
the time of reproduction. Mostly it concerns missing tarsi,
sometimes also missing tibiae or antennae. It is also
possible that these animals have lost these parts of the
body through their difficult digging work; I nevertheless
incline towards the assumption that also during the fight,
injuries can originate. On the one hand, the observations
speak for this interpretation for I repeatedly found injured
individuals full ovaries with mature eggs. This suggests
that the animals in question probably still had not even
dug. Also, the mutilated animals are particularly
aggressive; they are more frequently willing to fight the
others, especially strange species and perhaps stronger
individuals.
Undoubtedly the escape instinct supports, so far as it
concerns fights between individuals of the same species, the
fighting instinct with regard to its biological significance
in most favorable ways. Then firstly from the cooperation
both results in every advantage for the species which were
already reviewed in the last chapter.
f. Relations between partners of the pairs.
That the relationships between male and female during
digging are extremely loose has already been said. The fight
between beetles of the same sex round the carrion can in
effect bring about a change in the pair without releasing a
visible reaction to the fight by the indifferent remaining
partner . Only the presence of the body holds both sexes
together at this time. If the carrion is removed the animals
then leave the digging place and furthermore take no notice
of one another.
Male ------------------------
Female -------------------------
While this incident in the male is already over after
several seconds and repeated first after a considerable
length of time, I was able in three cases which were
observed completely in the wild, to overhear the loud
utterances of the gravediggers which really differ from the
first type described. It is a matter of a continuous common
chirping of males and females which lasts minutes long and
sounds very strange. The unusual sound picture arises that
each alternates a chirp of one animal with that of the
other, schematically expressed:
Male - - - - - - - - - - - -
Female - - - - - - - - - - - - -
Whether a firm task falls to the chirping of the gravedigger
at the digging and which it is, I cannot say with certainty.
In all probability the partners influence (perziperen) their
mutual chirping in some way. Copulation took place regularly
in the course of digging, suggesting that the mutual
chirping is an expression of sexual excitement. Moreover the
possibility exists that the chirping operates as a warning
noise and serves to frighten away related species or genera.
The common minutes-long chirping of males and females
especially give this impression. It was strengthened owing
to a pair chirping in this way being constantly in regular
activity: while the male digs one frequently sees the female
radiating from the prey, going out investigating.
As soon as the digging work is thriving over the first
start, the mating happens above ground. The male climbs the
female in a manner that in normal important relations his
foretarsi get a grip of the corners of the shoulders of the
female and the middle tarsi grip round roughly the
epipleurons. During the actual union of both beetles the
male performs a brushing movement with his foretarsi which
appears broadening in relation to the female. At the end of
3-4 seconds copulation is terminated. That same pair can
however still proceed to renewed mating before the
conclusion of the digging work.
a) Brood care
Examination of the food selected for the brood. As the
gravediggers are not choosy concerning their own food, they
are also satisfied with any carrion for the provision of the
brood. Indeed the size sets the upper and lower limit.
Beetles are not able to bury large pieces and small ones do
not supply a sufficient quantity of food for the numerous
hungry larvae. However, between those there remains
considerable latitude. If we disregard the strongest member
of the genus, N. germanicus (this species is rare in the
surroundings of Frankfurt and so therefore too small for
experimentation) carrion from the size of three day old
kittens to pieces of meat 1 cc were buried. They were
usually distributed such that N. humator naturally found the
largest, N. vespilloides the smallest pieces.
Unable to detach pieces from large carrion, the beetles are
directed in their care for the offspring predominantly to
the bodies of small vertebrates. When a sexually mature
gravedigger has located a carcass, he first reacts in a
wholly determined way. The dead body is climbed, the
mandibles beat here and there loosely in the carrion with
the maxillary palps clearly shaking, the antennae moving in
a swinging motion. At least twice the beetle steps in nearly
vertically one upon another, standing directly on the body.
Thereupon the beetle slips under the carrion and its
activity can only be guessed. He raises the prey a little
from the ground.
The single phases 1. Examination of the find with mouth
parts and antennae. 2. Inspection of length and breadth. 3.
Lifting of the carrion, take place in succession, being
constantly observed before the beetle begins the real
digging work. One gets the impression that the animal deals
with the act of examination of his find in this capacity. It
is conceivable that initially after the find through each
constantly recurring action the chemical condition, size and
shiftability are examined, thus first from the determined
combination of doses of excitement, the digging instinct is
released. A confirmation of this presumption is seen in the
fact that a Necrophorus not once attempted bury carrion
whose size exceeded his capacity.
b. Burying of a body
Naturally the direct observation of the digging activity was
impaired to a large measure because the largest part was
carried out in the soil and therefore remained invisible to
the observer. This circumstance explains why the analysis of
the digging technique of the burying beetle is still
outstanding. In order to be able to follow the occurrence
without a break the investigation must be completed in the
wild as such under known artificial conditions. To this end,
sexually mature beetles were brought pairwise into narrow
terraria to dig so that a direct observation of the activity
was possible through the glass walls.
The terraria (24 cm long, 18 cm high, 2.5 cm broad) have
wood borders on the narrow side-walls which are cut crossway
square (2.5 cm x 2.5 cm). On this wood border is screwed,
from below the soil on the long side, a right angled
aluminum strip. The long walls consist of glass. Each glass
plate leans on the right as well as the left woodborder so
that every time
However, under these conditions which differ strongly from
the natural, some of the beetles did not set about digging,
but rather exhausted themselves through continuous attempts
to escape. In order to be able to use these animals for
experimental purposes, I attempted a further trick. A box of
zinc plate was made of the size that therein holds the glass
vessels put against one another, tightly. All single cages
were filled full with soil and above that still a 2 cm thick
layer of earth spread out. It originated in this way a roomy
surface under which the glass terraria lay hidden. At the
start the beetles were in no way confined to their work. In
due course they came across the glass wall in the soil, but
this hindrance meant no more then a stone or a hard root in
the wild which the animals are able to avoid.
Actually all gravediggers went nearly immediately to the
work. They fall on the way in the depth with their prey
necessarily in one of the glass boxes which then with help
from the earth surface over-topping is raised out and the
observations made. The glass cage being used was replaced
immediately by another, and consequently the described
experimental order is ready for use so long as it does not
lack sexually mature beetles.
If a beetle found a carrion which proved after complete
examination to be suitable then the digging work began. Next
the beetle scraped more soil out from under the body. The
gravedigger thereby proceeds so that he carries soil
particles from behind with forelegs which is further given
to the second pair and then reaches the last. From there the
strong hind legs shove out the mass from under the body.
Though frequent repetition of the activity arises the
already mentioned small earth wall of Fabre (1899). However
the dead body is not ring-like, but only grasped round
partly. Correspondingly, only a part of the carrion is also
only dug under.
More frequently than on bare, soft ground the gravedigger
finds the carrion on hard leaf. This is especially true for
the the meadow-inhabiting N. vespillo. Under these
conditions are experienced the application of the already
well known (Fabre 1899) ability of the beetles to cut
through grass and roots with their mandibles. If one perhaps
removes the small animal body an hour after work begins, one
sees an approximately round place on which the grass stalks
are pressed on the sides and are bitten through the base so
that from the small area all sort of roots still travel
through, laying free in the soil. The size of the prepared
place always is dependent on but, importantly, smaller than
the object to be buried. Therefore, also in this case the
whole body is not dug under.
From this stage onwards the digging work is continued
independent of the existence of a ground covering always in
the same way. Even after the
Up to this time one is able to follow the work of the
Necrophorus from the shaking of the body, then suddenly this
ceases and remains completely motionless for a long time.
Through that the observer receives the impression that the
animal has ceased its activity, but in reality the work goes
ahead briskly and is continued in another way. The working
animal has begun from the hitherto cultivated, shallow
depression to dig out a passage sloping from beneath in the
soil. There the diameter of the hole behind the original
cavity falls behind with its increasing length more and more
so an automatic after sinking of the body does not come into
question (Fig. 8b). By this it appears that it is able to
arrive during the work of the beetle with no shaking of the
body. As tools for the digging of the pit, the animal is
served principally by legs, which forward the soil in the
already described species and manner. Now and again one also
sees the gravedigger push large lumps of soil with their
neck plates.
The newly excavated cavity reached finally a length of 3-4
cm, with very small carrion from about 2 cm, so begin again
a new phase of digging activity. The carrion now becomes
forced into the prepared hole. This represents an important
achievement for the beetles. For the body must be brought to
a small volume, otherwise it does not manage to pass along
the narrow passage. The start of this working phase the
beetle orients itself under the carrion so that its back is
turned towards the underground, the abdomen tip towards the
newly excavated, funnel-shaped hole. The animal now grips
the body with the tarsi and hauls it in with accurate
movements of the legs from behind and like-minded movements
from the head and thorax over itself off deeper into the
hole. In most cases, the gravedigger grasps the carrion in
the middle, and pulls it into the passage while the LAST
SENTENCE CUT OFF WHEN XEROXING! this way, out of it the
advantage results that the carrion is pressed together in a
very much concentrated mass. This is the first step in the
rounding of the body which is experienced later in the
crypt. The more the carrion squeezes itself in the slope in
the cavity leading to the funnel, the more effective the
effort of the working beetles which in order are firmly it
refutes gains on the funnel wall.
How successful this working method is, is shown by a chance
observation. A N. humator female buried a piece of horse
meat some 6-8 cm in a sunk flower pot filled to the higher
edge with earth. By aiming at the base of the vessel, the
female pressed its find through holes in the base of the
flowerpot which were big enough to allow through the N.
humator female a small example. By digging up the pot, the
meat was held tightly only still with a small point, while
the remaining part had already passed the narrow opening.
Only with difficulty did I succeed in pulling back the meat
the same way with forceps which is able to give an idea of
the working capacity of the beetle.
Still before the carrion has arrived at the end of the
prepared passage it is extended in the beginning successful
way. Fig. 8d. Scarcely does this happen round a tiny piece,
so the animal shoves itself along between carrion and soil
and walks round the prey and in doing so gives it the
advantage of a smooth body shape. In this way all parts of
the body become dragged here which still stick out of the
main lump as for example tail and legs. Thereby the whole
carcass is stuck together more and more. Fig. 8e. Both
activities, gradual lengthening of the passage and hauling
back the body with rounding off at the same time, alternates
at length with one another till the carrion has reached a
certain depth. The thrown out material is, as already Fabre
(1899) observed, in the meantime thrown together. Only a
small soil accumulation free of plant growth betrays the
activity of the gravediggers.
From the given description one presumes understandably that
the carrion is not vertically under the small hillock.
Moreover, one regularly finds several centimeters laterally
from the small heap, loose earth distant in the soil (Fig
Bb), an establishment, which again allows a conclusion about
the working method of the gravedigger that through digging
under the prey alone would hardly succeed in this place.
In the same way as N. vespillo goes about its work. so too
does N. humator, N. germanicus, N. investigator and N.
fossor; N. vespilloides is an apparent exception. This
inhabitor of drier woods buries his finds only under
mosslawns, leaf or needle litter which puts plenty of living
room at his disposal. His performance confines him thereupon
to carrying the body through the often several cm high
ground covering till the carrion is put on free earth. If
one finds such a buried carcass, one can be certain to have
the work of a N. vespilloides, Fig. 9. The instinct of this
species is so well adapted to the normal conditions of his
environment that N. vespilloides in a cage without ground
covering is very rarely brought to reproduction as I was
able to experience with my experimental animals. However as
soon as one puts plenty of moss or needle litter in the
rearing cages, they are no longer opposed to the natural
course. Thereby showing that the different phases of the
digging event, which were observed in the work of the other
species in the essentials are also the same in N.
vespilloides, only on account of the loose condition of his
materials differing in appearance.
A proof that the above described manner of digging is not
confined alone to the named native species, is given in an
observation of Osten-Sacken (1862). According to him, N.
americanus ought to produce a long sloping in the soil
leading to a pit in which he slips in the carcass. There
this beetle ought to be instructed in the main point of
snake carrion, Osten-Sacken considered this as he believed
from that of other species, differing in digging method as
adaption to the shape of prey animals. However this
assumption was invalidated for other Necrophorus species
proceed independent of the shape of the body likewise. Then
the extension of the prepared hollow with increasing size of
the buried object takes root, becomes understandable if the
long passages which are manufactured as a rule in N.
americanus to the taking up of snakes, is seen, while they
were able to ignore decidedly shorter cavities in the native
gravediggers average for small carcasses.
The depth up to which the gravediggers bury their finds,
varies with the species. In some 60 cases the distance of
the soil surface vertically to the roof of the crypt was
measured. Thereby proving that N. germanicus digs the
deepest. This species was only observed in captivity
however, in which the animal moved the carrion without
exception to the hottom of the cage which was 20 cm deep; it
is possible that this animal reaches greater depths in the
wild. A buried carcass of N. humator was found normally at
7.4 cm depth; the highest measured value for this species
amounted to some 13 cm. In the smaller interval from the
soil surface one finds the brood holes of the remaining
species.
The time which the burying of the carcass requires is
dependent on the size of the object, on the condition of the
ground of the physiologica competence and finally the number
of working beetles. The following observations which refer
to a female N. vespillo give an idea. The animal had carried
a mouse in loose garden soil, with peat mixed through to a
typical depth for the species N. vespillo in 3 hours. In
natural conditions in which a turf is penetrated through,
roots and stones are dealt with; this work takes up 8-10
hours in the above described narrow glass terraria the
beetles required likewise a whole night during which several
long rest pauses were observed. If several gravediggers take
part in the work, so the working time shortens in proportion
to their number.
One finds clumsily formed balls of carrion after the finish
of the digging activity in the ground. Fig lOa-f. show
carrion taken from the crypt. From all forms shows the
instinct of the beetle to adjust the carrion as far as
possible to a ball shape. The skeleton of vertebrate animals
frequently resists the rounding; from figures lOb and c this
problem is obvious with a bisected frog. Boneless pieces of
meat which can be shaped anyway by the beetles keep a
perfect ball shape well. The expediency of this round form
is obvious: with the smallest surface area the ball protects
the largest volume and therefore is particularly favorable
to preserve the food from desiccation.
Rounding of the carrion and building the crypt are closely
connected in their origin, and arise as a result of one and
the same activity. For hours on end the beetle wanders round
the buried prey step by step in all possible ways. With its
feet on the body and its back against the surrounding earth,
he lifts the mass continuously through stretching of its
strong bent legs away from itself. Through the counter
pressure of the soil which opposes the back of the beetle as
well as the opposite lying side of the body, the carrion is
strongly pressed together and obtains the ball shape from
the constant repeated kicking on all sides unless the strong
skeleton resists it. Necessarily through the constant
pressure of the surrounding soil, a cavity arises with hard
walls in which the fully rounded animal body rests. It is
the crypt. The diameter is dependent on the size of buried
object as well as that of the working beetles.
As was recognized already by Fabre (1899), one finds the
small mammal's bodies hairless, the bodies of birds
featherless also without tail or steering feathers in the
crypt. How this appearance ought to be explained, as though
the results of wind decay or as the result of active work of
the beetle, Fabre left open. However, none of these
possibilities alone give the truth; first through the
interlocking of both factors arise several admitted results.
Through the ungentle treatment which the body receives
during the digging activities in the passages of the narrow
cavity on the one hand the sedimentary hairs or feathers
which are only still loose in the skin of the body, fall out
without direct action of the beetles; on the other hand the
animals subscribe directly to their removal: after rounding
the body and at the same time completing the crypt, the
beetle continues to walk round over the carrion, however
with renewed activity. Like a spatula its head now stretches
with slightly open mandibles over the surface of the ball
and free the food provisions of particles for the larvae and
also still attached hair or feathers. For hours on end, only
broken by short rest pauses, one sees the beetle performing
these movements which are brought about through bending in
of the initial thrusting out of the head. In this way every
square millimeter is cleared stepwise--apart from direct
observations--as is recognized from an enormous quantity of
tiny points which were left behind by the mandibles in the
soft substrate and which by now cover the whole carrion
ball. The falling away--be it hairs or feathers--gather on
the base of the hole and there become pressed against the
walls of the brood rooms by the gravedigger so that he
covers them with a carpet similar to the base of the crypt.
This was observed at its most perfect on carrion which were
buried in soft peat, free from all hindrance. Under the
conditions which in the wild are most frequent, one finds
traces of hair or of feather which have been lost for the
large part already on the way through the firm soil.
d. Egglaying
When the carrion is rounded, cleaned and resting with a
smooth surface in a firm-walled crypt (which to reach this
portion of necessary activities requires in total between
12-48 hours) the female proceeds to egglaying while the male
repeats from time to time the above quoted work.
As first Hain (1927) and V. Lengerken (1928) have stated
independently the eggs of the gravedigger are not layed on
the carrion or in the carrion, but individually in the soil,
as I likewise confirmed. Unexceptionally I found in more
than 100 cases, the eggs of included species, singly in
small earthholes which show a striking arrangement. They
lie, as I was able to observe in the species N. vespillo (76
cases) in some 6-10 mm intervals to both sides of a round
shaped, smooth-walled against shock, indeed little resistant
passage which the mother animal has dug from the crypt out
into the soil. It is analogous to the parent gallery of the
barkbeetles, a space is quite big enough to let the female
move through it unhindered. The passage runs horizontal with
weak bends to the crypt in the soil (fig. 11 and 12). Its
length depends on the number of its flanking eggs, whic in
N. vespillo can fluctuate between 1-24, but averages about
14-15 eggs(Table 3.)
The small chambers in which the eggs lie, are built in all
probability through the activity of the extended laying
apparatus of the female, which releases frothy drops of
secretion at the same time the small hole receives an egg.
The barrel shaped eggs are white and shining. The size
directly after egglaying, an average of 2.958 x 1.836 mm, a
result which was calculated from the following values (table
4).
e. Moisture and digestion of the body
After egglaying the female hurries back again to the crypt
and moves the pile of earth from building the mother passage
to the side while it wanders round the carrion in the
explained way and presses the loose earth to the wall. This
as all previous earth working of the gravedigger requires
for success a certain moisture of the ground which the
animals find in the wild at their living places and which is
carefully supplied in the terrarium.
After the hole is tidied up, the female begins with a
remarkable activity: at the highest point of the carrion
ball the animal digs a circular hole which on average
measures 1/2 cm in diameter. First the surface of the body
is bitten to pieces with the mandibles then the fore tarsi
grip extended into the gaping split. With help from head and
forelegs which represent, with the strong tibial spurs,
excellent instruments, the cutting circle is pulled apart as
with hooks so the opening enlarges to a small crater. In
this hollow the beetle lowers its head and begins to eat, as
is concluded from the movement of the mouthparts. It is the
first meal taken from the buried body. This was certainly
not remarkable for an eating place and was found repeatedly
at the corresponding point of every carrion ball and at the
same time. The place was at that where later the young
sliding larvae gather. At the end of about 1/2 hour the
beetle appeared satiated. Still she never moved away without
shutting the hole and finally smoothing the surface for a
long time with the underside of the head. In fact this
precinct in no way differs from her later surroundings. The
excrement of the beetle, after completion of this work, was
distributed now on the upper half of the ball. The animal
which put the anal opening of the carrion surface to this
purpose, proceeded slowly over it then while the abdomen
carried out a sideways pendulum movement in this way the
liquid excrement succeeded in making a snake-like track on
the carrion which is recognizably shining with moisture and
first disappears after some time through blending.
In the change with hours of peace which the animal spends
pressing tight the meatball, this work fills up the period
in which the brood passes through embryological
development.
The question of whether this instinctive action is of use
for the descendants ought perhaps to be answered in the
affirmative. The smearing of the liquid excrement on the
ball of carrion doubtless serves the use on the one hand
that the food provisions, already protected against the
danger of drying out to a certain degree by their shape,
remain moist with greater probability as this then also
found, always independent of the weather, moisture
glittering in the crypt. On the other hand, for this reason,
it is possible for the carrion ball to become dried through
with the scent of animals which, is demonstrated in return
for our ability to smell the gravedigger in the excrement.
Be it that the Necrophorus scent acts to scare away other
likers of carrion or also the attraction of the small larvae
which emerge from the egg scattered in the neighboring soil,
which would also conceivably be an advantage in this
respect.
But also the taking up of food of females represents an
action of broodcare. Presumably, if the already previously
mentioned supposition of a preoral digestion of the
gravedigger is right, the food begins to decay with the
feeding activity of parental animals sharing digestive
enzymes with the environment. Thereby, the crater becomes
deeper and deeper with the passing of time on the inside of
the ball of carrion and the female renews the deposition of
fresh secretion which accelerates the otherwise slowly
advancing process.
That it is a matter of the disclosed consideration, not pure
speculation is proved by the behaviors of the female several
hours before the brood emerges. At this time the up to now
calm movements of the animal become hasty--nearly abrupt as
at the end clockwork. The ball of carrion hurriedly climbed
on and the eaten area opened with impetuous movements of the
head, which allow the onlooker to suspect rapid destruction
of work. Without turn, as hitherto again to close the beetle
now walks through, in many cases softly chirping, the
mothering passage from which it turns back to the crypt
after several minutes. After the beetle has walked through
the whole subterranean cavity one soon finds her sitting and
eating again on the ball of carrion at the crater thereupon
arranging the opening in a regular circle. This activity of
the female which coincides roughly with the end of embryonic
development in the brood, is closely connected with the
appearance of the descendants. The fact that the crater is
opened at this time and is no longer shut, proves that
external digested food is intended for the larvae. The
walking through the mother passage which is repeated several
times in the course of 20-30 minutes is suitable for
facilitating the hatching larvae in the soil in finding the
carrion. On the one hand the scent of the female makes it
possible for the larvae to orient better, on the other hand
the beetle roams out in the space of passages, throwing
pieces of earth. Thereby the young larvae opened a way in
which they are successful in obtaining a quick and certain
food source.
f.Male participation in brood care
Brood care is independent of the sex in sexually mature
gravedigger. Every male just as every female, is able to
bury carrion as experiments in which the sexes were isolated
have shown. Observations in the wild are however in
contradiction to this result. Males which set about the
digging work without the presence of a female stop their
work after a short time in order to attract a female through
"ventilation". If there is no result, the carrion remains
uncovered or buried only shallowly. This indicates the
dependence of the male on the presence of the female. We
verify this supposition with an observation occurring when a
female was removed after termination of the digging activity
and the male was left alone. In this case the body remained
unaltered, a fact whose meaning is unmistakable. A mistake
by the female affecting the brood and long preparation of
the body would cause a significant loss of time. The pair
remains together so that the male participates in all the
females work. However, in the majority of observed cases the
male left the crypt in the interval between egg-laying and
the hatching of the descendants--either of its own free will
or in the face of a hostile attitude from the female. Only
exceptionally is the male still found together with the
young brood, perhaps because the female is not always
successful in driving the male away. However, N.
vespilloides males are the exception to the rule because
males are found with broods in nearly half the cases.
Fabre (1899) supposed that after preparation of the body for
the brood the pairs left the crypt. The majority of obtained
observation in the wild oppose this (Table 5.) In the woods
and meadows in the surroundings of Frankfurt suitable
carrion was laid out for burying. Roughly 48 hours later I
visited the same place and noted which carcasses were buried
by the gravediggers which was recognized by the slight but
typical alteration in the ground. After a further 7-9 days
during which the building of the crypt, the preparation of
the carrion is completed the larval development must be more
or less advanced, the broodrooms were cautiously opened and
the finds resulted as follows--table 5.
After the combination the larvae from I to III are to be
found and the last stage nearly always in the company of a
pair or single female in the crypt. The moderate number of
exceptions (4 out of 59) lost most meaning when one thinks
how easily the results could be negatively influenced by
digging.
Also Fabre once found by opening a crypt of a Necrophorus
pair together with the larvae. This observation which
contradicts his acceptance that the parents shall leave the
crypt before the hatching of the brood, he explains in the
following way: "La periode de la parse est maintenant fise,
et la victuaille est copieuse. N'ayant pas aube chose a
faire, les romanicus se sort attables a cote des noumsaous"
Fabre (1899). The view arrived at in this quotation that at
the end of the brood period many parents stay with the
larvae on the carrion merely to satisfy their own feeding
requirements has till today prevailed. Through my
establishment that the parents, above all the female, are
found regularly and during the whole breeding period in the
company of the larvae, this is refuted. But this
presentation becomes entirely valid, if itself restricted a
close relationship between the female and her descendants is
disclosed over the care of the brood. Was the species
already as the mother attracted the scarcely hatched larvae,
already unusual in the beetle order, thus excelling in the
activity, which happens in crypt between the female and her
brood all that one knows up to now in this respect about
indigenous beetles.
The following observations were described using the cage in
Fig. 7, a glass cage which made insight into the crypt
possible. After completion of digging activity the cage was
brought from its vertical position to a side position, so
that the beetle was given the necessary room in the
horizontal to build the crypt. The larval nest was drawn
from the natural brood room and brought together with the
pursuing female into a small flower pot, which was shut from
above by a damp cloth. During the observation in softened
light this was removed or replaced by a glass disc so as not
to disturb the female. Before I was able to turn towards the
mentioned activity the brood must be accurately taken into
consideration.
g. The brood at the time of larval development.
In the extraordinarily small time interval of 7 days for the
development of beetle larvae, the necessary weight and size
increase of Necrophorus larvae to form the adult occurs. The
speed with which the development proceeds becomes more
astonishing if one takes into consideration the achieved
performance of growth in this time. While the increase in
length which normally is in no way exceeded (the N. vespillo
larvae grow an average from 0.5 to 2.8 cm), the increase in
weight is in proportion to developmental time. The enclosed
curve Fig. 14, whose values were obtained through daily
weighing of the larvae with the help of a Sarbornis
weighting analyzer, gives an insight into the weight
increase of three N. vespillo larvae during the 7 day
developmental period. On the X-axis is the age of larvae -
calculated from the moment of hatching-stated in hours. The
Y-axis bears the weight in mgs. The first weighing was
carried out immediately after the hatching and before the
first feeding and was for larva I 0.0035 g. After 7 hours
the starting weight had doubled. The last weighing took
place shortly after the wandering of the larvae from the
remains of the food carrion, and showed for larva I 0.3037
g, where afterward the total increase in weight was roughly
100 times the initial weight.
Disregarding the statement of absolute values, the graphical
representation gives us information about the rate of
increase of weight of age stages. Two rapid molts H1 and H2,
separate the three larval stages I, II & III from one
another (Fig. 15 for N. germanicus.) Larval stage III not
only takes up the longest time in the larval life of
Necrophorus, explaining why one encounters mostly larvae of
this age in the wild but also has the greatest rate of
increase in weight, which probably during the first 48 hours
was somewhat held back through the energy consumption in the
,short interval following moulting. The drop in the curve
towards its end (from L1 from 0.3573 - O.3037 g) has a
double cause. Firstly, the larvae cease feeding shortly
before leaving the crypt and more importantly they empty
their previously full guts.
The three larval stages, which do not significantly differ
with regard to morphology, can be recognized through certain
clear differences in their behaviors. At the end of
embryonic development, which takes 5 days, the larvae of the
grave digger hatch from the egg. In order to free the
sticking egg cover from part of their body, the animals try
with intaking movements of their longitudinal axis, in which
they succeed in about 1 min. Thereafter the egg chamber is
left immediately and the larvae walk to the stored up food.
The newly hatched larvae are white, very agile creatures,
which with the help of six legs and their hind segments
which function to push behind, move away smartly and
restlessly with antennae held upwards. The majority break
through the earth-layer which separates the egg chamber from
the mother passage following the leveled path of the female
and reach the crypt in this way in a comparatively short
time, where they are attracted to the scent of the carrion
and that of the female. Single individuals also try to
succeed directly through the soil by the shortest route to
the provisions of food where they likewise succeed. Indeed
then the small larvae require 2-3 hours to cover a stretch
of 4-5 cm which it does in constant motion, only stopping
for a few short rests. Every small stone poses an obstacle
to advance and must be gone round, till finally the animal
falls into the subterranean crypt by shoving through the
crypt wall. At once it climbs the ball of carrion to the
highest point and there enters into the crater together with
its brothers and sisters, which follow the same instinct to
the same place.
That the freshly hatched larvae, in fact react
chemotactically to the smell of the carrion which forces its
way through the mother passage and the soil to them, is
shown by the following experiment. A wall of carrion taken
from one of the crypts was laid on a piece of filter paper
and the center formed a circle of 25 cm radius from the
periphery in regular formation were distributed 10 N.
vespillo larva of the first stadium. Each larva was covered
with a small glass dish under which they were free to move
and could be put in any course. The experiment was carried
out in a room in which no noticeable air current was
recognized. The illumination of the otherwise dark room was
done with a lamp with strongly diverted light. After lifting
off the glass dish all 10 larvae wandered in roughly
circular courses to the ball of carrion. Thereby with the
exception of 6 larvae, all experimental animals appeared at
the carrion after short searching movements in the
direction, which on the whole were detained. Only 6 larvae
initially moved themselves in the opposite direction in
order to later hurry to the carrion in nearly straight
movement. In the immediate vicinity of the carrion they met
several larvae. These came to the residence shortly, whilst
these move round one another, making their separate ways
without exception to the source of the smell. The required
times for the ten are given in the following table.
The experiment was unsuccessful when repeated with larvae
III. Then their movement resembled a random walk in various
directions which took the larvae away from the carrion, as
well as toward it. Often the smell of the carrion was not
received by these larvae over large distances, although when
in the immediate vicinity these animals appear to be able to
perceive the carrion and walk into it.
The aimless wandering around, which restricted the
experiment involving older larvae, caused the animals to
meet frequently. Contrary to the behavior of larvae I,
larvae III remained closed to one another when they came
into contact; frequently they were attracted by their own
smell of carrion. This behavior gives rise to a cluster of
larvae, which is in constant motion through the circling
around one another of the larvae and as a whole rolls slowly
from the place. Fig. 16 shows one of these groups of larvae
which originated in this way. Although in this experiment
the food is only 9 cm distance from the larvae heap, the
animals remain crowded together, till after an hour the
carrion was put back 2 cm distance. First in this moment the
closely pressed group loosened and the animals clambered
singly onto the carrion. This observation shows for the
first time that the old larvae are attracted to the smell of
carrion only from a short distance, in striking contrast to
larval I stage.
With increasing age of larvae, weakness of the ability to
smell can occur in connection with the altering condition of
the larval environment. Thus the needs of the larvae
correspond throughout but the problem of finding carrion
from greater distances falls to the young larvae; larvae II
and III suffice with the kind of attraction which holds them
together in close combination with the food.
The portion of the ball of carrion from which the attraction
radiates is the higher pole, prepared by the parents, with
intestinal secretion to form the crater. Here the young
larvae appear and the old larvae remain. From here the
consumption of the carrion starts. How this takes place is
again remarkable. While the brood are in stage I and II they
contribute, through their feeding, to the broadening of the
crater in which the young larvae lie horizontal in the
casing with their heads pointing outwards. Soon the larvae
advance to their second moult inside the body. They rummage
about and feed themselves deeper and deeper into the
carrion. As long as there is no lack of food during the 7
day development, instinct allows the larvae to leave the
outside of the ball of carrion undamaged -indifferent to
conditions - so that they finally rest in a nest with both
shelter and food. In the wild in which the grave digger
directs in his brood care on small bodies, the center layer
is formed from the skin of the corpse giving mechanical
protection against advancing robbers. The rounding of the
ball therefore retains also the shape first bestowed on him.
presentation is given in Fig. 17 of how the larvae are able
to disappear into th depth of the body. If the provision of
food for the brood is less plentiful than in the quoted
example, the larvae are not able to completely conceal
themselves. They then put only the front of the body in the
carrion, while the hind end remains visible (Fig. 18).
The larvae II stage is also found if the ball of carrion was
big enough for them to allow a scattering of the larvae
inside its mass, nestled uniformly crowded with one another.
It is true the appearance was able simply to be gone back
on, that the larvae from a close undescribed precinct from
the crater in which they lie close together, begun the
advance into the ball of carrion. The following observations
however give us the right to perceive in every appearance
the utterance of a particular instinct.
It appears namely that larvae III are qualified for the
pursuing of oral drips : if one lays a larvae in a cupped
hand, they leave every where small brown drops where they
touch their closest surroundings, tasting with the head.
That allows conclusion of extra intestinal digestion. It
appears that the quantity of preorally digested food I
suppose is used for the young larvae not for the older
brood. If however the larvae III really digest preorally
then it is able to be of help to the animals, if they press
together with their heads in a small circle to feed.
Why?
We start from the condition of an isolated single larva. The
deposited digestive secretion from it is pressed constantly
in all ways loses very soon with distance from outlet point
the required concentration for effectiveness, through a
certain part of the digestive secretion going for the
conversion of food. Otherwise by some sense pressing
together of the brood. Now the resulting fall in
concentration from the oral drops of each larva overlaps
with the original fall in concentration around the quantity
of secretion of the neighboring larvae. At the place of
overlap however much too small quantity of enzymes reach
effective concentration to be able to contribute in this way
to the quick formation of the food.
This presentation finds yet further support in the fact that
one finds also other extraintestinal digesting insect larvae
in the same group. Larvae of Miastor metraloas were found to
lie radially, the head reaching from the middle of the
circle, by Springer (1917) who showed the above mentioned
advantage in their position.
If, however, the habit of remaining bundled together of
older burying-beetle larvae is an advantageous instinct, it
is doubtful through which kinds of attraction they would
intervene. It maybe a matter of mutual thigmotaxis of the
larvae. It is possible that the ball on the
Further, a most remarkable instinct of the larvae should now
be mentioned in the next chapter in connection with certain
instincts of the mother and the parents respectively.
h. Feeding of the larvae.
Soon after the first larvae hatch and have appeared in the
crypt the female employs the attraction of the brood from
the mother passage and makes for the ball of carrion in
which the larvae lie thickly pressed in the crater-like
cavity. In permanent motion they crawl over and through one
another. Their mouthparts touch the carrion - but only
tasting. They never eat from there; the transparent gut
remains empty. One gets the impression that the lengthily
prepared substrate will be in no way be the searched for
food of the brood.
Immediately however the observer presents himself with an
unexpected scene. Scarcely has the female approached the
crater when all larvae lift up the front of their bodies
steeply, so that their legs grasp in space. The beetle
remains standing directly over the brood and waves her
forelegs with shaking movement on the carrion or sometimes
on the larvae, which flock round her head. Now the female
closes her mandibles and rapidly lays the head of a larva
between the jaws itself pressing closely to the mouth
opening of the female. If allowed the chance, then one sees
a tiny drop of brown liquid on the mouth of the mother in
which the larva infringes. But already after a few seconds
the female turns and immediately tries to reach another
larva with his mouth held high. Without doubt, the brood is
fed by the female. Correspondingly the initially white,
almost see-through larvae after contact with the mouth of
the mother become dirty brown, a change which gradually
progresses from the front right to the hind end of the
larvae.
However, with what does the female feed the brood? Is the
food source the gut or a specialized gland. Specialized
glands, which are rare in beetles, was not found, the
feeding juice is derived from the gut. The administered
substance is also-gut contents presumably a mixture of food
and gut secretions. In fact, the female takes several feeds
of carrion at the crater shortly before the appearance of
the brood. The quantity of food taken up must be exhausted
with time both from feeding the larvae and from feeding the
beetle itself. To continue the activity of feeding for
several days, the female is compelled to fill the foregut
anew. With increasing age of the larvae, the female fed with
increasing frequency immediately before passing food at the
crater, an activity which claimed up to 1/4 hour.
The feeding can be observed during the first 48 hours of
larval development, but is seldom repeated later when the
brood gradually are made independent. The young larvae beg
impetuously for the food juice of the female. Three to four
larvae often reach her at the same time crowded before her
mouth and dispute their right against each other to this
place. Fig. 2, which allows a glance into the inside of the
crypt, shows directly the feeding of highly erect larvae. A
second is attacking to drive away the first. Larvae which by
chance find the hind end of the beetle, climb about
carelessly as if searching for the mouth. Most however,
immediately set out for the head of the female. Either the
larvae leave the crater to this purpose, in order to lay
back outside themselves hurriedly the prey or they make
their way along on the ventral side of the female. Here by
the larvae showed themselves, lying on their backs, forward
under the beetles and more likely did not rest till their
mouths had found the provision of food. Fig. 21 shows this
frequent performance. Rarely and only in the Larvae I stage
one catches sight of an initial attempt to reach the mouth
of the mother, namely from those forelegs held out. However,
what I saw never led to the result: The small animals
clambered with great skill onto the tibia, and lifted
themselves with their sucker - like after segment onto the
femur. Stretched forward, one then sees the larvae searching
for the mouth of the mother.
Fowlers declaration (1912), which characterized the larvae
of the grave digger as inactive maggots, is then in no way
correct for the young larvae. However, larvae were not
constantly attentitive to females. Moreover, a pattern of
use and disuse develops in relation to the molting
pattern.
Newly hatched larvae are apparently completely dependent on
the female for food. They beg extremely impatiently and
follow the female for short distances over the carrion with
great dexterity and ability. When the young brood scatter
themselves even more often over the upper surface part of
the ball of carrion, they gather again in an even shorter
time in the crater as soon as the female turns back there.
At the age of 5-6 hours the larvae then begin to feed
independently. With heads turned outwards They lie close to
one another in the crater. Their mandibles, whose points
have meanwhile colored light brown, beat continuously in the
carrion. These larvae are still ready in the vicinity of the
female to receive food, and will act as in the first hours
of life. Yet they rarely attempt to follow the mother in her
walks around the carrion; rather they use her absence in
order to eat independently. Also they stay permanently on
the inside of the boarder of the crater until the first
moult.
Rarely is this executed so the larvae arrange themselves
again searching upwards, without also only undertaking an
attempt to take up food independently so behaving just as in
the first hours of life. After the first moult there is a
second period of greater dependence on the female followed,
2-3 hours later, as in larva I, by greater independence.
After the second and last moult the larvae is directed in
feeding by the parent, in order to quickly complete female
care, and finally to grow completely independent.
Conditions during the feeding time, which stretches roughly
over the first 60-72 hours of larval development, are
summarized in Fig. 28. The ordinate shows the changing
dependence of the larvae on the female for food. On the
abscissa is marked the moment of hatching, the first and
second moults, and the end of the feeding period. Three
peaks show that the time span is distributed over three
periods in which the brood taxes the female for food: after
hatching, and after their first and second moults. Between
periods of greatest dependence on the female the larvae feed
independently. The female accomplishes certain typical
repeated instinctive actions during the entire feeding time
in the interval of 10-30 mins. she appears at the crater.
The female usually starts feeding, by moving her first pair
of legs, comparable to a running on the spot. It appears
similar to (The "Mittern") an alarm signal as a sign for the
brood to begin feeding. If this hypothesis is correct, the
Mittern ought particularly to be for the older brood, hidden
deep in the carrion, can be attracted through quick shock
waves. In fact the larvae III which are of no further
interest to the female also appear at the food after the
Mittern, if they have not already outgrown motherly care in
this respect.
After the brood is alarmed the female proceeds immediately
to feeding. The most remarkable of the activities has
already been revealed, only a little still remains to be
supplemented. The female's pose during feeding, varies,
depending upon the most favorable height for the begging
brood. While she bends deeply to young larvae, her head and
neck plate show a strong inclination downward, the long axis
of the body climbs forward moving around to feed the higher
reaching older larvae. The weight of the female rests in
this position on the last pair of legs. The forelegs are
either free and used to ward off obstructive larvae or grip
the tails of larvae being fed (Fig. 20). That the female
holds tight to the larvae on this occasion shows in the
following chance observation. An N. germanicus larvae III
was successful in begging for the food juice of the female
at the edge of the nest. During the feeding they fell down.
The female, continued to feed the larva which was lying on
its back held tightly between the female's tarsi. Indeed,
this pose of N. germanicus is nothing exceptional; they
often take up other poses in the consumption of fly
maggots.
The length of each feed to a larva, which was measured by
the period in which the head of the larva rested between the
mandibles of the female, is small. In N. germanicus the
feeding of older larvae can claim up to 18 seconds; but for
all Necrophorus species including N. germanicus, feeding
times are generally 2-4 seconds and not longer than 7
seconds. Feeding is terminated by a scarcely visible
sideways movement of the female's head. The mouth of the
female consequently slips from the larva and becomes free
for another larva. Five to six individuals are fed in
immediate succession. After a break the female again begins
feeding exactly the same way. The larvae of a nest are
provided with roughly the same amount of food juice from the
female. This is the suitable succession of a typical feeding
method. Everytime the beetle turns away after a short feed
the equal distribution of food to a normal sized number of
individuals is favored. Yet the instinct concerning this is
rigid and she allows herself to be converted easily into
useless activity by the experimentor, who reduced the number
of larvae. Nevertheless, the female acts in completely the
same way. Mittern feeding and again turning away following
one another at typical rates at which then the beetle often
enough holds her open mandibles in space, and just turns
away in the moment in which one of the larvae
approaches.
i. Ecology of Feeding.
Now the question is raised of what importance for the brood
is feeding by the females. For the larvae, is it a matter of
an activity necessary for living or of more secondary
importance? This question can be decided very easily
experimentally. If the female is removed before the first
larval feeding, and if the larvae are completely dependent
on the female, death of the entire brood must result. It
appears that such a radical result does not happen. Most of
the larvae grow up as if fed by the female.
The other hand points to two facts that the absence of the
female in any case operates disadvantageously. Whilst in 8
rearings, which together included 89 larvae, under the care
of females all individuals completed the 7 day development.
From 147 orphaned larvae which were distributed over 14
nests, 16 animals in stages I & II died, so that in each
rearing one dead larva was found on average. It is almost
certain that the early death of particular larvae is
connected with the absence of females, primarily pointing to
the loss of food. Secondly it appears that without the
female grown up larvae are able to pupate but very rarely do
young beetles hatch from the pupae; from 33 such controlled
pupae only one hatched. Under the unfavorable conditions of
artificial rearing, the brood perishes in the pupal stage in
roughly half of all cases even though fed by the female. The
very much higher death rate of the brood raised without the
female is certainly attributed to absence of parental care.
Therefore it appears that the larvae are able to develop
normally without the help of a parent animal, but that
through feeding they are favored or protected.
Is what exists now an impairment from deficiency of food?
Next comes the idea of something qualitative. The feeding
female was able to supply the larvae some substance - if
only a small quantity - which is important for development.
This assumption was not refuted but it was unpredictable. It
is assumed, that the deficiency of a specific substance in
the course of development represents here completely
expressed and believed to itself more serious aftermath if
this is in fact the case in orphaned larvae; then still the
larvae are able to complete their development to adult
without the food of the female. Moreover it is not very
probable, as p.570 showed, that the quality of food differs
from that which the female has prepared in the area of the
crater: presumably both show the same kind of mixture of
carrion and gut secretion.
More likely the impairment is quantitative. Possibly the
quantity of food which the female has already mixed through
with gut secretion before the appearance of the larvae, does
not suffice for the brood and must therefore be supplemented
through feeding by the female. However, the feeding of the
brood begins immediately after the appearance of the larvae
when a deficiency cannot yet exist in the predigested
food.
A second more distant possibility exists, why a sufficient
quantity of food was missing for the larvae in the female's
absence. The gravedigger used only small carrion as
provisions of food for the brood. This specialization brings
two-fold advantage: firstly, small carrion is more numerous
than large: secondly. the beetles are only able to bury and
work bodies of a small size.With these advantages however
must be taken a distinct disadvantage: the rapid transiency
of small carrion. The decaying process which proceeds from
the surface towards the inside as oxidation events are tied
to the equivalent of oxygen, in small carrion it leads in
time to complete destruction, which would not be the case in
large bodies. That only small carrion are used has been
proved; this assumes that the development of larvae is
highly accelerated. In fact the development time of
Necrophorus larvae is strikingly short; a 7 day development
with more than one year of each beetle larva subject to
quality food material. Even the carrion and meat eating
larvae of closely related Oecoploma thoracica took an
average 34 days for their development (Heymons). This larva,
unlike the Necrophorus larvae is not solely a carrion
feeder, but can always breed on new food when freely moving
around. Thus, the 7 day development period of Necrophorus
larvae depends not perhaps on quality or quantity of food,
but is fixed genotypically so that with or without the
female, development takes 7 days.
The necessary and obligatory shortness of the available time
to develop however requires complete exploitation. Just how
are hatched or freshly moulted Necrophorus larvae unable to
independently take up food because of the soft cuticle of
their mouth parts. Through this condition repeated failure
in the supply of food was effected, for the quantity of food
which the larvae can take up in the time allowed in their
condition, sinks below the optimum. It is as Fig. 22 shows,
just the named development phase in which the brood is fed
exclusively by the female. Therefore it is apparent that the
female prevents through the feeding a break in the supply of
food. The benefit lying therein is by no means
insignificant. During the 60-72 hours feeding period the
larvae are fed copiously by the female half the time. We see
also that all appearances after the feeding instinct of the
female serve the purpose, to secure for the larvae the
necessary quantity of food for their development despite
the--offered of other ground--shortness of their development
time.
The short development time results in yet a further use for
the species. The carrion which the grave digger must visit
to feed to maturity and reproduce are distributed sparsely
in distant parts. The problem of finding carrion becomes all
the greater the longer time placed at the beetles disposal
to visit the body; this time however becomes extended
through an acceleration of development. Above all the short
duration of development is of use for any species, such as
N. vespillo or N. vespilloides which rears two generations
in a year. The beetles of the second brood, which in N.
vespillo hatch in the middle to the end of September, still
find at this time favorable conditions to feed to maturity,
which would become absent for them with later hatching, so
that they would have to overwinter without feeding to
maturity.
j. Defense of the larvae and their food.
Also in the period between feedings or after the conclusion
of the entire feeding period one sees the nursing female
sitting mostly over the crater (Fig. 23). Very conspicuous
are the sudden movements with which the female turns here
and there as the single entrance to the larvae might be
protected by the towering female in all ways against
possible enemies. However what are the enemies of
Necrophorus larvae? Above all the male is mentioned. The
male is already drawn attention to by Soffel (1904) as an
occasional carrion eater. Although I was never able to catch
him red-handed, no doubt the male ate up carrion and larvae
in very many cases of my experimental nests. From an area of
about 1 hectare he had expelled outright, after robbing in 3
days not less than 36 buried carcasses which had fallen
victim, as was gathered from the existence and development
of typical male tunnels.
Naturally what happens when this superior enemy comes face
to face with a defense of the brood does not need to be
questioned. This is still of value for the hostile insects
which either as predators hunt after the Necrophorus larvae
themselves or as carrion-eaters were able to impair the
closely measured food for the brood.
One of the predators is the large Staphylinus olens Mull. It
was found in 6 cases in experimental nests of N.
vespilloides, among them 2 times in 2 examples. In all cases
the nest, in which I had established a normal number of
young larvae (I & II) only a few hours before, was found
empty and with no trace of the larvae in the near
surroundings. On the other hand in two of these cases elytra
and legs lying around gave away the fact that females had
also fallen victim to the superior enemy. Therefore the
successful defense of the brood against this enemy in the
event does not arise.
To the predators is further counted the Staphylinid
Philonthus whose determination Herr. Prof. Dr. van Ender
undertook in a loving way. Necrophorus eggs in the immediate
vicinity of an N. vespillo nest in the soil were brought up
together with the Necrophorus eggs but without the presence
of Necrophorus females. They hatched out extremely lively
Philonthus larvae clambered onto the ball of carrion, seized
and consumed any Necrophorus larvae which had already
arrived there. After 2-3 hours pause, in which the predator
concealed itself in a hiding place, further attacks on N.
larvae were observed. In 24 hours the whole N. brood was
destroyed. A further look about by the visibly growing
Philonthus larvae at this time failed as any suitable food
was absent. Carrion and pieces of meat were scorned as were
as older fly maggots. Suitable food, especially Necrophorus
larvae, were unfortunately not at that moment at my
disposal. Finally the restless, hunting Philonthus larvae
died with wide open mandibles. However by this time the
establishment of the species was frustrated so a possibility
to determine the species of the larvae of the genus
Philonthus did not exist at the time.
Philonthus eggs were found later several more times in the
vicinity of freshly laid Necrophorus vespillo nests, in
total 17 times. The danger to the Necrophorus brood through
predatory Philonthus larvae cannot be totally averted
through the vigilant female, but still substantially
reduced. For in the wild I saw repeatedly, as the beetle
succeeded now and again. she seized and killed the visible
predator.
The female had further success in defending her brood
against the hostile carrion-eaters than against predators.
Additionally, one is justly allowed to mention all those
insects which feed themselves or their descendants on
carrion. From such were used the following species for
experiment:
Necrophorus vespillo in 32 experiments
Necrophorus vespilloides in 32 experiments
Geotrupes sylvaticus 18
Oeceophoma thoracica 2
Silpha obscure 1
Lucilia caesar 1
In a weakly lit room a ball of carrion taken from a crypt,
in whose insides were N. vespillo larvae of stage III
together with the attendant female were put in the middle on
the floor of an uncovered glass terrarium (52 x 40 x 27 cm)
whose floor was covered up to a height of 1.5 cm with earth.
The experimental arrangement allowed the hostile insect to
be brought into the vicinity of the nest without the nursing
female being disturbed through other than from the new
arrival attraction.
All experimental results agree that a reaction of the female
first becomes perceptible if the enemy was or even had
climbed in the immediate vicinity of the ball of carrion.
With their particular generic name the defending female made
without exception the usual way with Necroporhus in the
fight. From this same species were the dung beetles (G.
sylvaticus) driven away, while Oeceophoma thoracica and
Silpha obscura were driven away only several times in all
cases and for this reason finally expelled. Only Lucilia
caesar did not let itself be finally expelled. Although the
Necrophorus female repeatedly rushed on the fly with quick
movements, and frightened it away for moments, they always
moved back again, till it finally succeeded in laying some
eggs on the carrion ball. These were sometimes eaten later
by the female as also fly maggots which developed in the
ball of carrion are consumed by the female. How effective in
fact must be the warding off of enemies of this species for
in the Frankfurt woods are found almost on any free-lying
carrion Geotrupes, Silphidae and often insects among them
not rarely also feeding Necrophorus. As soon as digging
Necrophorus are present, the remaining visitors disappear.
So have I then, never during the whole time of my
observations been able to discover any intruder within an
intact crypt. This observation which covers also the very
shallowly buried carrion of N. vespilloides, is all the more
remarkable as Geotrupes was also found on the carrion, that
I had covered several cm deep with earth and
leaves.
k. Maintenance of the crypt.
The necessary opening of the crypt in the wild for
observation led to the establishment of a remarkable
regulatory instinct of the grave digger. The animal repairs
the disturbed brood room. The following experimental
descriptions serve to show the below surface observed
activities.
It is unknown which kind of attraction caused the repair
instinct. Three different kinds of attraction which singly
or in combination were able to work, lie in the realm of
possibility.
1. With the disturbance of the crypt often a sudden lighting
of an entirely dark brood room occurs, consequently the
possible attraction to an attractive light exists.
3. A crypt can be sunk without connection with the outside
world, thus offering another kind of attraction. As a result
of the original kind of crypt the nursing female fits in the
space between the ball of carrion and surrounding earth,
touching the crypt wall only lightly with the back. By
collapse of the hole, the light on the back of the beetle
either suddenly intensifies or is completely lost. Any
intensive change of this kind is able to be the cause of
attraction.
Occasional support that, for the mentioned attractions of
contact for the activation of the repairing instinct are of
decisive meaning. A final difference however falls requires
thorough experiments which ought to be made in the coming
summer.
1. Defense of males at the nursing.
Although the males in the majority of cases were no longer
found together with the larvae in the nest (Table 5), N.
vespilloides and N. germanicus were observed feeding, and
presumably males of the remaining species do so as well. The
fact that male grave diggers are also equipped with a
broodcare instinct, is not surprising as the participation
of the male in the broodcare in beetles is a known
occurrence. One is reminded of the group of coprophagous
lamellicomier amongst whose representative are found
numerous genera with highly differentiated broodcare in
which the male of many species participates. I]hy,
inspite of the qualification of male grave diggers for
broodcare, was contribution of the male only observed in
exceptional cases. Two explanations are possible:
1. The male was able after fulfillment of his task, which on
the one hand in the capacity of working to bury the body, on
the other making the female to fly away in search of another
carcass and through renewed ingression of task be useful at
another place.
2. Certain signs suggest that broodcare instinct of the male
is less pronounced than that of the females. It is
conspicuous that feeding male offers his food juices to the
brood at greater intervals than the nursing female. While
the young beetles groomed themselves often and leaning of
the nursing grave diggers was confined to the cleaning of
eggs and antennae. When the larvae leave the nest, one sees
the female respectively the pair completely dirty (Fig. 23)
and often with mites sitting dotted in the rest of the body.
Without feeding or cleaning the parent animals spend for 2-3
hours of the carrion, till they finally remove themselves
from the crypt and return no longer.
m. Development of full grown larvae to young beetles.
Shortly before the conclusion of the 7 day period of
development the brood ceases to take up food. Up to this
point the near white looking larvae are colored yellowish
and at the same time become broader and flatter. The animal,
which at this stage can also be described as "prepupal",
shows a different behavior from the larvae III stage. The
continuous massing together of the larvae under one another
ceases. The animals bore through the outer layer of the ball
of carrion and advance singly into the soil to pupate. Fig.
25 illustrates an abandonded carrion which remains behind
hollowed out and with a hole through it. Over the depth as
far as to the dug in Necrophorus-larvae, Xambeu (1892) Chenn
and Desinarest (1851) have already made statements.
According to Xambeu (1892) the N. fossor pupae are found in
15 cm depth, from Chenn and Desinarest N. humator in 60 cm.
My results differ from the named works: the depth, up to
which the larvae were able to advance to pupate is dependent
naturally on the respective depths of the crypt. From the
brood cavity the pupae move predominantly in the horizontal
plane up to 30 cm from the nest and in doing so only bore
themselves a little deeper. So one finds for example the
pupae of N. vespilloides 1-5 cm under the earth surface,
however in a particular case directly under the most
respectively raw humus layer. But also for the deeper
digging species N. humator the estimated values of Chenn and
Desinarest are, at least for the individuals living in the
surroundings of Frankfurt, regarded as clearly too high. At
the end of their wanderings the prepupal stage pauses to
build the pupal cradle which originates through long hours
of rotation of the larva on its long axis, as already shown
by Fabre.
The pupal rest of the grave digger lasts 14-15 days. The
pupa rests only on the strongest bristles which are found on
the eyes and the pseudocerci and the ventral side turned
downwards, in the small subterranean cavity. At the
irritation of touching the abdomen the bristles beat or make
circular movements. Two days before the hatching of the
young beetles, the initially white pupa becomes brownish.
The young beetle which frees itself from the pupal covering,
already has gold/brown extremities and a likewise colored
thorax and scutellum. The elytra, are white/yellowish, are
only indistinctly marked. Under normal conditions, the
animal remains in the protection of his subterranean room.
until 4 days after the casting of the pupal skin when the
complete coloring and hardening of the chitinous skeleton
occurs. First then the young beetle breaks through the wall
of his pupal cradle and works his way through to the surface
of the earth. The light color and the silk smoothness of
this armor betrays the young beetle, which goes off without
delay in search of food. After a maturing feed, which is
completed in 10-14 days in N. vespillo and N. vespilloides,
the young beetle is ready to reproduce and qualified for the
combined performance.
The investigation extended to the species: Necrophorus
germanicus, L., N. humator, A., N. vespillo L., N.
vespilloides Herbst, N. fossor Er., N.investigator Zett.
1. N. vespillo is native in meadows. N. humator and N.
vespilloides are confined to wood areas. N. humator is found
predominantly in humid deciduous woods, N. vespilloides in
dry coniferous woods.
2. All species feed themselves on carrion and diptera
larvae; N germanicus also eats Geotrupes.
3. Extra intestinal digestion of the adult is probable.
4. Before the burial every carcass is examined for chemical
condition, size and ease with which it can be shifted.
5. Sexually mature males, which find themselves alone at the
burial of a suitable body' attract the female of their
species through "Ventilation".
6. The isolation of a single pair from a large number of
commonly burying individuals is based not on social instinct
(Fabre 1988, Reuter 1913, Schroder 1929) but on
fighting.
7.The burying does not advance alone through digging under
the body. This is rather conveyed in the course of the
digging activity in a slanting hole leading into the depths
at whose base lies the ball shaped brood chamber (crypt -
Fabre 1~899).
8. Simultaneously with the construction of the crypt, the
rounding and cleaning of the buried body advances.
9. The eggs are laid singly on both sides of a horizontal
outgoing passage from the crypt into the soil, the mothering
passage.
10. In the majority of cases the males which shared
broodcare with the female are driven away out of the crypt
after completion of egg-laying.
11. After egg-laying the ball of carrion is damped with
excrement from the female and on its upper pole mixed
through with gut secretions.
12. At this point, which is deepened by the female into a
circular crater shortly before the appearance of the
descendants, gather the hatched larvae at the end of 5 days
embryonic development, which distinguish themselves in the
first stage through good ability to smell.
13. The larvae go through two moults and are fully grown in
7
days.
14. A female is regularly found by the brood in the crypt,
rarely a pair. The grave digger practices highly developed
broodcare:
a) The brood is fed by the
female with gut contents. The time of feeding extends
over the first 60-72 hrs. of larval development.
b) The brood is defended against enemies.
c) The crypt is repaired after destruction.
Males which are not expelled hy the female, participate
in broodcare.
16. The species N. germanicus, N. humator, N. vespillo and N
. vespilloides overwinter as adults, N. fossor and N.
investigator in the prepual stage.
17. The pupal stage lasts 14-15 days.