VIEW THE MOBILE VERSION of www.bugsinsects.net Informational Site Network Informational
Privacy
   Home - Articles - Books

THE CETONIA-LARVA




The Scolia's feeding-period lasts, on the average, for a dozen days or so.
By then the victuals are no more than a crumpled bag, a skin emptied of the
last scrap of nutriment. A little earlier, the russet-yellow tint announces
the extinction of the last spark of life in the creature that is being
devoured. The empty skin is pushed back to make space; the dining-room, a
shapeless cavity with crumbling walls, is tidied up a little; and the
Scolia-grub sets to work on its cocoon without further delay.

The first courses form a general scaffolding, which finds a support here
and there on the earthen walls, and consist of a rough, blood-red fabric.
When the larva is merely laid, as required by my investigations, in a
hollow made with the finger-tip in the bed of mould, it is not able to spin
its cocoon, for want of a ceiling to which to fasten the upper threads of
its network. To weave its cocoon, every spinning larva is compelled to
isolate itself in a hammock slung in an open-work enclosure, which enables
it to distribute its thread uniformly in all directions. If there be no
ceiling, the upper part of the cocoon cannot be fashioned, because the
worker lacks the necessary points of support. Under these conditions my
Scolia-grubs contrive at most to upholster their little pit with a thick
down of reddish silk. Discouraged by futile endeavours, some of them die.
It is as if they had been killed by the silk which they omit to disgorge
because they are unable to make the right use of it. This, if we were not
watchful, would be a very frequent cause of failure in our attempts at
artificial rearing. But, once the danger has been perceived, the remedy is
simple. I make a ceiling over the cavity by laying a short strip of paper
above it. If I want to see how matters are progressing, I bend the strip
into a semicircle, into a half-cylinder with open ends. Those who wish to
play the breeder for themselves will be able to profit by these little
practical details.

In twenty-four hours the cocoon is finished; at least, it no longer allows
us to see the grub, which is doubtless making the walls of its dwelling
still thicker. At first the cocoon is a vivid red; later it changes to a
light chestnut-brown. Its form is that of an ellipsoid, with a major axis
26 millimetres in length, while the minor axis measures 11 millimetres.
(1.014 x .429 inch.--Translator's Note.) These dimensions, which
incidentally are inclined to vary slightly, are those of the female
cocoons. In the other sex they are smaller and may measure as little as 17
millimetres in length by 7 millimetres in width. (.663 x .273 inch.--
Translator's Note.)

The two ends of the ellipsoid have the same form, so much so that it is
only thanks to an individual peculiarity, independent of the shape, that we
can tell the cephalic from the anal extremity. The cephalic pole is
flexible and yields to the pressure of my tweezers; the anal pole is hard
and unyielding. The wrapper is double, as in the cocoons of the Sphex. (Cf.
"The Hunting Wasps": chapters 4 to 10 et passim.--Translator's Note.) The
outer envelope, consisting of pure silk, is thin, flexible and offers
little resistance. It is closely superimposed upon the inner envelope and
is easily separated from it everywhere, except at the anal end, where it
adheres to the second envelope. The adhesion of the two wrappers at one end
and the non-adhesion at the other are the cause of the differences which
the tweezers reveal when pinching the two ends of the cocoon.

The inner envelope is firm, elastic, rigid and, to a certain point,
brittle. I do not hesitate to look upon it as consisting of a silken tissue
which the larva, towards the end of its task, has steeped thoroughly in a
sort of varnish prepared not by the silk-glands but by the stomach. The
cocoons of the Sphex have already shown us a similar varnish. This product
of the chylific ventricle is chestnut-brown. It is this which, saturating
the thickness of the tissue, effaces the bright red of the beginning and
replaces it by a brown tint. It is this again which, disgorged more
profusely at the lower end of the cocoon, glues the two wrappers together
at that point.

The perfect insect is hatched at the beginning of July. The emergence takes
place without any violent effraction, without any ragged rents. A clean,
circular fissure appears at some distance from the top; and the cephalic
end is detached all of a piece, as a loose lid might be. It is as though
the recluse had only to raise a cover by butting it with her head, so exact
is the line of division, at least as regards the inner envelope, the
stronger and more important of the two. As for the outer wrapper, its lack
of resistance enables it to yield without difficulty when the other gives
way.

I cannot quite make out by what knack the Wasp contrives to detach the cap
of the inner shell with such accuracy. Is it the art practised by the
tailor when cutting his stuff, with mandibles taking the place of scissors?
I hardly venture to admit as much: the tissue is so tough and the circle of
division so precise. The mandibles are not sharp enough to cut without
leaving a ragged edge; and then what geometrical certainty they would need
for an operation so perfect that it might well have been performed with the
compasses!

I suspect therefore that the Scolia first fashions the outer sac in
accordance with the usual method, that is, by distributing the silk
uniformly, without any special preparation of one part of the wall more
than of another, and that it afterwards changes its method of weaving in
order to attend to the main work, the inner shell. In this it apparently
imitates the Bembex (Cf. "The Hunting Wasps": chapters 14 to 16.--
Translator's Note.), which weaves a sort of eel-trap, whose ample mesh
allows it to gather grains of sand outside and encrust them one by one in
the silky network, and completes the performance with a cap fitting the
entrance to the trap. This provides a circular line of least resistance,
along which the casket breaks open afterwards. If the Scolia really works
in the same manner, everything is explained: the eel-trap, while still
open, enables it to soak with varnish both the inside and the outside of
the inner shell, which has to acquire the consistency of parchment; lastly,
the cap which completes and closes the structure leaves for the future a
circular line capable of splitting easily and neatly.

This is enough on the subject of the Scolia-grub. Let us go back to its
provender, of whose remarkable structure we as yet know nothing. In order
that it may be consumed with the delicate anatomical discretion imposed by
the necessity of having fresh food to the last, the Cetonia-grub must be
plunged into a state of absolute immobility: any twitchings on its part--as
the experiments which I have undertaken go to prove--would discourage our
nibbling larva and impede the work of carving, which has to be effected
with so much circumspection. It is not enough for the victim to be unable
to move from place to place beneath the soil: in addition to this, the
contractible power in its sturdy muscular organism must be suppressed.

In its normal state, this larva, at the very least disturbance, curls
itself up, almost as the Hedgehog does; and the two halves of the ventral
surface are laid one against the other. You are quite surprised at the
strength which the creature displays in keeping itself thus contracted. If
you try to unroll it, your fingers encounter a resistance far greater than
the size of the animal would have caused you to suspect. To overcome the
resistance of this sort of spring coiled upon itself, you have to force it,
so much so that you are afraid, if you persist, of seeing the indomitable
spiral suddenly burst and shoot forth its entrails.

A similar muscular energy is found in the larvae of the Oryctes (Also known
as the Rhinoceros Beetle.--Translator's Note.), the Anoxia (A Beetle akin
to the Cockchafer.--Translator's Note.), the Cockchafer. Weighed down by a
heavy belly and living underground, where they feed either on leaf-mould or
on roots, these larvae all possess the vigorous constitution needed to drag
their corpulence through a resisting medium. All of them also roll
themselves into a hook which is not straightened without an effort.

Now what would become of the egg and the new-born grub of the Scoliae,
fixed under the belly, at the centre of the Cetonia's spiral, or inside the
hook of the Oryctes or the Anoxia? They would be crushed between the jaws
of the living vice. It is essential that the arc should slacken and the
hook unbend, without the least possibility of their returning to a state of
tension. Indeed, the well-being of the Scoliae demands something more:
those powerful bodies must not retain even the power to quiver, lest they
derange a method of feeding which has to be conducted with the greatest
caution.

The Cetonia-grub to which the Two-banded Scolia's egg is fastened fulfils
the required conditions admirably. It is lying on its back, in the midst of
the mould, with its belly fully extended. Long accustomed though I be to
this spectacle of victims paralysed by the sting of the Hunting Wasp, I
cannot suppress my astonishment at the profound immobility of the prey
before my eyes. In the other victims with flexible skins, Caterpillars,
Crickets, Mantes, Ephippigers, I perceived at least some pulsations of the
abdomen, a few feeble contortions under the stimulus of a needle. There is
nothing of the sort here, nothing but absolute inertia, except in the head,
where I see, from time to time, the mouth-parts open and close, the palpi
give a tremor, the short antennae sway to and fro. A prick with the point
of a needle causes no contraction, no matter what the spot pricked. Though
I stab it through and through, the creature does not stir, be it ever so
little. A corpse is not more inert. Never, since my remotest
investigations, have I witnessed so profound a paralysis. I have seen many
wonders due to the surgical talent of the Wasp; but to-day's marvel
surpasses them all.

I am doubly surprised when I consider the unfavourable conditions under
which the Scolia operates. The other paralysers work in the open air, in
the full light of day. There is nothing to hinder them. They enjoy full
liberty of action in seizing the prey, holding it in position and
sacrificing it; they are able to see the victim and to parry its means of
defence, to avoid its spears, its pincers. The spot or spots to be attained
are within their reach; they drive the dagger in without let or hindrance.

What difficulties, on the other hand, await the Scolia! She hunts
underground, in the blackest darkness. Her movements are laboured and
uncertain, owing to the mould, which is continually giving way all round
her; she cannot keep her eyes on the terrible mandibles, which are capable
of cutting her body in two with a single bite. Moreover, the Cetonia-grub,
perceiving that the enemy is approaching, assumes its defensive posture,
rolls itself up and makes a shield for its only vulnerable part, the
ventral surface, with its convex back. No, it cannot be an easy operation
to subdue the powerful larva in its underground retreat and to stab with
the precision which immediate paralysis requires.

We wish that we might witness the struggle between the two adversaries and
see at first hand what happens, but we cannot hope to succeed. It all takes
place in the mysterious darkness of the soil; in broad daylight, the attack
would not be delivered, for the victim must remain where it is and then and
there receive the egg, which is unable to thrive and develop except under
the warm cover of vegetable mould. If direct observation is impracticable,
we can at least foresee the main outlines of the drama by allowing
ourselves to be guided by the warlike manoeuvres of other burrowers.

I picture things thus: digging and rummaging through the heap of mould,
guided perhaps by that singular sensibility of the antennae which enables
the Hairy Ammophila to discover the Grey Worm (The caterpillar of the
Turnip Moth. Cf. "The Hunting Wasps": chapters 18 to 20.--Translator's
Note.) underground, the Scolia ends by finding a Cetonia-larva, a good
plump one, in the pink of condition, having reached its full growth, just
what the grub which is to feed on it requires. Forthwith, the assaulted
victim, contracting desperately, rolls itself into a ball. The other seizes
it by the skin of the neck. To unroll it is impossible to the insect, for I
myself have some trouble in doing so. One single point is accessible to the
sting: the under part of the head, or rather of the first segments, which
are placed outside the coil, so that the grub's hard cranium makes a
rampart for the hinder extremity, which is less well defended. Here the
Wasp's sting enters and here only can it enter, within a narrowly
circumscribed area. One stab only of the lancet is given at this point, one
only because there is no room for more; and this is enough: the larva is
absolutely paralysed.

The nervous functions are abolished instantly; the muscular contractions
cease; and the animal uncoils like a broken spring. Henceforth motionless,
it lies on its back, its ventral surface fully exposed from end to end. On
the median line of this surface, towards the rear, near the brown patch due
to the alimentary broth contained in the intestine, the Scolia lays her egg
and without more ado, leaves everything lying on the actual spot where the
murder was committed, in order to go in search of another victim.

This is how the deed must be done: the results prove it emphatically. But
then the Cetonia-grub must possess a very exceptional structure in its
nervous organization. The larva's violent contraction leaves but a single
point of attack open to the sting, the under part of the neck, which is
doubtless uncovered when the victim tries to defend itself with its
mandibles; and yet a stab in this one point produces the most thorough
paralysis that I have ever seen. It is the general rule that larvae possess
a centre of innervation for each segment. This is so in particular with the
Grey Worm, the sacrificial victim of the Hairy Ammophila. The Wasp is
acquainted with this anatomical secret: she stabs the caterpillar again and
again, from end to end, segment by segment, ganglion by ganglion. With such
an organization the Cetonia-grub, unconquerably coiled upon itself would
defy the paralyser's surgical skill.

If the first ganglion were wounded, the others would remain uninjured; and
the powerful body, actuated by these last, would lose none of its powers of
contraction. Woe then to the egg, to the young grub held fast in its
embrace! And how insurmountable would be the difficulties if the Scolia,
working in the profound darkness amid the crumbling soil and confronted by
a terrible pair of mandibles, had to stab each segment in turn with her
sting, with the certainty of method displayed by the Ammophila! The
delicate operation is possible in the open air, where nothing stands in the
way, in broad daylight, where the sight guides the scalpel, and with a
patient which can always be released if it becomes dangerous. But in the
dark, underground, amidst the ruins of a ceiling which crumbles in
consequence of the conflict and at close quarters with an opponent greatly
her superior in strength, how is the Scolia to guide her sting with the
accuracy that is essential if the stabs are to be repeated?

So profound a paralysis; the difficulty of vivisection underground; the
desperate coiling of the victim: all these things tell me that the Cetonia-
grub, as regards its nervous system, must possess a structure peculiar to
itself. The whole of the ganglia must be concentrated in a limited area in
the first segments, almost under the neck. I see this as clearly as though
it had been revealed to me by a post-mortem dissection.

Never was anatomical forecast more fully confirmed by direct examination.
After forty-eight hours in benzine, which dissolves the fat and renders the
nervous system more plainly visible, the Cetonia-grub is subjected to
dissection. Those of my readers who are familiar with these investigations
will understand my delight. What a clever school is the Scolia's! It is
just as I thought! Admirable! The thoracic and abdominal ganglia are
gathered into a single nervous mass, situated within the quadrilateral
bounded by the four hinder legs, which legs are very near the head. It is a
tiny, dull-white cylinder, about three millimetres long by half a
millimetre wide. (.117 x .019 inch.--Translator's Note.) This is the organ
which the Scolia's sting must attack in order to secure the paralysis of
the whole body, excepting the head, which is provided with special ganglia.
>From it run numbers of filaments which actuate the feet and the powerful
muscular layer which is the creature's essential motor organ. When examined
merely through the pocket-lens, this cylinder appears to be slightly
furrowed transversely, a proof of its complex structure. Under the
microscope, it is seen to be formed by the close juxtaposition, the
welding, end to end, of the ganglia, which can be distinguished one from
the other by a slight intermediate groove. The bulkiest are the first, the
fourth and the tenth, or last; these are all very nearly of equal size. The
rest are barely half or even a third as large as those mentioned.

The Interrupted Scolia experiences the same hunting and surgical
difficulties when she attacks, in the crumbling, sandy soil, the larvae of
the Shaggy Anoxia or of the Morning Anoxia, according to the district; and
these difficulties, if they are to be overcome, demand in the victim a
concentrated nervous system, like the Cetonia's. Such is my logical
conviction before making my examination; such also is the result of direct
observation. When subjected to the scalpel, the larva of the Morning Anoxia
shows me its centres of innervation for the thorax and the abdomen,
gathered into a short cylinder, which, placed very far forward, almost
immediately after the head, does not run back beyond the level of the
second pair of legs. The vulnerable point is thus easily accessible to the
sting, despite the creature's posture of defence, in which it contracts and
coils up. In this cylinder I recognize eleven ganglia, one more than in the
Cetonia. The first three, or thoracic, ganglia are plainly distinguishable
from one another, although they are set very close together; the rest are
all in contact. The largest are the three thoracic ganglia and the
eleventh.

After ascertaining these facts, I remembered Swammerdam's investigations
into the grub of the Monoceros, our Oryctes nasicornis. (Jan Swammerdam
(1637-1680), the Dutch naturalist and anatomist.--Translator's Note.) I
chanced to possess an abridgement of the "Biblia naturae," the masterly
work of the father of insect anatomy. I consulted the venerable volume. It
informed me that the learned Dutchman had been struck, long before I was,
by an anatomical peculiarity similar to that which the larvae of the
Cetoniae and Anoxiae had shown me in their nerve-centres. Having observed
in the Silk-worm a nervous system formed of ganglia distinct one from the
other, he was quite surprised to find that, in the grub of the Oryctes, the
same system was concentrated into a short chain of ganglia in
juxtaposition. His was the surprise of the anatomist who, studying the
organ qua organ, sees for the first time an unusual conformation. Mine was
of a different nature: I was amazed to see the precision with which the
paralysis of the victim sacrificed by the Scolia, a paralysis so profound
in spite of the difficulties of an underground operation, had guided my
forecast as to structure when, anticipating the dissection, I declared in
favour of an exceptional concentration of the nervous system. Physiology
perceived what anatomy had not yet revealed, at all events to my eyes, for
since then, on dipping into my books, I have learnt that these anatomical
peculiarities, which were then so new to me, are now within the domain of
current science. We know that, in the Scarabaeidae, both the larva and the
perfect insect are endowed with a concentrated nervous system.

The Garden Scolia attacks Oryctes nasicornis; the Two-banded Scolia the
Cetonia; the Interrupted Scolia the Anoxia. All three operate below ground,
under the most unfavourable conditions; and all three have for their victim
a larva of one of the Scarabaeidae, which, thanks to the exceptional
arrangement of its nerve-centres, lends itself, alone of all larvae, to the
Wasp's successful enterprises. In the presence of this underground game, so
greatly varied in size and shape and yet so judiciously selected to
facilitate paralysis, I do not hesitate to generalize and I accept, as the
ration of the other Scoliae, larvae of Lamellicorns whose species will be
determined by future observation. Perhaps one of them will be found to give
chase to the terrible enemy of my crops, the voracious White Worm, the grub
of the Cockchafer; perhaps the Hemorrhoidal Scolia, rivalling in size the
Garden Scolia and like her, no doubt, requiring a copious diet, will be
entered in the insects' "Who's Who" as the destroyer of the Pine-chafer,
that magnificent Beetle, flecked with white upon a black or brown ground,
who of an evening, during the summer solstice, browses on the foliage of
the fir-trees. Though unable to speak with certainty or precision, I am
inclined to look upon these devourers of Scarabaeus-grubs as valiant
agricultural auxiliaries.

The Cetonia-larva has figured hitherto only in its quality of a paralysed
victim. We will now consider it in its normal state. With its convex back
and its almost flat ventral surface, the creature is like a semi-cylinder
in shape, fuller in the hinder portion. On the back, each of the segments,
except the last, or anal, segment, puckers into three thick pads, bristling
with stiff, tawny hairs. The anal segment, much wider than the rest, is
rounded at the end and coloured a deep brown by the contents of the
intestine, which show through the translucent skin; it bristles with hairs
like the other segments, but is level, without pads. On the ventral
surface, the segments have no creases; and the hairs, though abundant, are
rather less so than on the back. The legs, which are quite well-formed, are
short and feeble in comparison with the animal's size. The head has a
strong, horny cap for a cranium. The mandibles are powerful, with bevelled
tips and three or four teeth on the edge of the bevel.

Its mode of locomotion marks it as an idiosyncratic, exceptional, fantastic
creature, having no fellow, that I know of, in the insect world. Though
endowed with legs--a trifle short, it is true, but after all as good as
those of a host of other larvae--it never uses them for walking. It
progresses on its back, always on its back, never otherwise. By means of
wriggling movements and the purchase afforded by the dorsal bristles, it
makes its way belly upwards, with its legs kicking the empty air. The
spectator to whom these topsy-turvy gymnastics are a novelty thinks at
first that the creature must have had a fright of some sort and that it is
struggling as best it can in the face of danger. He puts it back on its
belly; he lays it on its side. Nothing is of any use; it obstinately turns
over and resumes its dorsal progress. That is its manner of travelling over
a flat surface; it has no other.

This reversal of the usual mode of walking is so peculiar to the Cetonia-
larva that it is enough in itself to reveal the grub's identity to the
least expert eyes. Dig into the vegetable mould formed by the decayed wood
in the hollow trunks of old willow-trees, search at the foot of rotten
stumps or in heaps of compost; and, if you come upon a plumpish grub moving
along on its back, there is no room for doubt: your discovery is a Cetonia-
larva.

This topsy-turvy progress is fairly swift and is not less in speed to that
of an equally fat grub travelling on its legs. It would even be greater on
a polished surface, where walking on foot is hampered by incessant slips,
whereas the numerous hairs of the dorsal pads find the necessary support by
multiplying the points of contact. On polished wood, on a sheet of paper
and even on a strip of glass, I see my grubs moving from point to point
with the same ease as on a surface of garden mould. In the space of one
minute, on the wood of my table, they cover a distance of eight inches. The
pace is no swifter on a horizontal bed of sifted mould. A strip of glass
reduces the distance covered by one half. The slippery surface only half
paralyses this strange method of locomotion.

We will now place side by side with the Cetonia-grub the larva of the
Morning Anoxia, the prey of the Interrupted Scolia. It is very like the
larva of the Common Cockchafer. It is a fat, pot-bellied grub, with a
thick, red cap on its head and armed with strong, black mandibles, which
are powerful implements for digging and cutting through roots. The legs are
sturdy and end in a hooked nail. The creature has a long, heavy, brown
paunch. When placed on the table, it lies on its side; it struggles without
being able to advance or even to remain on its belly or back. In its usual
posture it is curled up into a narrow hook. I have never seen it straighten
itself completely; the bulky abdomen prevents it. When placed on a surface
of moist sand, the ventripotent creature is no better able to shift its
position: curved into a fish-hook, it lies on its side.

To dig into the earth and bury itself, it uses the fore-edge of its head, a
sort of weeding-hoe with the two mandibles for points. The legs take part
in this work, but far less effectually. In this way it contrives to dig
itself a shallow pit. Then, bracing itself against the wall of the pit,
with the aid of wriggling movements which are favoured by the short, stiff
hairs bristling all over its body, the grub changes its position and
plunges into the sand, but still with difficulty.

Apart from a few details, which are of no importance here, we may repeat
this sketch of the Anoxia-grub and we shall have, if the size be at least
quadrupled, a picture of the larva of Oryctes nasicornis, the monstrous
prey of the Garden Scolia. Its general appearance is the same: there is the
same exaggeration of the belly; the same hook-like curve; the same
incapacity for standing on its legs. And as much may be said of the larva
of Scarabaeus pentodon, a fellow-boarder of the Oryctes and the Cetonia.





Next: THE PROBLEM OF THE SCOLIAE

Previous: A DANGEROUS DIET



Add to del.icio.us Add to Reddit Add to Digg Add to Del.icio.us Add to Google Add to Twitter Add to Stumble Upon
Add to Informational Site Network
Report
Privacy
SHAREADD TO EBOOK