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.