THE LARVA AND THE NYMPH
The egg of the Yellow-winged Sphex is white, elongated, cylindrical,
slightly bow-shaped and measures three to four millimetres [17] in
length. So far from being laid anywhere on the victim, at random, it is
deposited on a specially favoured spot, which is always the same; in
short, it is placed across the Cricket’s breast, a little to one side,
between the first and second pair of legs. The egg of the White-edged
Sphex and that of the Languedocian Sphex occupy a similar position: the
first on the breast of a Locust, the second on the breast of an
Ephippiger. [18] The point selected must present some peculiarity of
great importance to the young larva’s safety, for I have never known it
to vary.
The egg hatches after three or four days. A very delicate wrapper tears
asunder; and there lies before our eyes a feeble grub, transparent as
crystal, a little attenuated and as it were compressed in front,
slightly swollen at the back and adorned on either side with a narrow
white thread formed of the principal trachean ducts. The frail creature
occupies the same position as the egg. Its head is, so to speak,
planted at the very spot where the upper end of the egg was fixed; and
all the remainder simply rests upon the victim, without being fastened
to it. The grub’s transparency enables us readily to distinguish rapid
undulations inside it, ripples which follow one upon the other with
mathematical regularity and which, beginning in the middle of the body,
spread some forward and some backward. These fluctuating movements are
due to the digestive canal, which takes long draughts of the juices
drawn from the victim’s body.
Let us dwell for a moment upon a sight which cannot fail to attract our
attention. The Wasp’s prey lies on its back, motionless. In the cell of
the Yellow-winged Sphex it is a Cricket, or rather three or four
Crickets stacked one atop the other; in the cell of the Languedocian
Sphex it is a single head of game, but large in proportion, a
fat-bellied Ephippiger. The grub is lost should it happen to be torn
from the spot whence it derives life; a fall would be the end of it,
for, weak as it is and deprived of all means of motion, how could it
make its way back to the spot at which it slakes its appetite? The
slightest movement would enable the victim to rid itself of the atom
gnawing at its entrails; and yet the gigantic prey submits meekly,
without the least quiver of protest. I well know that it is paralysed,
that it has lost the use of its legs through the sting of its
murderess; but still, recent victim that it is, it retains more or less
power of movement and sensation in the regions not affected by the
dart. The abdomen throbs, the mandibles open and close, the abdominal
filaments wave to and fro, as do the antennæ. What would happen if the
worm were to bite into one of the still impressionable parts, near the
mandibles, or even on the belly, which, being more tender and more
succulent, seems as though it ought, after all, to supply the first
mouthfuls of the feeble grub? Bitten to the quick, the Cricket, Locust
or Ephippiger would at least shiver; and this faint tremor of the skin
would be enough to shake off the tiny larva and bring it to the ground,
where it would no doubt perish, for it might at any moment find itself
in the grips of those dreadful mandibles.
But there is one part of the body where no such danger is to be feared,
the part which the Wasp has wounded with her sting—in short, the
thorax. Here and here alone, on a victim of recent date, the
experimenter can rummage with a needle, driving it through and through,
without producing a sign of suffering in the patient. Well, it is here
that the egg is invariably laid; it is here that the young larva always
takes its first bite at its prey. Gnawed at a point no longer
susceptible to pain, the Cricket remains motionless. Later, when the
wound has reached a sensitive point, he will doubtless toss about to
such extent as he can; but then it will be too late: his torpor will be
too deep; and besides the enemy will have gained strength. This
explains why the egg is laid on a spot which never varies, near the
wounds caused by the sting—in short, on the thorax: not in the middle,
where the skin would perhaps be too thick for the new-born grub, but on
one side, towards the juncture of the legs, where it is much thinner.
What a judicious choice, how logical on the part of the mother when,
underground, in complete darkness, she discerns the one suitable spot
on the victim and selects it for her egg!
I have reared Sphex-grubs by giving them, one after the other, the
Crickets taken from the cells; and I was then able to follow day by day
the rapid progress of my nurselings. The first Cricket, the one on whom
the egg was laid, is attacked, as I have said, near the point where the
huntress administered her second sting, that is to say, between the
first and second pair of legs. In a few days the young larva has dug in
the victim’s breast a hollow large enough to admit half its body. It is
not uncommon to see the Cricket, bitten to the quick, uselessly waving
his antennæ and his abdominal threads, opening and closing his
mandibles on space and even moving a leg. But the enemy is safe and is
ransacking his entrails with impunity. What an awful nightmare for the
paralysed Cricket!
The first ration is finished in six or seven days’ time; none of it
remains but the framework of skin, with all its parts more or less in
position. The larva, whose length is now twelve millimetres, [19]
leaves the Cricket’s body through the hole in the thorax which it made
to start with. During this operation it moults; and its cast skin often
remains caught in the opening through which it made its exit. It rests
after the moulting and then attacks a second ration. Being stronger
now, the larva has nothing to fear from the feeble movements of the
Cricket, whose daily-increasing torpor has had time to extinguish the
last glimmers of resisting-power during the week and more that has
elapsed since the dagger-thrusts were given. It is therefore assailed
with no precautions, usually at the belly, which is the tenderest part
and the richest in juices. Soon the turn comes of the third Cricket and
lastly of the fourth, who is devoured in ten hours or so. Of these last
three victims all that remains is the tough integuments, whose various
parts are severed one by one and carefully emptied. If a fifth ration
be presented, the larva scorns it, or hardly touches it, not from
abstemiousness, but from imperious necessity. For observe that hitherto
the larva has ejected no excrement and that its intestines, into which
four Crickets have been crammed, are distended to bursting-point. A new
ration cannot therefore tempt its gluttony; and henceforth it thinks
only of making itself a silken tabernacle.
In all, its repast has lasted from ten to twelve days without
cessation. At this period the larva’s length measures from twenty-five
to thirty millimetres [20] and its greatest breadth from five to six.
[21] Its general outline, spreading a little at the back and gradually
tapering in front, conforms with the usual type of Hymenopteron-grubs.
Its segments are fourteen in number, including the head, which is very
small and armed with weak mandibles that would appear unequal to the
part which they have just played. Of these fourteen segments the middle
ones are supplied with stigmata, or breathing-holes. Its livery
consists of a yellowish-white ground, studded with innumerable dots of
a chalky white.
We have seen the larva begin its second Cricket with the belly, the
juiciest and softest part. Like a child, which first licks the jam off
its bread and then bites into the crumb with a disdainful tooth, the
larva makes straight for the best part, the abdominal viscera, and
leaves until later the meat that has to be patiently extracted from its
horny sheath: a task for a leisure hour, when it is comfortably
digesting the earlier meal. Nevertheless, the grub, when quite young,
when newly hatched, is not so dainty: it goes for the bread first and
the jam afterwards. It has no choice: it is obliged to bite its first
mouthful right out of the breast, at the spot where the mother fixed
the egg. The food here is a little harder, but the place is safe,
because of the profound inertia into which the thorax has been plunged
by three thrusts of the dagger. Elsewhere there would be, if not
always, at least often, spasmodic shudders which would dislodge the
feeble grub and expose it to terrible hazards among a heap of victims
whose hind-legs, toothed like saws, might give an occasional jerk and
whose mandibles might still be capable of snapping. It is therefore the
question of safety and not of the grub’s likes or dislikes that
determines the mother’s choice in placing the egg.
And here a suspicion occurs to my mind. The first ration, the Cricket
on whom the egg is laid, exposes the grub to more parlous risks than do
the others. To begin with, the larva is still but a frail worm; and
then the victim is quite a recent one and therefore most likely to give
evidence of a spark of life. This first victim has to be paralysed as
completely as possible: consequently it receives the Wasp’s three
dagger-thrusts. But the others, whose torpor deepens the older they
grow, the others whom the larva attacks after it has gained in
strength: do they need to be operated on as carefully? Might not one
prick be enough, or two pricks, the effects of which would spread
little by little while the grub is consuming its first ration? The
poison-fluid is too precious for the Wasp to lavish it unnecessarily:
it is hunting-ammunition, to be employed with due economy. At any rate,
though I have witnessed three consecutive stabs given to the same
victim, at other times I have seen only two administered. It is true
that the quivering tip of the Sphex’ abdomen seemed to be seeking the
favourable spot for a third wound; but, if it was really given, it
escaped me. I should therefore be inclined to think that the victim
forming the first ration is always stabbed thrice, whereas the others,
from motives of economy, receive only two stings. Our study of the
Ammophilæ, who hunt Caterpillars, will confirm this suspicion later.
After devouring the last Cricket the larva sets about weaving its
cocoon. The work is finished well within forty-eight hours. Henceforth
the skilful worker, safe within her impenetrable shelter, can yield to
the irresistible lethargy that invades her, to that nameless mode of
existence, neither sleep nor waking, neither death nor life, from which
she will emerge, ten months from now, transfigured. Very few cocoons
are so complicated as hers. It consists, in fact, in addition to a
coarse outer network, of three distinct layers, presenting the
appearance of three cocoons one inside the other. Let us examine in
detail these several courses of the silken edifice.
There is first an open woof, of a rough cobweb texture, whereon the
larva begins by isolating itself, hanging as in a hammock, to work more
easily at the cocoon proper. This unfinished net, hastily woven to
serve as a builder’s scaffolding, is made of threads flung out at
random, which hold together grains of sand, bits of earth and the
leavings of the larva’s feast: the Cricket’s thighs, still braided with
red, his shanks and pieces of his skull. The next covering, which is
the first covering of the cocoon proper, consists of a much-creased
felted tunic, light-red in colour, very fine and very flexible. A few
threads flung out here and there join it to the previous scaffolding
and to the second wrapper. It forms a cylindrical wallet, closed on
every side and too large for its contents, thus causing the surface to
wrinkle.
Next comes an elastic sheath, distinctly smaller than the wallet that
contains it, almost cylindrical, rounded at the upper end, towards
which the larva’s head is turned, and finishing in a blunt cone at the
lower end. Its colour is still light-red, save towards the cone at the
bottom, where the shade is darker. Its consistency is pretty firm;
nevertheless, it yields to moderate squeezing, except in its conical
part, which resists the pressure of the fingers and seems to contain a
hard substance. On opening this sheath, we see that it is formed of two
layers closely applied one to the other, but easily separated. The
outer layer is a silk felt, exactly like that of the wallet which comes
before; the inner layer, the third layer of the cocoon, is a sort of
shellac, a shiny wash of a dark violet-brown, brittle, very soft to the
touch, and of a nature apparently quite different from the rest of the
cocoon. We see, in fact, under the microscope that, instead of being a
felt of silky threads like the previous wrapper, it is a homogeneous
coating of a peculiar varnish, whose origin is rather singular, as we
shall see. As for the resistance of the cone-shaped end of the cocoon,
we discover that this is due to a plug of crumbly matter, violet-black
and sparkling with a number of black particles. This plug is the dried
mass of the excrement which the larva ejects, once and for all, inside
the cocoon itself. The same stercoral kernel also causes the darker
shade of the cone-shaped end of the cocoon. The complicated dwelling
averages twenty-seven millimetres in length, while its greatest width
is nine millimetres. [22]
Let us return to the violet varnish that lines the inside of the
cocoon. I thought at first that I must attribute it to the silk-glands,
which, after giving a glossy coat to the double wrapper of silk and the
scaffolding, have still a secret store of the fluid. To convince
myself, I opened some larvæ which had just finished their work as
weavers and had not yet begun to apply their lacquer. At that period I
saw no trace of violet fluid in the silk-glands. This shade is found
only in the digestive canal, which bulges with a purple-coloured pulp;
we find it also, but later, in the stercoral plug relegated to the
lower end of the cocoon. With this exception, everything is white, or
faintly tinged with yellow. Far be it from me to suggest that the larva
plasters its cocoon with its excreta; and yet I am convinced that this
plaster is a product of the digestive organs and I suspect, though I
cannot say for certain—having been clumsy enough several times to miss
a favourable opportunity of making sure—that the larva disgorges and
applies with its mouth the quintessence of the purple pulp from its
stomach in order to form the shellac glaze. Only after this last
performance would it reject its digestive residuum in a single lump;
and this would explain the unpleasant necessity in which the larva
finds itself of making room for its excreta inside its actual
habitation.
Be this as it may, there can be no doubt about the usefulness of the
coating of shellac; its complete impermeability must protect the larva
against the damp which would certainly attack it in the precarious
refuge dug for it by the mother. Remember that the larva is buried only
a few inches down in uncovered, sandy ground. To judge to what extent
the cocoons thus varnished are able to resist the damp, I kept some
steeped in water for several days on end, without afterwards finding a
trace of moisture inside them. Compare the Sphex’ cocoon, with its
manifold linings, which are so well adapted for the protection of the
larva in an unprotected burrow, with the cocoon of the Great Cerceris,
lying under the dry shelter of a slab of sandstone and at a distance of
eighteen or twenty inches underground: this cocoon has the shape of a
very long pear, with the narrow end lopped off. It consists of a single
silken wrapper, so thin and fine that the larva shows through it. In my
numerous entomological investigations I have always seen the larva’s
industry and the mother’s thus making good each other’s deficiencies.
In a deep, well-sheltered abode, the cocoon is of a light material; in
a surface dwelling, exposed to the inclemencies of the weather, the
cocoon is stoutly built.
Nine months elapse, during which a task is performed wherein all is
mystery. I skip this period, filled with the dead secret of the
transformation, and, to come to the nymph, pass at once from the end of
September to the first days of the following June. The larva has cast
its withered slough; the nymph, that transitory organism, or rather
that perfect insect in swaddling-bands, motionlessly awaits the
awakening which will not take place for another month to come. The
legs, the antennæ, the exposed mouth-parts and the wing-stumps have the
appearance of clearest crystal and lie evenly spread under the thorax
and the abdomen. The rest of the body is an opaque white, very faintly
smeared with yellow. The middle four segments of the abdomen carry a
narrow and blunt extension on either side. The last segment,
terminating above in a blade-like expansion shaped like the sector of a
circle, is equipped below with two conical protuberances set side by
side: this makes in all eleven appendages studding the outline of the
abdomen. Such is the delicate creature which, to become a Sphex, must
don a motley livery of black and red and throw off the fine skin in
which it is closely swathed.
I was curious to follow from day to day the appearance and the progress
of the nymph’s colouring and to test whether the light of the sun, that
rich palette whence nature derives her colours, could influence that
progress. With this object, I took pupæ from their cocoons and put them
in glass tubes, of which some, kept in complete darkness, realized the
natural conditions of the nymphs and served me as a standard of
comparison, while the others, hung against a white wall, received a
strong diffused light throughout the day. Under these diametrically
opposed conditions, the evolution of the colours remained absolutely
uniform in both cases, or, if there were some slight discrepancies,
these were to the disadvantage of the pupæ exposed to the light. It is,
therefore, exactly the reverse of what happens in the case of plants:
light does not affect the colouring of insects, does not even
accelerate the process; and this must be so, because, in the species
which are the most brilliant in colouring, the Buprestes and
Ground-beetles, for instance, the wondrous hues which one would imagine
to be stolen from a sunbeam are really elaborated in the dusky bowels
of the earth or deep down in the decaying trunk of some venerable tree.
The first outlines of colour show on the eyes, whose faceted cornea
changes successively from white to fawn, next to slate-grey, lastly to
black. The simple eyes at the top of the forehead, the ocelli, share in
this colouring, in their turn, before the rest of the body has yet lost
any of its neutral, white tint. It should be remarked that this early
development of the most delicate organ, the eye, is general in all
animals. Later, a smoky line appears on the upper part of the groove
separating the mesothorax and the metathorax; and, twenty-four hours
later, the whole back of the metathorax is black. At the same time, the
edge of the prothorax becomes shaded, a black dot appears in the
central and upper part of the metathorax, and the mandibles assume a
rusty tinge. Gradually a deeper and deeper shade creeps over the two
end segments of the thorax and finally reaches the head and the
hind-quarters. A day is enough to turn the smoky hue of the head and of
the end segments deep black. Thereupon the abdomen begins to share in
the rapidly-increasing coloration. The edge of its front segments is
tinted saffron; and its hinder segments acquire a dull-black border.
Lastly, the antennæ and legs, after passing through darker and darker
shades, turn black; the lower part of the abdomen is now entirely
orange-red and the tip black. The livery is complete except for the
tarsi and the mouth-parts, which are a transparent red, and the
wing-stumps, which are dull black. In four-and-twenty hours the nymph
will burst its fetters.
It takes the nymph only six or seven days to don its final tints,
omitting the eyes, whose colouring precedes that of the rest of the
body by fourteen or fifteen days. The law governing the insect’s
chromatic evolution is easily gathered from this brief sketch. We see
that, with the exception of the eyes and the ocelli, whose early
development recalls what takes place in the higher animals, the
starting-point of the coloration is a central spot, the mesothorax,
whence it gradually invades, by centrifugal progression, first the rest
of the thorax, then the head and abdomen, lastly the different
appendages, the legs and antennæ. The tarsi and the mouth-parts colour
later still; and the wings do not assume their hue until after they are
taken from their cases.
We now have the Sphex arrayed in her livery. She has yet to cast her
nymphal wrapper. This is a very fine tunic, moulded exactly in
accordance with the smallest structural details and scarcely veiling
the shape and colours of the perfect insect. As a prelude to the last
act of the metamorphosis, the Sphex, suddenly shaking off her torpor,
begins to move about violently, as though to call her long-numbed limbs
to life. The abdomen is alternately lengthened and shortened; the legs
are abruptly extended, then bent, then extended again; and their
different joints are stiffened with an effort. The insect, using its
head and the tip of its abdomen as a lever, with the ventral surface
underneath, repeatedly distends with vigorous jerks the joint of the
neck and that of the peduncle connecting the abdomen and the thorax. At
last its efforts are crowned with success; and, after a quarter of an
hour of these rough gymnastics, the scabbard, tugged in every
direction, rips open at the neck, at the point where the legs are
attached and near the peduncle of the abdomen, in short, wherever the
mobility of the parts has permitted any violent dislocation to take
place.
All these rents in the veil that is being cast result in a number of
irregular shreds, whereof the largest envelops the abdomen and runs up
the back of the thorax. To this shred belong the wing-cases. A second
shred covers the head. Lastly, each leg has its own sheath, more or
less badly treated near the base. The large shred, which in itself
forms the best part of the wrapper, is thrown off by means of alternate
contractions and expansions of the abdomen. By this mechanical process
it is slowly forced backwards, where it ends by forming a little pellet
that for some time remains fastened to the insect by the tracheal
gills. The Sphex then once more becomes motionless; and the operation
is over. However, the head, antennæ and legs are still more or less
veiled. It is evident that the legs in particular cannot be freed all
in one piece, because of the numerous excrescences or spines with which
they are armed. These different shreds of skin dry up on the insect and
are removed afterwards by rubbing the legs. It is not until the Sphex
has acquired her full vigour that she finishes her moulting by
brushing, smoothing and combing her whole body with her tarsi.
The way in which the wings come out of their sheaths is the most
remarkable part of the sloughing. In their incomplete stump stage they
are folded lengthwise and are very much compressed. It is easy to
extract them from their cases a little while before the normal date of
their appearance; but then they remain permanently contracted and do
not fill out. On the other hand, when once the large strip of skin to
which the sheaths of the wings belong is pushed back by the movements
of the abdomen, we see the wings come slowly out of their cases and
straightway, as they become free, assume dimensions out of all
proportion to the narrow prison whence they emerge. They are therefore
the seat of an abundant rush of vital fluids which swell them and
spread them out, and which, owing to the inflation which they provoke,
must be the chief cause of the wings’ emergence from their cases. When
newly expanded, the wings are heavy, full of juices and of a very pale
straw-colour. If the rush of the fluids takes place irregularly, we
then see the end of the wing weighed down by a little yellow drop
contained between the two scales.
After stripping herself of the abdominal sheath, which carries the
wing-cases with it, the Sphex relapses into immobility for about three
days. During this time the wings assume their normal hue, the tarsi
become coloured, and the mouth-parts, at first extended, adopt their
proper position. After twenty-four days spent in the nymphal stage, the
insect has achieved the perfect state. It tears the cocoon that holds
it captive, opens itself a passage through the sand and comes out one
fine morning into the light of day, undazzled by that hitherto unknown
radiance. Bathed in sunshine, the Sphex brushes her antennæ and her
wings, passes and repasses her legs over her abdomen, washes her eyes
with her front tarsi wetted with saliva, like a cat; and, her toilet
finished, flies away joyfully: she has two months to live.
You pretty Sphex-wasps hatched before my eyes, brought up by my hand,
ration by ration, on a bed of sand in an old quill-box; you whose
transformations I have followed step by step, starting up from my sleep
in alarm lest I should have missed the moment when the nymph is
bursting its swaddling-bands or the wing leaving its case; you who have
taught me so much and learned nothing yourselves, knowing without
teachers all that you have to know: O my pretty Sphex-wasps, fly away
without fear of my tubes, my boxes, my bottles, or any of my
receptacles, through this warm sunlight beloved of the Cicadæ; [23] go,
but beware of the Praying Mantis, [24] who is plotting your ruin on the
flowering heads of the thistles, and mind the Lizard, who is lying in
wait for you on the sunny slopes; go in peace, dig your burrows, stab
your Crickets scientifically and continue your kind, to procure one day
for others what you have given me: the few moments of happiness in my
life!