THE LARVA AND THE NYMPH

The Hunting Wasps by Jean-Henri Fabre, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. THE LARVA AND THE NYMPH

Chapter VI. THE LARVA AND THE NYMPH

The egg of the Yellow-winged Sphex is white, elongated, cylindrical, slightly bow-shaped and measures three to four millimetresin 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.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,3 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 millimetres4 and its greatest breadth from five to six.5 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.6

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;7 go, but beware of the Praying Mantis,8 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!

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