Insect life: Souvenirs of a naturalist

by Jean-Henri Fabre

We saw that the larva began on the stomach of the second cricket, this being the most juicy and fattest part. Like a child who first licks off the jam on his bread, and then bites the slice with contemptuous tooth, it goes straight to what is best, the abdominal intestines, leaving the flesh, which must be extracted from its horny sheath, until it can be digested deliberately. But when first hatched it is not thus dainty: it must take the bread first and the jam later, and it has no choice but to bite its first mouthful from the middle of the victim’s chest, exactly where its mother placed the egg. It is rather tougher, but the spot is a secure one, on account of the deep inertia into which three stabs have thrown the thorax. Elsewhere, there would be, generally, if not always, spasmodic convulsions which would detach the feeble thing and expose it to terrible risks amid a heap of victims whose hind legs, toothed like a saw, might occasionally kick, and whose jaws could still grip. Thus it is motives of [[106]]security, and not the habits of the grub, which determine the mother where to place its egg.

A suspicion suggests itself to me as to this. The first cricket, the ration on which the egg is laid, exposes the grub to more risks than do the others. First, the larva is still a weakly creature; next, the victim was only recently stung, and therefore in the likeliest state for displaying some remains of life. This first cricket has to be as thoroughly paralysed as possible, and therefore it is stabbed three times. But the others, whose torpor deepens as time passes,—the others which the larvæ only attack when grown strong,—have they to be treated as carefully? Might not a single stab, or two, suffice to bring on a gradual paralysis while the grub devours its first allowance? The poison is too precious to be squandered; it is powder and shot for the Sphex, only to be used economically. At all events, if at one time I have been able to see a victim stabbed thrice, at another I have only seen two wounds given. It is true that the quivering point of the Sphex’s abdomen seemed seeking a favourable spot for a third wound; but if really given, it escaped my observation. I incline to believe that the victim destined to be eaten first always is stabbed three times, but that economy causes the others only to be struck twice. The study of the caterpillar-hunting Ammophila will later confirm this suspicion.

The last cricket being finished, the larva sets to work to spin a cocoon. In less than forty-eight hours the work is completed, and henceforward the skilful worker may yield within an impenetrable shelter to the overpowering lethargy which is stealing [[107]]over it—a state of being which is neither sleeping nor waking, death nor life, whence it will issue transfigured ten months later. Few cocoons are so complex as is this one. Besides a coarse outer network, there are three distinct layers, forming three cocoons, one within another. Let us examine in detail these various courses of the silken edifice. First comes an open network, coarse and cobwebby, on which the larva places itself and hangs as in a hammock to work more easily at the cocoon properly so called. This incomplete net, hastily spun to serve as a scaffolding, is made with threads carelessly placed and holding grains of sand, bits of earth, and remains from the larva’s banquet—cricket’s thighs, still banded with red, feet, and skull. The next covering, which is the first of the real cocoon, is a felted wrapper, light red, very fine, very supple, and somewhat crumpled. A few threads cast here and there connect it with the preceding scaffolding and the following covering. It forms a cylindrical purse, with no opening and too large for what it contains, thus causing the surface to wrinkle. Then comes an elastic case, markedly smaller than the purse which contains it, almost cylindrical, and rounded at the upper end, toward which is turned the head of the larva, while at the lower it makes a blunt cone. Its colour is light red, except towards the lower end, where the shade is darker. It is fairly firm, though it yields to a moderate pressure, except in the conical part, which resists and seems to contain a hard substance. On opening this sheath it is seen to be formed of two layers closely pressed together, but easily separable. The outer is a silken felt [[108]]precisely like that of the preceding purse, the inner one, the third of the cocoon, is a kind of lacquer—a brilliant violet-brown varnish, fragile, very soft to the touch, and of quite a different nature to the rest of the cocoon. The microscope shows that instead of being a felt of silky filaments like the other coverings, it is a homogeneous covering of a peculiar varnish, whose origin is, as we shall see, sufficiently strange. As for the resistance of the conical end of the cocoon, one finds it caused by a load of friable matter, dark violet, and shining with numerous black particles. This load is the dry mass of excrement, ejected once for all by the larva, inside its cocoon, and to it is due the darker colour of the conical end. The average length of this complex dwelling is twenty-seven millimetres, and its greatest width nine.

Let us return to the purple varnish which covers the interior of the cocoon. At first, I thought it should be attributed to the silk glands, which, after serving to spin the double wrapper of silk and the scaffolding, must finally have secreted it. To convince myself, I opened larvæ which had just completed their task of weaving, and had not yet begun to lay on the lacquer. At that period I found no trace of violet fluid in their glands. It is only seen in the digestive canal, which is swelled with a purple pulp, and later in the stercorous load sent down to the lower end of the cocoon. Elsewhere all is white, or faintly tinged with yellow. I am far from suggesting that the larva plasters its cocoon with excrement, yet I am convinced that this wash is produced by the digestive organs, and I [[109]]suspect—though I cannot positively assert it, having several times missed the moment to ascertain it—that the larva disgorges and applies with its mouth the quintessence of the purple pulp in its stomach to make the wash of lacquer. Only after this last piece of work would it eject the remains of digestion in a single mass, and thus is explained the disgusting necessity of storing the excrement within the larva’s habitation.

At all events the usefulness of this layer is clear; its absolute impermeability protects the larva from the damp which would certainly penetrate the poor shelter hollowed for it by its mother. Recollect that it is buried but a few inches deep in sandy, open ground. To judge how far cocoons thus varnished are capable of resisting damp, I have plunged them in water for several days, yet never found any trace of moisture within them. Let us compare the Sphex cocoon, with manifold coverings to protect the larva in a burrow itself unprotected, with that of Cerceris tuberculata, sheltered by a layer of sandstone, more than half a yard down in the ground. This cocoon has the form of a very long pear, with the small end cut off. It is composed of a single silken wrap, so fine that the larva is seen through it. In my many entomological researches I have always found the labour of larva and mother supplement each other. In a deep well-sheltered dwelling the cocoon is of light materials; for a surface abode, exposed to wind and weather, it is strongly constructed.

Nine months pass, during which a work is done which is quite hidden. I pass over this period, [[110]]occupied by the unknown mystery of transformation, and to come to the nymph, go from the end of September to the first days of the following July. The larva has thrown aside its faded vestment, and the chrysalis, a transitory organisation, or rather, a perfect insect in swaddling bands, awaits motionless the awakening which is still a month off. Feet, antennæ, the visible portions of the mouth, and the undeveloped wings, look like clearest crystal, and are regularly spread out under the thorax and abdomen. The rest of the body is of an opaque white, slightly tinged with yellow; the four intermediary segments of the abdomen show on either side a narrow, blunt prolongation; the last segment has above a blade-like termination, shaped like the section of a circle, furnished below with two conical protuberances, side by side, thus making in all eleven appendages starring the contour of the abdomen. Such is the delicate creature which, to become a Sphex, must assume a particoloured livery of black and red, and throw off the fine skin which swaddles it so closely.

I have been curious to follow day by day the progress and coloration of the chrysalid, and to experiment whether sunlight—that rich palette whence Nature draws her colours—could influence their progress. With this aim I have taken chrysalids out of their cocoon and kept them in glass tubes, where some, in complete darkness, realised natural conditions, while others, hung up against a white wall, were all day long in a strong light. These diametrically opposed conditions did not affect the colouring, or if there were some slight difference, it was to the disadvantage of those exposed to light. [[111]]Quite unlike to what occurs with plants, light does not influence insect-colouring, nor even quicken it. It must be so, since in the species most gifted with splendid colour—Buprestids and Carabids for instance—the wonderful hues that would seem stolen from a sunbeam are really elaborated in darkness, deep in the ground, or in the decayed trunk of some aged tree.

The first indication of colour is in the eyes, whose horny facets pass successively from white to tawny, then to a slaty hue, and lastly to black. The simple ones at the top of the forehead share in their turn in this coloration before the rest of the body has at all lost its whitish tint. It should be noted that this precocity in the most delicate of organs, the eye, is general in animals. Later a smoky line appears in the furrow separating the mesothorax from the metathorax, and four-and-twenty hours later the whole back of the mesothorax is black. At the same time the division of the prothorax grows shaded, a black dot appears in the central and upper part of the metathorax, and the mandibles are covered with a rusty tint. Gradually a deeper and deeper shade spreads over the last segments of the thorax, and finally reaches the head and sides. One day suffices to turn the smoky tint of the head and the furthest segments of the thorax into deep black. Then the abdomen shares in the rapidly increasing coloration. The edge of the anterior segments is tinted with daffodil, while the posterior segments acquire a band of ashy black. Then the antennæ and feet take a darker and darker tint, till they become black, all the base of the abdomen turns [[112]]orange-red, and the tip black. The livery would then be complete, but that the tarsi and mouthpieces are transparent red and the stumps of wings ashy black. Four-and-twenty hours later the chrysalis will burst its bonds. It only takes six or seven days to acquire its permanent tints; the eyes have done so a fortnight before the rest of the body. From this sketch the law of chromatic evolution is easily apprehended. We see that, omitting the eyes and ocelli, whose early perfection recalls what takes place in the higher animals, the starting-point of coloration is a central one, the mesothorax, whence it invades progressively by centrifugal progression—first the rest of the thorax, then the head and abdomen, and finally the various appendages, antennæ, and feet. The tarsi and mouthpieces take colour later still, and the wings only on coming out of their cases.

Now we have the Sphex in full costume, but she still has to free herself from the chrysalis case. This is a very fine wrap, enfolding every smallest detail of structure, and hardly veiling the shape and colours of the perfect insect. As prelude to the last act of metamorphosis, the Sphex, rousing suddenly from her torpor, begins to shake herself violently, as if to call life into her long-benumbed limbs. The abdomen is alternately lengthened and contracted, the feet are suddenly spread, then bent, then spread again, and their various joints are stiffened with effort. The creature, curved backwards on its head and the point of the abdomen, with ventral surface upward, distends by vigorous shakes the jointing of its neck and of the petiole [[113]]attaching the abdomen to the thorax. At last its efforts are crowned with success, and after half an hour of these rough gymnastics the sheath, pulled in every direction, ruptures at the neck, at the insertion of the feet and petiole, and, in short, wherever the body has been movable enough to allow of sufficiently violent displacement.

All these tears leave several irregular strips, the chief of which envelops the abdomen and comes up the back of the thorax. To it belong the wing sheaths. A second strip covers the head. Lastly, each foot has its own sheath, more or less dilapidated toward the base. The biggest, which forms the chief part of the whole covering, is got off by alternate dilatations and contractions of the abdomen, which gradually push it back into a little ball connected for some time with the animal by tracheal filaments. Then the Sphex again becomes motionless, and the operation is over, though head, antennæ, and feet are still more or less covered. It is clear that the feet cannot be freed in one piece on account of the roughnesses and thorns with which they are armed. These rags of skin dry up and are got rid of later by rubbing the feet together, and by brushing, smoothing, and combing the whole body with the tarsi when the Sphex has acquired full vigour.