The Hunting Wasps

by Jean-Henri Fabre

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 [[93]]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 [[94]]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 [[95]]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 [[96]]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 [[97]]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 [[98]]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 [[99]]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 [[100]]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 [[101]]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.