This stubborn refusal is not due to gastronomical causes. I have stated that the larvae reared by my own hands feed on young Grasshoppers as readily as on young Mantes; they do not seem to perceive any difference between the two dishes; they thrive equally on the game chosen by me and that selected by the mother. If the mother sets no value on the Grasshopper, what then can be the reason of her refusal? I can see only one: this quarry, which is not hers, perhaps inspires her with fear, as any unknown thing might do; the ferocious Mantis does not alarm her, but the peaceable Grasshopper terrifies her. And then, if she were to overcome her apprehensions, she does not know how to master the Acridian and, above all, how to operate upon him. To every man his trade, to every Wasp her own way of wielding her sting. Modify the conditions ever so slightly; and these skilful paralysers are at an utter loss.
To every insect also its own art of fashioning the cocoon, an art which varies greatly, an art in which the larva displays all the resources of its instincts. The Tachytes, the Bembeces, the Stizi, the Palari and other burrowers build composite cocoons, hard as fruit-stones, formed of an encrustation of sand in a network of silk. We are already acquainted with the work of the Bembex. I will recall the fact that their larva first weaves a conical, horizontal bag of pure white silk, with wide meshes, held in place by interlaced threads which fix it to the walls of the cell. I have compared this bag, because of its shape, with a fishtrap. Without leaving this hammock, stretching its neck through the orifice, the worker gathers from without a little heap of sand, which it stores inside its workshop. Then, selecting the grains one by one, it encrusts them all around itself in the fabric of the bag and cements them with the fluid from its spinnerets, which hardens at once. When this task is finished, the house has still to be closed, for it has been wide open all this time to permit of the renewal of the store of sand as the heap inside becomes exhausted. For this purpose a cap of silk is woven across the opening and finally encrusted with the materials which the larva has retained at its disposal.
The Tachytes builds in quite another fashion, although its work, once finished, does not differ from that of the Bembex. The larva surrounds itself, to begin with, about the middle of its body with a silken girdle which a number of threads, very irregularly distributed, hold in place and connect with the walls of the cell. Sand is collected, within reach of the worker, on this general scaffolding. Then begins the work of minor masonry, with grains of sand for rubble and the secretion of the spinnerets for cement. The first course is laid upon the fore-edge of the suspensory ring. When the circle is completed, a second course of grains of sand, stuck together by the fluid silk, is raised upon the hardened edge of what has just been done. Thus the work proceeds, by ring-shaped courses, laid edge to edge, until the cocoon, having acquired half of its proper length, is rounded into a cap and finally is closed. The building-methods of the Tachytes-larva remind me of a mason constructing a round chimney, a narrow tower of which he occupies the centre. Turning on his own axis and using the materials placed to his hand, he encloses himself little by little in his sheath of masonry. In the same way the worker encloses itself in its mosaic. To build the second half of the cocoon, the larva turns round and builds in the same way on the other edge of the original ring. In about thirty-six hours the solid shell is completed.
I am rather interested to see the Bembex and the Tachytes, two workers in the same guild, employ such different methods to achieve the same result. The first begins by weaving an eel-trap of pure silk and next encrusts the grains of sand inside; the second, a bolder architect, is economical of the silk envelope, confines itself to a hanging girdle and builds course by course. The building-materials are the same: sand and silk; the surroundings amid which the two artisans work are the same: a cell in a soil of sandy gravel; yet each of the builders possesses its individual art, its own plan, its one method.
The nature of the food has no more effect upon the larva's talents than the environment in which it lives or the materials employed. The proof of this is furnished by Stiza ruficornis, another builder of cocoons in grains of sand cemented with silk. This sturdy Wasp digs her burrows in soft sandstone. Like the Mantis-killing Tachytes, she hunts the various Mantides of the countryside, consisting mainly of the Praying Mantis; only her large size requires them to be more fully developed, without however having attained the form and the dimensions of the adult. She places three to five of them in each cell.
In solidity and volume her cocoon rivals that of the largest Bembex; but it differs from it, at first sight, by a singular feature of which I know no other example. From the side of the shell, which is uniformly smoothed on every side, a rough knob protrudes, a little clod of sand stuck on to the rest. The work of Stizus ruficornis can at once be recognized, among all the other cocoons of a similar nature, by this protuberance.
Its origin will be explained by the method which the larva follows in constructing its strong-box. At the beginning, a conical bag is woven of pure white silk; you might take it for the initial eel-trap of the Bembeces, only this bag has two openings, a very wide one in front and another, very narrow one at the side. Through the front opening the Stizus provides itself with sand as and when it spends this material on encrusting the interior. This strengthens the cocoon; and the cap which closes it is made next. So far it is exactly like the work of the Bembex. We now have the worker enclosed, engaged in perfecting the inner wall. For these final touches a little more sand is needed. It obtains it from outside by means of the aperture which it has taken the precaution of contriving in the side of its building, a narrow dormer-window just large enough to allow its slender neck to pass. When the store has been taken in, this accessory orifice, which is used only during the last few moments, is closed with a mouthful of mortar, thrust outward from within. This forms the irregular nipple which projects from the side of the shell.
For the present I shall not expatiate further upon Stizus ruficornis, whose complete biography would be out of place in this chapter. I will limit myself to mentioning its method of constructing strong-boxes in order to compare it with that of the Bembex and above all with that of the Tachytes, a consumer, like itself, of Praying Mantes. From this parallel it seems to me to follow that the conditions of life in which men see to-day the origin of instincts—the type of food, the surroundings amid which the larval life is passed, the materials available for a defensive wrapper and other factors which the evolutionists are accustomed to invoke—have no actual influence upon the larva's industry. My three architects in glued sand, even when all the conditions, down to the nature of the provisions, are the same, adopt different means to execute an identical task. They are engineers who have not graduated from the same school, who have not been educated on the same principles, though the lesson of things is almost the same for all of them. The workshop, the work, the provisions have not determined the instinct. The instinct comes first; it lays down laws instead of being subject to them.