I obtained the same success in offering young Mantes for food. One of the larvae thus served would even incline me to believe that it preferred the new dish to the traditional diet of its race. Two Eristales, or Drone-flies, and a Praying Mantis an inch long composed its daily allowance. The Drone-flies are disdained from the first mouthful; and the Mantis, already tasted and apparently found excellent, causes the Fly to be completely forgotten. Is this an epicure's preference, due to the greater juiciness of the flesh? I am not in a position to say. At all events, the Bembex is not so infatuated with Fly as to refuse to abandon it for other game.
The failure which I foresaw has proved a magnificent success. It is fairly convincing, is it not? Without the evidence of experiment, what can we rely upon? Beneath the ruins of so many theories which appeared to be most solidly erected I should hesitate to admit that two and two make four if the facts were not before me. My argument had the most tempting probability on its side, but it had not the truth. As it is always possible to find reasons after the event in support of an opinion which one would not at first admit, I should now argue as follows:
The plant is the great factory in which are elaborated, with mineral materials, the organic principles which are the materials of life. Certain products are common to the whole vegetable series, but others, far less numerous, are prepared in special laboratories. Each genus, each species has its trade-mark. Here essential oils are manufactured; here alkaloids; here starches, fatty substances, resins, sugars, acids. Hence result special energies, which do not suit every herbivorous animal. It assuredly requires a stomach made expressly for the purpose to digest aconite, colchicum, hemlock or henbane; those who have not such a stomach could never endure a diet of that sort. Besides, the Mithridates fed on poison resist only a single toxin. (Mithridates VI. King of Pontus (d. B.C. 63) is said to have secured immunity from poison by taking increased doses of it.—Translator's Note.) The caterpillar of the Death's-head Hawk-moth, which delights in the solanin of the potato, would be killed by the acrid principle of the tithymals that form the food of the Spurge-caterpillar. The herbivorous larvae are therefore perforce exclusive in their tastes, because different genera of vegetables possess very different properties.
With this variety in the products of the plant, the animal, a consumer far more than a producer, contrasts the uniformity in its own products. The albumen in the egg of the Ostrich or the Chaffinch, the casein in the milk of the Cow or the Ass, the muscular flesh of the Wolf or the Sheep, the Screech-owl or the Field-mouse, the Frog or the Earth-worm: these remain albumen, casein or fibrin, edible if not eaten. Here are no excruciating condiments, no special acridities, no alkaloids fatal to any stomach other than that of the appointed consumer; so that animal food is not confined to one and the same eater. What does not man eat, from that delicacy of the arctic regions, soup made of Seal's blood and a scrap of Whale-blubber wrapped in a willow-leaf for a vegetable, to the Chinaman's fried Silk-worm or the Arab's dried Locust? What would he not eat, if he had not to overcome the repugnance dictated by habit rather than by actual necessity? The prey being uniform in its nutritive principles, the carnivorous larva ought to accommodate itself to any sort of game, above all if the new dish be not too great a departure from consecrated usage. Thus should I argue, with no less probability on my side, had I to begin all over again. But, as all our arguments have not the value of a single fact, I should be forced in the end to resort to experiment.
I did so the next year, on a larger scale and with a greater variety of subjects. I shrink from a continuous narrative of my experiments and of my personal education in this new art, where the failure of one day taught me the way to succeed on the morrow. It would be long and tedious. Enough if I briefly state my results and the conditions which must be fulfilled in order to run the delicate refectory as it should be run.
And, first, we must not dream of detaching the egg from its natural prey to lay it on another. The egg adheres pretty firmly, by its cephalic pole, to the quarry. To remove it from its place would inevitably jeopardize its future. I therefore let the larva hatch and acquire sufficient strength to bear the removal without peril. For that matter, my excavations most often provide me with my subjects in the form of larvae. I adopt for rearing-purposes the larvae that are a quarter to a half developed. The others are too young and risky to handle, or too old and limited to a short period of artificial feeding.
Secondly, I avoid bulky heads of game, a single one of which would suffice for the whole growing-stage. I have already said and I here repeat how nice a matter it is to consume a victim which has to keep fresh for a couple of weeks and not to finish dying until it is almost entirely devoured. Death here leaves no corpse; when life is extinct, the body has disappeared, leaving only a shred of skin. Larvae with only one large prey have a special art of eating, a dangerous art, in which a clumsy bite would prove fatal. If bitten before the proper time at such a point, the victim becomes putrid, which promptly causes death by poisoning in the consumer. When diverted from its plan of attack, deprived of its clue, the larva is not always able to rediscover the lawful morsels in good time and is killed by the decomposition of its badly dissected prey. What will happen if the experimenter gives it a game to which it is not accustomed? Not knowing how to eat it according to rule, the larva will kill it; and by next day the victuals will have become so much toxic putrescence. I have already told how I found it impossible to rear the Two-banded Scolia on Oryctes-larvae, fastened down to deprive them of movement, or even on Ephippigers, paralysed by the Languedocian Sphex. In both cases the new diet was accepted without hesitation, a proof that it suited the nurseling; but in a day or two putrescence supervened and the Scolia perished on the fetid morsel. The method of preserving the Ephippiger, so well known to the Sphex, was unknown to my boarder; in this was enough to convert a delicious food into poison.
Even so did my other attempts miscarry wretchedly, attempts at feeding with the single dish consisting of one big head of game to replace the normal ration. Only one success is recorded in my notebooks, but that was so difficult that I would not undertake to obtain it a second time. I succeeded in feeding the larva of the Hairy Ammophila with an adult black Cricket, who was accepted as readily as the natural game, the caterpillar.
To avoid putrefaction of victuals which last overlong and are not consumed according to the method indispensable to their preservation, I employ small game, each piece of which can be finished by the larva at a single sitting, or at most in a single day. It matters little then that the victim is slashed and dismembered at random; decomposition has no time to seize upon its still quivering tissues. This is the procedure of those larvae which gulp down their food, snapping at random without distinguishing one part from another, such as the Bembex-larvae, which finish the Fly into which they have bitten before beginning another in the heap, or the Cerceris-larvae, which drain their Weevils methodically one after another. With the first strokes of the mandibles the victim broached may be mortally wounded. This is no disadvantage: a brief spell suffices to make use of the corpse, which is saved from putrefaction by being promptly consumed. Close beside it, the other victims, quite alive though motionless, await their respective turns and supply reserves of victuals which are always fresh.
I am too unskilful a butcher to imitate the Wasp and myself to resort to paralysis; moreover, the caustic liquid injected into the nerve-centres, ammonia in particular, would leave traces of smell or flavour which might put off my boarders. I am therefore compelled to deprive my insects of the power of movement by killing them outright. This makes it impracticable to provide a sufficiency of provisions beforehand in a single supply: while one item of the ration was being consumed the rest would spoil. One expedient alone remains to me, one which entails constant attendance: it is to renew the provisions each day. When all these conditions are fulfilled, the success of artificial feeding is still not without its difficulties; nevertheless, with a little care and above all plenty of patience, it is almost certain.