To decide positively whether honey is or is not repugnant to the grubs of the Philanthus was hardly practicable by the method just explained. The first meals consisted of flesh, and after that nothing in particular occurred. The honey is encountered later, when the bee is largely consumed. If hesitation and repugnance were manifested at this point they came too late to be conclusive; the sickness of the larvæ might be due to other causes, known or unknown. We must offer honey at the very beginning, before artificial rearing has spoilt the grub's appetite. To offer pure honey would, of course, be useless; no carnivorous creature would touch it, even were it starving. I must spread the honey on meat; that is, I must smear the dead bee with honey, lightly varnishing it with a camel's-hair brush.
Under these conditions the problem is solved with the first few mouthfuls. The grub, having bitten on the honeyed bee, draws back as though disgusted; hesitates for a long time; then, urged by hunger, begins again; tries first on one side, then on another; in the end it refuses to touch the bee again. For a few days it pines upon its rations, which are almost intact, then dies. As many as are subjected to the same treatment perish in the same way.
Do they simply die of hunger in the presence of food which their appetites reject, or are they poisoned by the small amount of honey absorbed at the first bites? I cannot say; but, whether poisonous or merely repugnant, the bee smeared with honey is always fatal to them; a fact which explains more clearly than the unfavourable circumstances of the former experiment my lack of success with the freshly killed bees.
This refusal to touch honey, whether poisonous or repugnant, is connected with principles of alimentation too general to be a gastronomic peculiarity of the Philanthus grub. Other carnivorous larvæ—at least in the series of the Hymenoptera—must share it. Let us experiment. The method need not be changed. I exhume the larvæ when in a state of medium growth, to avoid the vicissitudes of extreme youth; I collect the bodies of the grubs and insects which form their natural diet and smear each body with honey, in which condition I return them to the larvæ. A distinction is apparent: all the larvæ are not equally suited to my experiment. Those larvæ must be rejected which are nourished upon one single corpulent insect, as is that of the Scolia. The grub attacks its prey at a determined point, plunges its head and neck into the body of the insect, skilfully divides the entrails in order to keep the remains fresh until its meal is ended, and does not emerge from the opening until all is consumed but the empty skin.
To interrupt the larva with the object of smearing the interior of its prey with honey is doubly objectionable; I might extinguish the lingering vitality which keeps putrefaction at bay in the victim, and I might confuse the delicate art of the larva, which might not be able to recover the lode at which it was working or to distinguish between those parts which are lawfully and properly eaten and those which must not be consumed until a later period. As I have shown in a previous volume, the grub of the Scolia has taught me much in this respect. The only larvæ acceptable for this experiment are those which are fed on a number of small insects, which are attacked without any special art, dismembered at random, and quickly consumed. Among such larvæ I have experimented with those provided by chance—those of various Bembeces, fed on Diptera; those of the Palaris, whose diet consists of a large variety of Hymenoptera; those of the Tachytus, provided with young crickets; those of the Odynerus, fed upon larvæ of the Chrysomela; those of the sand-dwelling Cerceris, endowed with a hecatomb of weevils. As will be seen, both consumers and consumed offer plenty of variety. Well, in every case their proper diet, seasoned with honey, is fatal. Whether poisoned or disgusted, they all die in a few days.
A strange result! Honey, the nectar of the flowers, the sole diet of the apiary under its two forms and the sole nourishment of the predatory insect in its adult phase, is for the larva of the same insect an object of insurmountable disgust, and probably a poison. The transfiguration of the chrysalis surprises me less than this inversion of the appetite. What change occurs in the stomach of the insect that the adult should passionately seek that which the larva refuses under peril of death? It is no question of organic debility unable to support a diet too substantial, too hard, or too highly spiced. The grubs which consume the larva of the Cetoniæ, for example (the Rose-chafers), those which feed upon the leathery cricket, and those whose diet is rich in nitrobenzine, must assuredly have complacent gullets and adaptable stomachs. Yet these robust eaters die of hunger or poison for no greater cause than a drop of syrup, the lightest diet imaginable, adapted to the weakness of extreme youth, and a delicacy to the adult! What a gulf of obscurity in the stomach of a miserable worm!
These gastronomic experiments called for a counter-proof. The carnivorous grub is killed by honey. Is the honey-fed grub, inversely, killed by carnivorous diet? Here, again, we must make certain exceptions, observe a certain choice, as in the previous experiments. It would obviously be courting a flat refusal to offer a heap of young crickets to the larvæ of the Anthophorus and the Osmia, for example; the honey-fed grub would not bite such food. It would be absolutely useless to make such an experiment. We must find the equivalent of the bee smeared with honey; that is, we must offer the larva its ordinary food with a mixture of animal matter added. I shall experiment with albumen, as provided by the egg of the hen; albumen being an isomer of fibrine, which is the principal element of all flesh diet.
Osmia tricornis will lend itself to my experiment better than any other insect on account of its dry honey, or bee-bread, which is largely formed of flowery pollen. I knead it with the albumen, graduating the dose of the latter so that its weight largely exceeds that of the bee-bread. Thus I obtain pastes of various degrees of consistency, but all firm enough to support the larva without danger of immersion. With too fluid a mixture there would be a danger of death by drowning. Finally, on each cake of albuminous paste I install a larva of medium growth.
This diet is not distasteful; far from it. The grubs attack it without hesitation and devour it with every appearance of a normal appetite. Matters could not go better if the food had not been modified according to my recipes. All is eaten; even the portions which I feared contained an excessive proportion of albumen. Moreover—a matter of still greater importance—the larvæ of the Osmia fed in this manner attain their normal growth and spin their cocoons, from which adults issue in the following year. Despite the albuminous diet the cycle of evolution completes itself without mishap.
What are we to conclude from all this? I confess I am embarrassed. Omne vivum ex ovo, says the physiologist. All animals are carnivorous in their first beginnings; they are formed and nourished at the expense of the egg, in which albumen predominates. The highest, the mammals, adhere to this diet for a considerable time; they live by the maternal milk, rich in casein, another isomer of albumen. The gramnivorous nestling is fed first upon worms and grubs, which are best adapted to the delicacy of its stomach; many newly born creatures among the lower orders, being immediately left to their own devices, live on animal diet. In this way the original method of alimentation is continued—the method which builds flesh out of flesh and makes blood out of blood with no chemical processes but those of simple reconstruction. In maturity, when the stomach is more robust, a vegetable diet may be adopted, involving a more complex chemistry, although the food itself is more easily obtained. To milk succeeds fodder; to the worm, seeds and grain; to the dead or paralysed insects of the natal burrow, the nectar of flowers.