Some years ago, with a success which delighted the observer that I am, I changed the diet of various carnivorous larvæ. To those which lived on Weevils I gave Locusts; to those which lived on Locusts I gave Flies. My nurslings unhesitatingly accepted the food unknown to their race and were none the worse for it; but I would not undertake to rear a caterpillar with the first sort of leaves that came to hand: it would starve sooner than touch them.
Animal matter having undergone a more thorough refinement than vegetable substances, enables the stomach to pass from one dish to another without gradually becoming accustomed to each, whereas vegetable food, being comparatively refractory, calls for an apprenticeship on the part of the consumer. To turn Sheep’s flesh into Wolf’s flesh is an easy matter: a few minor transmutations are enough; but to make mutton out of grass is a complicated process of digestive chemistry, for which the ruminant’s four stomachs are none too many. The carnivorous insect is able to vary its diet, all sorts of game being of equal value.
Vegetable food involves other conditions. With its starches, oils, essences and spices [[218]]and often with its poisons, each plant tried would be a perilous innovation, to which the insect, repelled by the first mouthfuls, would never consent. How greatly preferable to these dangerous novelties is the invariable dish consecrated by ancient custom! This, no doubt, is why the vegetarian insect is faithful to its plant.
How is this division of the earth’s abundance among its consumers effected? We can hardly hope to understand the problem; it is too far beyond our methods of research. The most that we can do is, by experimental methods, to explore this corner of the unknown a little, to seek to discover how far the insect’s diet is fixed and to note its variations, if any. This will give us data which the future will employ to carry the problem farther.
Towards the end of the autumn, I had placed in the vivarium two couples of the Stercoraceous Geotrupes, with an ample heap of provender obtained from the Mules. I had no plans as regards my captives; I had put them there because it was an old habit of mine never to lose an opportunity. Chance had set them within my reach; chance would do the rest. [[219]]
With the sumptuous provision which I had bestowed upon them, the Geotrupes had had plenty wherewith to attend to their domestic affairs. They were overlooked all the winter, without any further intervention on my part. On the approach of spring, curiosity impelled me, in a leisure moment, to inspect them. It had been raining as hard through the sides of the cage, which consisted of a metal trellis, as it had in the streets; and, as the water could not trickle away through the wooden floor, the soil in the vivarium had turned to mud.
The sausages of food prepared by the parents were numerous, in spite of everything, but in a shocking state. Soaked by the rain, drenched to the very centre by continual infiltration, they fell into fragments if I moved them. Nevertheless, each contained, in the tattered chamber beneath it, an egg laid about the end of autumn; and this egg, spared by the ice-cold mud of winter, was so plump, so healthy and glossy, that an imminent hatching seemed evident.
What shall I give the grubs when they come out? I dare not count on the remnants of the regulation sausages, reduced to bales of fibre by the rains. As well give the new-born [[220]]larvæ an old rope’s-end. What is to be done? We will resort to a crazy artifice and serve a dish of our own invention, one absolutely unknown to the Geotrupes.
The mess prepared for my larvæ is made of leaves decaying on the ground: hazel-, cherry-, mulberry-, elm-, quince-leaves and others. I steep them in water to soften them and then shred them like fine-cut tobacco. The egg is placed at the bottom of a test-tube; and I pack a column of my vegetable mince-meat on the top. For purposes of comparison, other eggs are similarly lodged, but with a thankless ration of the normal preserves soaked by the rains.
Hatching occurs during the first week in March. I have before my eyes, when it leaves the egg, the larva which astonished me so greatly when I first realized, many years ago, that it was a cripple. In once more referring to this strange abnormality, I will confine myself to a few words on the subject of the head, which is remarkably bulky, swollen as it is by the motor muscles of the mandibular shears with broad, flat blades, notched at the tip and bearing a strong spur at the base. It is enough to see this dental armoury to recognize the new-born [[221]]grub as one that will not object to tackling ligneous fibres. With such a mincing-machine, a bit of straw must be a luxury.