Let us consider a few examples. The Death’s-head Hawk-moth (Acherontia atropos, Linn.) has the leaves of the potato for her caterpillar’s portion. She is a foreigner, who seems to have come from America together with her food-plant. I have tried to rear her caterpillar on various plants belonging, like the potato, to the family of the Solanaceæ. Henbane, datura and tobacco were obstinately refused, despite the acute hunger displayed when the normal food was served. The violent alkaloids with which these plants are saturated may perhaps explain this refusal. We will therefore keep the true genus Solanum and we will replace the too active poisons by solanin, which is not so virulent. The leaves of the tomato (Solanum lycopersicum), the egg-plant (S. melongena), the black-berried nightshade (S. nigrum), the orange-berried nightshade (S. villosum), a native of New Zealand, and the common bittersweet of our country-sides (S. dulcamara) are, on the other hand, accepted with the same relish as the potato. These contradictory results leave me perplexed. Since the caterpillar of the Death’s-head Hawk-moth requires food flavoured with solanin, why are certain species of the same genus Solanum gluttonously devoured and others refused? Can it be because the dose of solanin is unequal, being weaker here and more abundant there? Or are there other reasons? I am utterly at a loss. The magnificent caterpillar of the Spurge Hawk-moth, La Belle, as Réaumur calls it, knows nothing of these inexplicable preferences. It welcomes any species whose wounds exude the sap of the tithymals, the white milky liquid with the fiery flavour. In my neighbourhood it is often found on the tall spurge of these parts, Euphorbia characias; but it is just as happy on smaller species, such as the narrow notch-leaved spurge (Euphorbia serrata) and Gerard’s spurge (E. Gerardiana). Under my bell-jars it thrives on the first spurge that comes to hand. Anything except these caustic foods, which no other caterpillar would accept, it abhors. It turns away in disdain from the insipid lettuce of our gardens, from peppermint, from the Cruciferæ, rich in sulphurous juices, the caustic ranunculus and other more or less highly flavoured plants. It will have nothing but the spurge, whose milky sap would corrode any gullet but its own. An insect that can feed with pleasure on such acrid fare must obviously be predisposed that way. For that matter, consumers devoted to pungent flavours are not scarce. The grub of Brachycerus algirus is as fond of aioli as the Provençal peasant; it thrives and grows fat in a clove of garlic, without other nourishment.