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, [[226]]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.
What is more, I have found the larvæ of I know not what insect on Nux vomica, the terrible poison with which our municipal authorities flavour the sausages used for destroying Wolves and stray dogs. These strychnine-eaters have certainly not accustomed [[227]]themselves by degrees to this terrible diet: they would perish at the first mouthful, if they had not a specially constructed stomach at their service.
This exclusive taste for such or such a vegetable, sometimes harmless and sometimes poisonous, has many exceptions. Some vegetarian insects are omnivorous. The destructive Locust nibbles every green thing; our common Grasshoppers eat the tips of any sort of grass without distinction. Kept in a cage to divert the children, the Field Cricket feasts on a leaf of lettuce or endive, new foodstuffs that help it to forget the tough grasses of his meadows.
In April, on the green banks by the road-side, we meet with squads of an ugly, fat, bronze-black creature, which, when we tease it, plays the Tortoise, shrinking into a ball. It walks heavily on six feeble legs, while the end of the intestine, becoming a supplementary foot, acts as a lever and pushes it forward. It is the larva of a large black Chrysomela (Timarcha tenebricosa, Fab.), an unpleasant Beetle which, in self-defense, disgorges an orange spittle.
I amused myself last spring by following a flock of these larvæ to their grazing-grounds. [[228]]The favourite plant was one of the Rubiaceæ, the cheese-rennet (Galium verum), in the stage of young shoots. Various other plants were eaten no less readily on the way, including especially Cichoriaceæ such as Pterotheca nemansensis, Chondrilla juncea or gum-succory, and cut-leaved podospermum (P. laciniatum), and Leguminosæ such as Medicago falcata, or yellow medick and Trifolium repens, or white clover. The acrid flavours did not in the least discourage the flock. A Gerard’s spurge was met with, trailing its flower on the ground. A few larvæ stopped and nibbled the tender tops as eagerly as the clover. In short, the fat crippled larva varies its meal greatly.
Examples abound of insects equally omnivorous of vegetable substances; there is no need to linger over them. Let us pass on to the exploiters of woody materials. The larva of Ergates faber lives exclusively in decayed pine-stumps; the hideous caterpillar of the Moth inappropriately known as the Cossus eats into old willow-trees, in company with the Ægosoma.
These two are specialists.
The lesser Capricorn, Cerambyx cerdo, entrusts her grubs to the hawthorn, the sloe, [[229]]the apricot-tree and the cherry-laurel, all of which trees or shrubs belong to the family of the Rosaceæ. She varies her domain a little, while remaining faithful to woody vegetation characterized by a faint flavour of prussic acid.