The Zeuzera, or Leopard-moth, a large and beautiful white Moth with blue spots, is more general. She is the scourge of most of the trees and shrubs in my enclosure. I find her caterpillar chiefly in the lilac-tree; also in the elm, the plant-tree, the quince, the guelder-rose, the pear-tree and the chestnut. In these, always working upwards, it bores itself straight galleries which turn a branch the thickness of a good-sized bottle-neck into a fragile sheath soon broken by the winter wind.
To return to the specialists: the Shagreen Saperda exploits the black poplar and accepts nothing else, not even the white poplar; the Spotted Saperda has the elm for its domain; the Scalary Saperda is faithful to the dead cherry-tree.[6] The Great Capricorn lodges her grubs in the oak, sometimes the English oak and sometimes the evergreen oak, or ilex. [[230]]This last Beetle, being easily reared with slices of pear for food and sticks of wood in which to establish her family, lends herself to an experiment of some interest.
I collect the eggs which the mother’s pointed, groping oviduct has slipped into the irregular crevices of the bark. The number obtained enables me to make a variety of tests. Will the new-born larvæ accept the first wood that offers after they are hatched. That is the problem.
I select freshly-cut billets measuring two or three fingers’-breadths in diameter. They include the ilex, elm, lime, robinia, cherry, willow, elder, lilac, fig, laurel and pine. To avoid falls, which would confuse the new-born grubs if they had to wander about in search of the spot at which to bore, I do my best to imitate the natural conditions. The mother Capricorn lodges her eggs, one at a time, here and there in the fissures of the bark, fixing them with a thin varnish. I cannot gum the eggs in this way: my glue would perhaps endanger the vitality of the egg; but I can resort to the firm support of a furrow. With the point of a penknife I make this furrow, that is to say, a tiny cleft [[231]]into which the egg sinks half-way. This precaution succeeds admirably.
In a few days the eggs hatch without falling off, each at the spot decided by the point of my penknife. I watch in amazement the first wriggles of the feeble little creature’s body, the first strokes of its plane, as it attacks the thankless material, the bark and the wood, still dragging its white egg-shell behind it. By the following day, each grub has disappeared beneath a fine sawdust, the result of the work accomplished. The mound is still very small, matching the weakness of the excavator. Let us leave the grub at work. For a fortnight we see the mound grow bigger and bigger, until it is almost the size of a pinch of snuff. Then everything stops. The amount of sawdust does not increase, except in the oak-billet.
This activity at the outset, which is everywhere the same, in media differing so greatly in aroma and flavour, would lead us to suppose, at first thoughts, that the young Cerambyx is endowed with a highly complaisant stomach and can feed on the fig-tree, oozing with acrid milk, the laurel, aromatic with essential oils, and the pine, saturated with [[232]]resin, as well as on the oak, seasoned with tannin. Reflection persuades us that we are mistaken. The little creature is not engaged in eating: it is toiling to make itself a deep lodging in which it can feast in peace.
When examined through the lens, the sawdust confirms our theory: this dust has not passed through the digestive canal; it has played no part in feeding the grub. It is only so much meal, crumbled by the mandibles, and nothing more.
When appetite has come and the requisite depth has been reached, the grub at last begins to eat. If it finds the traditional food ready to its teeth, the sap-wood of the oak, with its astringent flavour, it gorges itself and proceeds to digest. If it finds nothing of the sort, it abstains from eating. This is certainly the reason why the heap of sawdust grows larger on the billet of oak but remains indefinitely stationary on the others.
What do they do in their little galleries, these grubs subjected to a strict fast in the absence of suitable victuals? In March, six months after the hatching, I look into the matter. I split the billets. There they are, the little grubs, no larger, but still lively, [[233]]swaying their heads to and fro if I disturb them. This persistence of life in such puny creatures deprived of food rouses our astonishment. It reminds us of the grubs of the Attelabus-beetle, which, subjected to the drought of summer in their little kegs made of a strip of oak-leaf, cease eating and slumber, half-dead, for four or five months, until the autumn rains have softened their food.
When I myself produced rain, a thing not beyond my power, so far as the needs of a grub are concerned, when I softened the rigid kegs and made them edible by a brief immersion in water, the recluses used to return to life, begin to eat again and continue their larval development without further check. Similarly, after six months’ fasting in the heart of inacceptable sticks, the Capricorn grubs would have recovered their strength and activity if I had removed them and put before them a freshly-cut billet of oak. I did not do it, so certain did the success of the experiment appear.