Social Leaf-Miners.
The preceding descriptions apply to caterpillars who construct their mines in solitude, there being seldom more than one on a leaf or leaflet, unless when two mother-flies happen to lay their eggs on the same leaf; but there are others, such as the miners of the leaves of the henbane (Hyoscyamus niger), which excavate a common area in concert—from four to eight forming a colony. These are very like flesh-maggots, being larger than the common miners; the leaves of this plant, from being thick and juicy, giving them space to work and plenty to eat.
Most of the solitary leaf-miners either cannot or will not construct a new mine, if ejected by an experimenter from the old, as we have frequently proved; but this is not the case with the social miners of the henbane-leaf. Bonnet ejected one of these, and watched it with his glass till it commenced a new tunnel, which it also enlarged with great expedition; and in order to verify the assertion of Réaumur, that they neither endeavour nor fear to meet one another, he introduced a second. Neither of them manifested any knowledge of the other’s contiguity, but both worked hard at the gallery, as did a third and a fourth which he afterwards introduced; for though they seemed uneasy, they never attacked one another, as the solitary ones often do when they meet.[CB]
Bark-mining Caterpillars.
A very different order of mining caterpillars are the progeny of various beetles, which excavate their galleries in the soft inner bark of trees, or between it and the young wood (alburnum). Some of these, though small, commit extensive ravages, as may readily be conceived when we are told that as many as eighty thousand are occasionally found on one tree. In 1783 the trees thus destroyed by the printer-beetle (Tomicus typographus, Latr.), so called from its tracks resembling letters, amounted to above a million and a half in the Hartz forest. It appears there periodically, and confines its ravages to the fir. This insect is said to have been found in the neighbourhood of London.
On taking off the bark of decaying poplars and willows, we have frequently met with the tracks of a miner of this order, extending in tortuous pathways, about a quarter of an inch broad, for several feet and even yards in length. The excavation is not circular, but a compressed oval, and crammed throughout with a dark-coloured substance like sawdust—the excrement no doubt of the little miner, who is thereby protected from the attacks of Staphylinidæ, and other predaceous insects from behind. But though we have found a great number of these subcortical tracks, we have never discovered one of the miners, though they are very probably the grubs of the pretty musk-beetle (Cerambyx moschatus), which are so abundant in the neighbourhood of the trees in question, that the very air in summer is perfumed with their odour. (J. R.)
[Mr. Rennie is undoubtedly right in his suggestion. I have found similar holes in old willow trees, and have traced them throughout their varied ramifications. They contain the larvæ and pupæ of the musk-beetle, some of which may be seen in the Museum at Oxford. On these trees, which mostly grow along the banks of the Cherwell, the perfect beetle was so abundant that it might be taken in any number, and, as described by Mr. Rennie, the air was perfumed with its powerful and agreeable odour. So strong is the scent of this beetle, that I have known it adhere to gloves after the lapse of many weeks, and I have often caught the scent when passing along the road, and merely by the aid of the nostrils discovered the insect.
On account of the vast number of carpenter-beetles, it is impossible to notice more than a few of them, and we will therefore select some of the most conspicuous. One of them, belonging to the genus Ptilinus, is very familiar to us as boring into wooden furniture, and producing the effect which is popularly called “worm-eaten.” Fortunately, the little creatures can be easily ejected, and the wood rendered free of them ever afterwards. All that is needed is to take a syringe with a very fine aperture—an injecting syringe is the best—and by its aid to force into the holes a solution of corrosive sublimate in spirits of wine—say a large teaspoonful of the powdered salt to a pint of spirits. The rapidity with which the poisoned spirit permeates the wood is wonderful, and in a short time it may be seen oozing out of twenty or thirty holes at once. This solution is peculiarly effective, as it kills all the insects, destroys every egg that it touches, and renders the wood poisonous to the grubs that happen to escape. I used to be greatly plagued with the Ptilinus among my ethnological collection, until I tried the corrosive sublimate, and ever since my spears, bows and arrows, and clubs have remained intact.
Another troublesome insect is the Scolytus destructor, which makes its radiating tunnels between the bark and the tree. Whole forests have been destroyed by this voracious little beetle, the bark having been completely detached, and the tree necessarily killed. The habits of this beetle are well described in the following passage.]
