According to the particular species of caterpillar, the silk will vary in strength and fineness, and also in colour, but seldom will it be found strong enough, or in sufficient quantity to be of use to us. The insects themselves employ it in many ways for their own safety and shelter. How common it is to find some of the leaves of a lilac-tree made up into little rolls, or folded together at their edges, where they stick so fast that it requires some little force to pull them asunder. This is the work of a small caterpillar, whose subsistence is found on that tree. A single egg has been laid on each leaf selected by the parent moth, and soon after it is hatched, the caterpillar begins rolling up the lilac leaf, and fastening the edges with silk. This is a work of time, but gradually proceeds as the caterpillar increases in strength, until at last it is shut up in a gallery of its own making, safe from the attacks of birds and larger insects. The leaves of various other trees are rolled in a somewhat similar manner, as those of the oak, the willow, and the rose, and also those of humbler plants, as the plantain, nettle, thistle, &c. The mode in which these leaf-rolling caterpillars set to work to form convenient habitations of the leaves of the plants on which they feed, is well described by Kirby and Spence.
OAK LEAF ROLLED BY A CATERPILLAR.
“Some of these merely connect together with a few silken threads several leaves, so as to form an irregular packet, in the centre of which the little hermit lives. Others confine themselves to a single leaf, of which they simply fold one part over the other. A third description form and inhabit a sort of roll, by some species made cylindrical, by others conical, resembling the papers in which grocers put their sugar, and as accurately constructed; only there is an opening left at the smaller extremity for the egress of the insect in case of need. If you were to see one of these rolls, you would immediately ask by what mechanism it could possibly be made—how an insect, without fingers, could contrive to bend a leaf into a roll, and to keep it in that form until fastened with the silk which holds it together? The following is the operation: the little caterpillar first fixes a series of silken cables from one side of the leaf to the other: she next pulls at these cables with her feet; and when she has forced the sides to approach, she fastens them together with stronger threads of silk. If the insect finds that one of the larger nerves of the leaf is so strong as to resist her efforts, she weakens it by gnawing it here and there half through. What engineer could act more sagaciously? To form one of the conical or horn-shaped rolls, which are not composed of a whole leaf, but of a long triangular portion cut out of the edge, some other manœuvres are requisite. Placing herself upon the leaf, the caterpillar cuts out with her jaws the piece which is to compose her roll. She does not, however, entirely detach it; it would then want a base. She detaches that part only which is to form the contour of the horn. This portion is a triangular strap, which she rolls as she cuts. When the body of the horn is finished, as it is intended to be fixed upon the leaf in nearly an upright position, it is necessary to elevate it. To effect this, she proceeds as we should with an inclined obelisk. She attaches threads, or little cables, towards the point of the pyramid, and raises it by the weight of her body.”
Other larvæ form their habitations wholly of silk: one that inhabits the leaves of pear-trees forms a little tent; another a sort of cloak; another, as in the case of the silkworm, a complete ball of silk.
PENDULOUS NESTS OF CATERPILLARS.
Among leaf-rolling caterpillars, one of the most curious is described by Bonnet, in which the nest hangs suspended from the branch of a fruit-tree by a strong silken thread. It is formed of one or two leaves neatly folded and fastened together with silk, and in this small enclosure several caterpillars live harmoniously together.
LEAF CUTTING CATERPILLAR.