Chapter IX Insects Forming Habitations of Detached Leaves
The habitations of the insects which we have just described consist of growing leaves, bent, rolled, or pressed together, and fixed in their positions by silken threads. But there are other habitations of a similar kind, which are constructed by cutting out and detaching a whole leaf, or a portion of a leaf. We have already seen how dexterously the upholsterer-bees cut out small parts of leaves and petals with their mandibles, and fit them into their cells. Some of the caterpillars do not exhibit quite so much neatness and elegance as the leaf-cutting bees, though their structures answer all the purposes intended; but there are others, as we shall presently see, that far excel the bees, at least in the delicate minutiæ of their workmanship. We shall first advert to those structures which are the most simple.
Not far from Longchamps, in a road through the Bois de Boulogne, is a large marsh, which M. Réaumur never observed to be in a dry state even during summer. This marsh is surrounded with very lofty oaks, and abounds with pondweed, the water-plant named by botanists potamogeton. The shining leaves of this plant, which are as large as those of the laurel or orange-tree, but thicker and more fleshy, are spread upon the surface of the water. Having pulled up several of these about the middle of June, M. Réaumur observed, beneath one of the first which he examined, an elevation of an oval shape, which was formed out of a leaf of the same plant. He carefully examined it, and discovered that threads of silk were attached to this elevation. Breaking the threads, he raised up one of the ends, and saw a cavity, in which a caterpillar (Hydrocampa potamogeta) was lodged. An indefatigable observer, such as M. Réaumur, would naturally follow up this discovery; and he has accordingly given us a memoir of the pondweed tent-maker, distinguished by his usual minute accuracy.
In order to make a new habitation, the caterpillar fastens itself on the under side of a leaf of the Potamogeton. With its mandibles it pierces some part of this leaf, and afterwards gradually gnaws a curved line, marking the form of the piece which it wishes to detach. When the caterpillar has cut off, as from a piece of cloth, a patch of leaf of the size and shape suited to its purpose, it is provided with half of the materials requisite for making a tent. It takes hold of this piece by its mandibles, and conveys it to the situation on the under side of its own or another leaf, whichever is found most appropriate. It is there disposed in such a manner that the under part of the patch—the side which was the under part of the entire leaf—is turned towards the under part of the new leaf, so that the inner walls of the cell or tent are always made by the under part of two portions of leaf. The leaves of the potamogeton are a little concave on the under side; and thus the caterpillar produces a hollow cell, though the rims are united.
The caterpillar secures the leaf in its position by threads of white silk. It then weaves in the cavity a cocoon, which is somewhat thin, but of very close tissue. There it shuts itself up, to appear again only in the form of the perfect insect, and is soon transformed into a chrysalis. In this cocoon of silk no point touches the water; whilst the tent of leaves, lined with silk, has been constructed underneath the water. This fact proves that the caterpillar has a particular art by which it repels the water from between the leaves.
When the caterpillar, which has thus conveyed and disposed a patch of leaf against another leaf, is not ready to be transformed into a chrysalis, it applies itself to make a tent or habitation which it may carry everywhere about with it. It begins by slightly fixing the piece against the perfect leaf, leaving intervals all round, between the piece and leaf, at which it may project its head. The piece which it has fixed serves as a model for cutting out a similar piece in the other leaf. The caterpillar puts them accurately together, except at one end of the oval, where an opening is left for the insect to project its head through. When the caterpillar is inclined to change its situation, it draws itself forward by means of its scaly limbs, riveted upon the leaf. The membraneous limbs, which are riveted against the inner sides of the tents, oblige it to follow the anterior part of the body, as it advances. The caterpillar, also, puts its head out of the tent every time it desires to eat.
There is found on the common chickweed (Stellaria media), towards the end of July, a middle-sized smooth green caterpillar, having three brown spots bordered with white on the back, and six legs and ten pro-legs, whose architecture is worthy of observation. When it is about to go into chrysalis, towards the beginning of August, it gnaws off, one by one, a number of the leaves and smaller twigs of the chickweed, and adjusts them into an oval cocoon, somewhat rough and unfinished externally, but smooth, uniform, and finely tapestried with white silk within. Here it undergoes its transformation securely, and, when the period of its pupa trance has expired in the following July, it makes its exit in the form of a yellowish moth, with several brown spots above, and a brown band on each of its four wings below. It is also furnished with a sort of tail.
On the cypress-spurge (Euphorbia cyparissias), a native woodland plant, but not of very common occurrence, may be found, towards the end of October, a caterpillar of a middle size, sparely tufted with hair, and striped with black, white, red, and brown. The leaves of the plant, which are in the form of short narrow blades of grass, are made choice of by the caterpillar to construct its cocoon, which it does with great neatness and regularity, the end of each leaf, after it has been detached from the plant, being fixed to the stem, and the other leaves placed parallel, as they are successively added. The other ends of all these are bent inwards, so as to form a uniformly rounded oblong figure, somewhat larger at one end than at the other.
A caterpillar which builds a very similar cocoon to the last-mentioned may be found upon a more common plant—the yellow snap-dragon or toad-flax (Antirrhinum linaria)—which is to be seen in almost every hedge. It is somewhat shaped like a leech, is of a middle size, and the prevailing colour pearl-grey, but striped with yellow and black. It spins up about the beginning of September, forming the outer coating of pieces of detached leaves of the plant, and sometimes of whole leaves placed longitudinally, the whole disposed with great symmetry and neatness. The moth appears in the following June.
It is worthy of remark, as one of the most striking instances of instinctive foresight, that the caterpillars which build structures of this substantial description are destined to lie much longer in their chrysalis trance than those which spin merely a flimsy web of silk. For the most part, indeed, the latter undergo their final transformation in a few weeks; while the former continue entranced the larger portion of a year, appearing in the perfect state the summer after their architectural labours have been completed. (J. R.) This is a remarkable example of the instinct which leads these little creatures to act as if under the dictates of prudence, and with a perfect knowledge of the time, be it long or short, which will elapse before the last change of the pupa takes place. That the caterpillar, while weaving its cocoon and preparing to assume the pupa state, exercises any reflective faculties, or is aware of what is about to occur relative to its own self, we cannot admit. It enters upon a work of which it has had no previous experience, and which is performed, as far as contingencies allow, in the same manner by every caterpillar of the same species. Its labours, its mode of carrying them on, and the very time in which they are to be commenced, are all pre-appointed; and an instinctive impulse urges and guides; and with this instinct its organic endowments are in precise harmony; nor does instinct ever impel to labours for which an animal is not provided. “The same wisdom,” says Bonnet, “which has constructed and arranged with so much art the various organs of animals, and has made them concur towards one determined end, has also provided that the different operations which are the natural results of the economy of the animal should concur towards the same end. The creature is directed towards his object by an invisible hand; he executes with precision, and by one effort, those works which we so much admire; he appears to act as if he reasoned, to return to his labour at the proper time, to change his scheme in case of need. But in all this he only obeys the secret influence which drives him on. He is but an instrument which cannot judge of each action, but is wound up by that adorable Intelligence, which has traced out for every insect its proper labours, as he has traced the orbit of each planet. When, therefore, I see an insect working at the construction of a nest, or a cocoon, I am impressed with respect, because it seems to me that I am at a spectacle where the Supreme Artist is hid behind the curtain.”BM
There is a small sort of caterpillar which may be found on old walls, feeding upon minute mosses and lichens, the proceedings of which are well worthy of attention. They are similar, in appearance and size, to the caterpillar of the small cabbage-butterfly (Pontia rapæ), and are smooth and bluish. The material which they use in building their cocoons is composed of the leaves and branchlets of green moss, which they cut into suitable pieces, detaching at the same time along with them a portion of the earth in which they grow. They arrange these upon the walls of their building, with the moss on the outside, and the earth on the inside, making a sort of vault of the tiny bits of green moss turf, dug from the surface of the wall. So neatly, also, are the several pieces joined, that the whole might well be supposed to be a patch of moss which had grown in form of an oval tuft, a little more elevated than the rest growing on the wall. When these caterpillars are shut up in a box with some moss, without earth, they construct with it cells in form of a hollow ball, very prettily plaited and interwoven.
In May last (1829), we found on the walls of Greenwich Park a great number of caterpillars, whose manners bore some resemblance to those of the grub described by M. Réaumur. (J. R.) They were of middle size, with a dull-orange stripe along the back; the head and sides of the body black, and the belly greenish. Their abodes were constructed with ingenuity and care. A caterpillar of this sort appears to choose either a part where the mortar contains a cavity, or it digs one suited to its design. Over the opening of the hollow in the mortar it builds an arched wall, so as to form a chamber considerably larger than is usual with other architect caterpillars. It selects grains of mortar, brick, or lichen, fixing them, by means of silk, firmly into the structure. As some of these vaulted walls were from an inch to an inch and a half long, and about a third of an inch wide and deep, it may be well imagined that it would require no little industry and labour to complete the work; yet it does not demand more than a few hours for the insect to raise it from the foundation. Like all other insect architects, this caterpillar uses its own body for a measuring-rule, and partly for a mould, or rather a block or centre to shape the walls by, curving itself round and round concentrically with the arch which it is building.
We afterwards found one of these caterpillars, which had dug a cell in one of the softest of the bricks, covering itself on the outside with an arched wall of brick-dust, cemented with silk. As this brick was of a bright-red colour, we were thereby able to ascertain that there was not a particle of lichen employed in the structure.
The neatness mentioned by Réaumur, as remarkable in his moss-building caterpillars, is equally observable in that which we have just described; for, on looking at the surface of the wall, it would be impossible for a person unacquainted with those structures to detect where they were placed, as they are usually, on the outside, level with the adjoining brick-work; and it is only when they are opened by the entomologist, that the little architect is perceived lying snug in his chamber. If a portion of the wall be thus broken down, the caterpillar immediately commences repairing the breach, by piecing in bits of mortar and fragments of lichen, till we can scarcely distinguish the new portion from the old.