Homes Without Hands: Being a Description of the Habitations of Animals, Classed According to Their Principle of Construction
"The first that may be mentioned is one of the most beautiful examples of insect workmanship I ever saw. It is a cocoon, about the size of a sparrow's egg, woven by a caterpillar in broad meshes, of either buff or rose-colored silk, and is frequently seen in the narrow alleys of the forest, suspended from the extreme tip of an outstanding leaf by a strong silken thread, five or six inches in length. It forms a very conspicuous object, hanging thus in mid-air. The glossy threads with which it is knitted are stout, and the structure is therefore not liable to be torn by the beaks of birds, while its pendulous position makes it doubly secure against the attacks of insectivorous birds, while its pendulous position makes it doubly secure against their attacks, the apparatus giving way when they peck at it. There is a small orifice at each end of the egg-shaped bag, to admit of the escape of the moth when it changes from the little chrysalis which sleeps tranquilly in its airy cage. The moth is of a dull slaty color, and belongs to the Lithosiidæ group of the silkworm family (Bombycidæ).
“When the caterpillar begins its work, it lets itself down from the tip of the leaf which it has chosen by spinning a thread of silk, the thickness of which it slowly increases as it descends. Having given the proper length to the cord, it proceeds to weave its elegant bag, placing itself in the centre, and spinning rings of silk at regular intervals, connecting them at the same time by means of the loose thread; so that the whole, when finished, forms a loose web, with quadrangular meshes of nearly equal size throughout. The task occupies about four days: when finished, the inclosed caterpillar becomes sluggish, its skin shrivels and cracks, and there then remains a motionless chrysalis of narrow shape, leaning against the sides of its silken cage.”
Some other lepidopterous insects suspend themselves by single threads, but most of them make their habitations of leaves, so that, when suspended, they do not attract much attention, looking like chance leaves that have fallen from the branches and caught in a stray piece of spider’s web. Sometimes these nests are made from single leaves, the edges of which are drawn together by the silken threads spun by the caterpillar that takes refuge within, and sometimes they are made from several leaves, which are fastened to each other by similar threads. Some of these pensile nests are inhabited by a number of caterpillars, which live together in perfect harmony. Some nests are not uncommon in tropical countries, and one or two of them will be described in the chapter on Social Insects. One traveler describes some of these nests by comparing them to the white paper bags in which grapes are tied, when ripe, in order to preserve them from wasps and other marauders.
He also mentions that the interior contained a quantity of green leaves, which afforded food to the inhabitants, but does not tell us whether the leaves were actually growing on the tree and surrounded by the nest, or whether they had been cut from the boughs outside, and carried into the interior by the inhabitants. The latter supposition is implied, but it can hardly be a correct one, as it is directly contrary to our present knowledge of the habits of caterpillars. I believe that no lepidopterous larva is known to fetch food from a distance, and to store it for future consumption. As far as we know at present, the caterpillar has not the least thought for the morrow, but simply devours the leaves where they grow.
There are many species, such as the larva of the common BROWN-TAIL MOTH (Porthesia auriflua), or of the SMALL ERMINE MOTH (Yponomeuta padella), which travel by day to considerable distances in their search after food, and return at night to their common habitation, guided by the threads which they continually spin as they crawl along. But no caterpillar is known which is gifted with the instinct of cutting off leaves and bringing them home for food, and we may therefore infer that the leaves in question were growing on the branches, and that the nests had been purposely spun round them.
THERE are, however, one or two species of British insects, belonging to the lepidoptera, which do cut off leaves and use them for the construction of the cocoon, though they do not employ them for food. These insects are moths, belonging to the genus Acronycta, and popularly called SPURGE MOTHS, on account of the plant on which they reside. One of these species makes a really curious pensile cocoon from the leaves of the cypress spurge (Euphorbia cyparissias), rather a scarce perennial plant about a foot in height, growing about woods and the borders of the fields. The leaves of the stem are lance-shaped, and those of the branches almost linear, like grass blades, and it is of these latter that the insect makes its habitation.
About October, the caterpillar begins to make its house, and does so in a very curious manner. Detaching a leaf from a branch, it fastens one end to the stem, and then bends the leaf so as to form a loop, and fastens the other end in a similar manner. A number of the leaves are placed nearly parallel to each other, so that when they are firmly woven together they form a bag-like cocoon, fixed to the stem of the plant by one side, and being upright like that of the burnet moth. Its texture is, however, very unlike that of the burnet, being loose, almost wholly composed of vegetable matter, and comparatively flimsy.
It has well been remarked that the strength, or, at all events, the weather-resisting power of a cocoon, depends upon the length of time which is occupied by the insect in undergoing its transformation, those creatures which only spend a few weeks in the pupal state being content with a mere web or hammock of silk, while those which pass the winter in the pupal condition make habitations which are comparatively substantial.
This rule, however, is not without its exceptions, as we find the pupæ of several butterflies, the common cabbage butterfly, for example, merely hung against walls, etc., without any protection around them. Instinct leads them to choose such spots as can best afford them shelter, as every one knows who has a tool-house or a summer-house in the garden, but there are many cases in which no such protection can be found, and the insects are forced to content themselves with the southern side of a tree-trunk, or the least windy side of a paling.
The caterpillar of the Spurge Moth is rather prettily marked, being striped longitudinally with white, red, and brown, relieved with black, and furnished with some scanty tufts of hair on each segment.
One species of insect suspends the cocoon by a thread at each end, so that the resemblance to a hammock is exact. This is the Acrolepia autumnella, one of the minute moths called Micro-Lepidoptera. The larva of this species is naked. It is a native of England.
WE now pass to the enormous variety of caterpillars which are popularly called Leaf-rollers, because they make their homes in leaves which they curl up in various methods.
Some use a single leaf, and others employ two or more in the construction of their nests. Even the single-leaf insects display a wonderful variety in their modes of performing an apparently simple task. Some bend the leaf longitudinally, and merely fasten the two edges together, while others bend it transversely, fixing the point to the middle nervure. Some roll it longitudinally, so as to make a hollow cylinder corresponding with the entire length of the leaf, while others roll it transversely, so that the cylinder is only as long as the leaf is wide, and a few species cut a slit in the leaf and roll up only a small portion of it.
The leaf-roller caterpillars belong to numerous species, and are plentiful enough—too plentiful, indeed, to please the gardener, who finds the leaves of his favorite trees curled up and permanently disfigured by these little marauders. All of them are of small size, and some so minute that the mere fact of their ability to roll up a leaf is something wonderful.
They flourish best during mild and rather rainy seasons, because the leaves are charged with moisture, and are so soft that they can easily be rolled, and, moreover, contain a plentiful supply of food. During the present year the Leaf-rollers have suffered greatly, the continual drought having dried up the leaves, rendering them both stiff and innutritious. The lilacs in my garden, which are usually covered with these cylindrical nests of Leaf-roller caterpillars, are comparatively free from them, and the few which exist are very poor specimens, several having been abandoned in a half-made state. In the lilacs of a friend, however, where the soil is about one hundred and twenty feet lower than my own garden, there are plenty of Leaf-roller nests, the ground being much moister than in more elevated situations, and being, moreover, on a different soil.
The mechanics of the Leaf-roller nest are very curious, and will be presently mentioned.
ONE of the most common among the Leaf-rollers is the pretty OAK MOTH (Tortrix viridana), which must not be confounded with the oak-egger moth already mentioned. It is a little creature with four rather wide delicate wings, the upper pair of a soft leaf-green, and the under pair of a grayish hue. In some seasons, the moths, or rather their larvæ, are so plentiful that great damage is done to the oak forests, tree after tree being so covered with them that scarcely a leaf escapes destruction, and the growth of the tree is consequently checked.
Like all Leaf-rollers, they feed on the green substance, or parenchyma of the leaf, and, being ensconced within their tubular home, can eat without fear of molestation. They are not very much afraid even of the small birds, for as soon as a bill is pushed into one end of the leafy cylinder, the caterpillar hastily “bundles” out of the other—there is no other word which so fully expresses the peculiar action of the larva—and lowers itself toward the ground by a silken thread which proceeds from its mouth. In fact, it acts like a spider in similar circumstances.
Where these insects are plentiful, an absurd effect can be produced by tapping the branches of oak trees with a stick. As the stroke reverberates through the branch, the leaves, which appear to the casual passenger to be in their ordinary condition, give forth their inhabitants, and hundreds of tiny caterpillars descend in hot haste, each lowering itself by a thread and dropping in little jerks of an inch or two each. Some of them are more timid than the others, and descend nearly to the ground; but the general mass of them remains at about the same height. Another tap will cause them all to drop a foot or two lower, the stroke being felt even at the end of the suspending thread; and by administering a succession of such taps they will all be induced to come to the ground. There they will wait a considerable time; but presently one of them will begin to reascend, working its way upward along the slender and scarcely visible line as easily as if it were crawling upon level ground. The least alarm will cause them to drop again, for they are then very timid; but, if allowed to remain in peace, they speedily reach their cells, and enter them with a haste that very much resembles the quick jerk with which a soldier-crab enters the shell from which he has been ejected.