Homes Without Hands: Being a Description of the Habitations of Animals, Classed According to Their Principle of Construction

If a tolerably smart breeze be blowing, the sight is still more curious, for the caterpillars are swung about through very large arcs, and, if the wind be steady, are all blown in one direction, so that their line forms quite a large angle with the level of the leaf to which the upper end is attached. The caterpillars, however, seem to be quite indifferent in the matter, and ascend steadily, whether the line be simply perpendicular, or whether it be violently blown about by the wind.

At the proper season of year, the moths are as plentiful as the larvæ, and a shake with the hand will cause a whole cloud of the green creatures to issue forth, producing a strangely confused effect to the eye as they flutter about with an uncertain and devious flight. A sweep with an ordinary entomological net will capture plenty of them, but in a few minutes they all disappear, some of them returning to the branches whence they had come, and others dropping to the ground. During the summer of 1864 they were very plentiful in Darenth Wood, the heavy growth of oaks giving them every encouragement.

THE insect which commits such devastation on the lilacs is generally the little chocolate-colored moth called the LILAC MOTH (Lozotænia ribeana), though there are other allied species which infest the same plant. Any one may see the damaged leaves for himself, and therefore I shall not particularly describe them, but pass at once to the mechanical powers which are involved in the task of curling the elastic leaf into cylindrical form.

Compare the size of the lilac leaf and of the newly-hatched caterpillar, the latter being about as large as the capital letter I. That so minute a creature should roll up the leaf by main strength is of course an impossibility, and the method by which that consummation is attained is so remarkable an instance of practical mechanics that I must describe the operation at length.

If the reader will procure one of the rolled leaves, he will see that the cylindrical portion is retained in its place by a row of silken threads, which are individually weak, but collectively strong, holding the elastic leaf as firmly as Gulliver was held by the multitudinous cords with which he was fastened to the ground. That they should hold the cylinder in shape is to be expected, but the manner in which the cylinder is made is not so clear. The following is the process:

First, the caterpillar attaches a number of threads to the point and upper edges of the leaf, and fastens the other ends to the middle of the leaf itself. It now proceeds to perform an operation which is precisely similar to the nautical method of “bowsing” up a rope. In order to “bowse” a rope taut, two men are employed, one of them pulling the nearly tightened rope at right angles so as to bend it, while the other continually belays it to the cleats. Now the caterpillar performs precisely this operation, but without requiring the aid of an assistant, the “bowsing” being performed by its feet, and the belaying by its spinneret. By thus hauling at and tightening each line in succession, the caterpillar bends the leaf over slightly, and then attaches a fresh series of threads to keep it in its place. By repeating this process, and by continually adding fresh lines, the creature fairly bends the leaf into a hollow cylinder, and then crawls inside to enjoy its well-earned home.

I may here point out that the whole process of rolling the leaf affords an admirable example of mechanics as exhibited in nature, and that it is achieved by the well-known principle of exchanging space and time for power. Although the caterpillar can not by any exertion of strength roll up the leaf in one minute, it is enabled to do so by dividing the work into a multitude of parts, and taking much longer time about it, just as a man who can not lift a single weight of a thousand pounds may do so by dividing it into ten parts, and, in consequence, by taking up a considerable time in lifting the separate parts.

Again, in the silken bands which hold the rolled and elastic leaf in its place, we have an excellent example of accumulated power; neither of the threads being alone capable of enduring the tension, but their united strength being more than sufficient for the task.

As soon as the caterpillar has entered its new home, it begins to feed, eating the green substance of the leaf, and generally leaving the nervures untouched. Sometimes the caterpillar lives for so short a time that a single leaf is sufficient for its subsistence; but there are some species which are obliged to repeat the task more than once.

THERE are other insects which also make their habitations in leaves; but, instead of rolling up the leaf and living inside the cylinder, they make their way between the two membranes, and there remain until they have undergone their transformation.

The reader must often have seen the leaves of garden plants and trees, especially those of the rose, traversed by pale winding marks, that look something like the rivers upon a map, and having mostly a narrow dark line running exactly along the middle. These curious marks are the tracks which are made by the various leaf-mining insects, while eating their way through the leaf in which they pass their larval state. In most cases, when the insect has completed its term of larval existence, one end of the track is found to be greatly widened, and to contain either the pupa itself or its empty case.

The track differs considerably in shape, according to the insect which makes it. Sometimes it winds about in the middle of the leaf, crossing itself more than once in its progress. Sometimes it proceeds in a nearly straight line across the leaf, and very frequently, especially in deeply-cut leaves, it follows the outline, keeping to the edge, and not trenching at all on the central portions.

Insects belonging to three orders are known to make these curious habitations, namely, the Lepidoptera, the Coleoptera, and the Diptera. Of these, the Lepidoptera are by far the most numerous, and belong to that group which is called, on account of their very minute dimensions, the Micro-Lepidoptera. These are all little moths, so small that on the wing they can scarcely be recognized as moths, and look more like little flies. They are all very beautiful, and many of the species are truly magnificent when seen through a microscope, their plumage glittering as if made of burnished gold and silver. Indeed, one genus in which these leaf-miners are comprised is named Argyromiges, a title based on a Greek word signifying silver.

The species which is most common in the leaves of the rose-tree is the RED-HEADED PIGMY (Microsetia ruficapitella). The larva of this insect seems not to possess even the rudiments of legs, and forces itself through the leaves by means of certain projections of the skin, which are sharp and angular, and serve as instruments of progression, like the abdominal scales of the serpent and the bristles of the earth-worm. A species which is found in the leaf of the oak is known to collectors by the name of CRAMER’S PIGMY (Argyromiges Cramerella). The caterpillars of the DAGGER MOTH (Diurnea) also live between the membranes of leaves, and are remarkable for the last pair of feet, which are shaped like a couple of very minute battledores. These feet are spread out greatly in the act of walking, and the creature is farther aided in its progress by the hair-covered warts upon the body.

As for the beetle leaf-miners, they are to be found among the weevils; and it is a remarkable fact that one of these insects belongs to the genus Cionus, which has already been mentioned on page 297 as the weaver of certain beautiful pensile cocoons.

Of the Diptera, the CELERY FLY (Zephritis onopordinis) is a good example. The larva of this really pretty fly, with its green eyes and black and white spotted wings, feeds not only on the celery, but on the parsnip, and does great harm to both plants. Gardeners often employ little boys to examine the celery plants, and whenever they find a “blister,” as they technically call it, to crush the inclosed maggot between the fingers. The color of this larva is pale green, so that it is not readily seen even when the blister is opened. If allowed to have its own way, the larva remains in the bag until it has finished its eating, and then descends into the ground, where it changes into the pupal state, and remains until the following spring. In such a case the leaves are often much damaged, the blisters being yellowish-white, and the leaf itself drooping and half withered.

Our last examples of pensile nests are taken from the Arachnidæ, being formed by several species of spiders.

It may perhaps be necessary to remark that the threads with which the spiders make their webs are in some respects similar to those which are produced by various caterpillars, and in other respects are exceedingly dissimilar. In both cases the threads are formed from a semi-liquid secretion, which is produced in the internal organs, is forced through minute apertures at the will of the animal, and hardens into a thread as soon as it comes in contact with the air.

Here, however, the resemblance ceases. The threads of the caterpillar are double, or rather are composed of two lines fused together throughout their length, the two half lines proceeding from a large silk-secreting tube at either side of the body, and uniting at the mouth, where they become fused together by passing through a short tube common to both. The threads of the spider are much more complex, each being formed of a vast number of smaller lines, which are produced from a peculiar organ termed the “spinneret,” which is placed at the extremity of the body. In consequence of its position, the spider always hangs with its head downward while lowering itself by means of its line.

The spinnerets are externally like little rounded projections, arranged in pairs, and four, six, or eight in number. They are variable in shape, mostly being rounded, but sometimes being so long that they have been mistaken for feelers. The spinnerets are covered with a multitude of very minute hair-like appendages, which are, in fact, the tubes through which the liquid secretion is forced into the air. All the threads which proceed from these tubes are joined into a single line; and it will be at once seen that very great strength is obtained by making the line compound instead of single.

The best known of these creatures is the common GARDEN SPIDER (Epeira diadema), sometimes called the GEOMETRIC SPIDER, whose beautifully radiated net is so familiar that its general shape requires no description. Suffice it to say that the spider exhibits wonderful skill in placing its web, making a frame-work of very strong threads or ropes, and then spinning the net itself between them. Very great elasticity is thus obtained, for the threads are exceedingly elastic; so that, although stretched tolerably tightly, they will yield to pressure, and immediately recover themselves. This property is very needful, in order to enable them to resist the wind, to which they are so fully exposed.

These spiders have, moreover, a most singular plan of strengthening their web when the wind is more than ordinarily violent. If they find that the wind stretches their nets to a dangerous extent, they hang pieces of wood, or stone, or other substances to the web, so as to obtain the needful steadiness. I have seen a piece of wood which had been thus used by a Garden Spider, and which was some two inches in length, and thicker than an ordinary drawing-pencil. The spider hauled it to a height of nearly five feet; and when by some accident the suspending thread was broken, the little creature immediately lowered itself to the ground, attached a fresh thread, ascended again to the web, and hauled the piece of wood after it.