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

Many other trees besides the oak are chosen by certain species of gall-fly, and even the herbs and flowers do not escape the ravages of these remarkable insects. The white poppy, from which is obtained the opium of commerce, is attacked by a species of gall-fly, which lays its eggs in the large head, or pod, and sometimes does much damage to the plant, the delicate divisions between the seed-vessels being rendered quite hard and solid, and the pod itself deformed. Mr. Westwood has described a species of gall-fly which infests the turnips, and another species is known to lay its eggs upon wheat.

As if to show that the family of Cynipidæ is really related to the ichneumons, it has been discovered that some species of this family are actually parasitic upon other insects. In treating of this remarkable fact, Mr. Westwood writes as follows: “The relations of these insects with the following families (i. e., Evanidæ and Ichneumonidæ) have been already noticed. It had always appeared to me contrary to nature that a tribe of vegetable-feeding insects should be arranged in the midst of parasites; nor was it until I had an opportunity of ascertaining the parasitic habits of some of the species of the family that I was enabled to form a just notion as to the true value of the parasitic or herbivorous nature of these insects. In June, 1838, I detected a minute species, Allotria victrix, in the act of ovipositing in the body of a rose aphis, and I subsequently succeeded in hatching specimens of the perfect insect from infested aphides.”

A figure of the tiny insect is given, as it appeared while in the act of depositing its eggs, and has a rather remarkable effect from the fact that the very minute dimensions of the parasite make the aphis look quite a large insect. Other species of this family are also known to be parasites. The rose aphis is certainly infested by two species of gall-fly, and probably more, while the aphides which are found on the willow, the cow-parsnip, and other plants, also fall victims to the Cynipidæ. There is one genus of this family, called Figites, which is parasitic on the larva or pupa of certain dipterous insects.

THE Cynipidæ are not the only insects that produce galls upon different plants. For example, several species of beetle are known to pass their earlier stages in swellings produced by the puncture of the parent insect. There is a little weevil of a grayish-brown, which is mentioned by Mr. Rennie as forming a gall upon the hawthorn.

“In May, 1829, we found on a hawthorn at Lee, in Kent, the leaves at the extremity of a branch neatly folded up in a bundle, but not quite so closely as is usual in the case of leaf-rolling caterpillars. On opening them up, there was no caterpillar to be seen, the centre being occupied with a roundish, brown-colored woody substance, similar to some excrescences made by gall insects (Cynips).

“Had we been aware of its real nature, we should have put it immediately under a glass, or in a box, till the contained insect had developed itself; but instead of this, we opened the ball, where we found a small yellow grub coiled up, and feeding on the exuding juices of the tree. As we could not replace the grub in its cell, part of the wall of which we had unfortunately broken, we put it in a small pasteboard box with a fresh shoot of hawthorn, expecting that it might construct a fresh cell. This, however, it was probably incompetent to perform; it did not, at least, make the attempt, and neither did it seem to feed on the fresh branch, keeping in preference to the ruins of its former cell.

“To our great surprise, although it was thus exposed to the air, and deprived of a considerable portion of its nourishment, both from the fact of the cell having been broken off, and from the juices of the branch having been dried up, the insect went through its regular changes, and appeared in the form of a small grayish-brown beetle of the weevil family.

“The most remarkable circumstance in the case in question was the apparent inability of the grub to construct a fresh cell after the first was injured, proving, we think, beyond a doubt, that it is the puncture made by the parent insect when the egg is deposited that causes the exudation and subsequent concretion of the juices forming the gall.” Although the insect in question succeeded in attaining the perfect state, it would probably be of stunted growth in consequence of the deprivation of food. Such, at all events, is the case with insects of other orders, when their supply of food is at all checked while they are in the larval state.

THERE is another weevil, scientifically called Cleonus sulcirostris, which is one of the gall-makers. It is one of the largest of the British weevils, being more than half an inch in length, and is very simply clad in gray and black.

If the reader desires to discover the larva of the beetle he may probably be successful by going to any waste spot where thistles are allowed to grow, and examining them carefully about the stems and roots. Nothing is more common than to find the stems of thistles swollen in parts, and in many cases the root is affected as well as the stem. Fortunately for the gardener, who hates thistles, even though he should be a Scotchman, as is so often the case with skilled gardeners, the larva of the Cleonus feeds on the juices of the plant at the expense of its life, so that the thistle dies just before the seed is developed, and a farther extension of the plant is thereby prevented.

THERE are also gall-making insects among the Diptera. Such, for example, is the THISTLE-GALL FLY (Urophora Cardui), which produces large and hard woody galls upon the thistle, as well as several species of the larger genus Tephritis, some species of which live in the parts of fructification of several flowers, the common dandelion being infested by them.

WE may now describe, at fuller length than has hitherto been done, another group of insects, which live between the membranes of leaves, and which belong to different orders.

Leaf Miners and Rollers.

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If the reader will carefully examine the leaves of any rose-tree which grows in the open air, he will certainly remark that many of them are notable for certain curious markings, which look something like the rivers in a map, and which traverse the leaf in various ways. They all, however, agree in one point, namely, their gradual and regular increase in diameter. At their origin they are so small that the finest thread could hardly pass through them, but in proportion as they increase in length they increase in width, so that at their termination they are sometimes the twelfth of an inch in width.

These marks are the tracks made by very small larvæ, which live between the membranes of the leaves, and feed upon the parenchyma, or soft substance which lies between the two membranes. They follow no rule in their meanderings, but traverse the leaf in a variety of ways. Sometimes they never leave the edge, but follow every little serration of the leaf with perfect accuracy. Sometimes they form a kind of spiral, and sometimes they wander irregularly over the whole leaf. Generally, the insect does not cross the track which it has once made, being diverted from doing so by some wonderful instinct. There are instances, however, where the insect has crossed its own track, not only once, but several times.

If the little gallery be opened at the widest extremity, one of three things will be found. Sometimes there is a tiny white grub, very much resembling the larva of certain beetles, and having the rings which represent the thorax rather wider than those which will afterward be developed into the abdomen. As the little creature is able to live between the membranes of a leaf so thin as that of the rose or oak, it is evident, to the most superficial observer, that the insect which will be developed from it must be of very minute dimensions.

The larva of all winged insects is very large in proportion to the same insects when they have obtained their perfect form, much of the substance being taken up by the wings. As a natural consequence, it follows that the larger the wings, the larger must be the grub, the size of the body being quite a secondary consideration. In the present case, the larvæ which we are supposed to examine belong to the lepidopterous order, in all of which insects the wings, when present at all, are of great comparative size. If, then, the full-grown larva is so small that it can lie concealed between the membranes of a leaf without causing any conspicuous alteration in its outline, it is evident that the perfect insect must be of almost microscopical minuteness. Accordingly, it has been found that the little moths which have been bred from such caterpillars are so small that they have almost escaped observation until comparatively late years.

How small these insects are may be imagined from the fact that many species of the Micro-lepidoptera, as they are fitly named, do not occupy, even with their wings spread, a space larger than is taken up by the capital letter at the beginning of this sentence. To “set” these tiny creatures is necessarily an extremely difficult task, and can not be accomplished by the ordinary plan of running a pin through the thorax, and extending the wings on the “setting-board.” The only method of displaying them is to set them on white cardboard by means of gum, which is strengthened by many entomologists with various substances. A sheet of cardboard covered with specimens of Micro-lepidoptera neatly set is a very pretty sight, but needs the aid of the microscope before it can be perfectly seen.

Even to the unaided eye, the tiny moths are seen to be beautifully decorated, their wings gleaming in favorable lights like the throat of the humming-bird. But when placed under the microscope, especially if it be furnished with a binocular tube, and illuminated by a suitable light, the wings are positively dazzling in their brightness, and hues that formerly seemed to be but dun and bronze or brown, suddenly flash out into gold and emerald, each scale distinct and shining as if of burnished metal.

Sometimes, when opening the extremity of the leafy tunnel, we find a tiny chrysalis lying in the little chamber, and awaiting the time for the shell to burst and the perfect insect to emerge. Later in the year we shall find neither larva nor pupa, but shall see a little hole in the leafy chamber, from which issues the shattered end of an empty chrysalis shell, showing that the moth has made its escape into the outer air.

Two examples of other mined leaves may be seen upon the illustration, both drawn from the actual object. The specimen in the right-hand upper corner was taken from the bramble, and has been mined by the larva of a little moth called Nepticula anomella. It is a very pretty little creature, though its hues are not brilliant without the aid of the microscope. The upper wings are brown, but their tips are beautifully colored with bright chestnut. The lower wings are pale gray, without any of the brilliancy that distinguishes the upper pair. They possess, however, a compensating beauty in the long, feathery fringe with which they are edged, and which, when subjected to the microscope, is seen to consist of the ordinary scales of the wings exceedingly developed both in length and width.