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

The BROWN-TAILED MOTH is another of the arboreal insects, and spins a web very like that of the gold-tailed moth, which has already been described. In some seasons it is more numerous than in others, and occasionally seen in vast multitudes. This phenomenon is often observable among insects, as is well known to all practical entomologists, and in more than one instance the caterpillars of the Brown-tailed Moth have been so plentiful as to become a positive pest.

They are social larvæ, and, as they are hatched late in the autumn, they spin a joint web, in which they can be secure throughout the winter months. As the brood is mostly numerous, and as two or more broods may unite in forming a common dwelling, their habitation is extremely large, often enveloping several branches, together with their twigs and leaves. Like the nest of the gold-tailed moth, it is divided into chambers, and is externally irregular in form, depending entirely for its shape upon the locality in which it is constructed.

Even in this country it is sometimes plentiful enough to annoy the farmer, who does not like to see his hedgerows disfigured by the silken tents spun by these caterpillars; but in France it has occurred in such hosts as to entail a serious loss upon the agriculturist, whole rows of trees having been stripped of their leaves, and the denuded branches covered with the sheets of web in which lay the destroying armies.

On the accompanying illustration may be seen a number of curious nests, composed of long hexagonal cells set side by side. These are made by several species of a hymenopterous insect belonging to the genus Icaria, and may be advantageously compared with the central figure in the illustration on page 586.

These nests, or rather these series of cells, are made after a singular fashion. First, the insect attaches to the branch a foot-stalk composed of the same material as that with which the cells are formed. This foot-stalk, although slender, is very hard, solid, and tough, and can uphold a considerable weight, as is necessary from the manner of constructing the nest. She then makes a cell after the ordinary wasp-fashion, attaching it to the foot-stalk with its mouth downward, and at first making it comparatively short. When the cell has nearly attained its due length, a second is placed alongside the first, and a third is added in like manner, each being lengthened as required. As the cells at the base of the series are finished first, it is evident that they gradually diminish toward the end, those at the extremity being often not one quarter so long as those at the base.

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The material employed in making these cells is woody fibre, like that which is used by our common British wasps, and the color is rather dark yellowish-brown, so that, in spite of the curious method in which the nest-groups project from the branches, they are not seen so readily as might be imagined from their eccentric form.

In these, as in many other forms of cells made by hymenopterous insects, is to be found an enigma which as yet is unsolved, and for the mention of which I am indebted to Mr. F. Smith, of the British Museum: all the cells are of equal size.

Now this point, which would not particularly strike an ordinary observer, is of the greatest importance to those who have studied the economy of insects, and have bestowed much thought upon them. If we examine the nest of a hive-bee, and take any single comb, we shall find that the cells are extremely variable in size—the largest being those which are occupied by the future queens, the smallest those which are the nurseries of the worker bees, and the intermediate cells those in which the drones or males are hatched.

If we examine the nest of the common wasp or hornet, we still find the cells of various sizes, corresponding with the sexes and uses of the occupants; and if we look at that of any species of humble-bee, the same fact is clearly perceptible. But in the nests of the Icarias and similar insects no such variation is discoverable, and no distinction can be found between the male and female cells. The natural question therefore arises, whether all the members of each brood, or rather each cell group, are of the same sex; or whether one nest produces males and another females, just as one portion of the comb is given to males, another to females, and another to neuters, in the case of the hive-bee? No matter how large may be the nest of an Icaria, or how full of cells it may be, the cells are all so alike in shape and size that they must apparently be the cradles of insects belonging to the same sex.

This fact is curiously brought out in a remarkable series of cell groups which have been placed on a single leaf, some of which are shown in the lower part of the illustration. The leaf is rather long, and, being dry, is now curved by its own force. This leaf seems to have possessed some fascination for the Icarias, as upon the upper surface no less than fifteen nests have been established, none of great length, and all nearly or quite completed. In none are the cells perfectly straight, all having a slight curve downward on account of the delicate material of which they are made.

The insect which builds these curious cells is a commonplace-looking creature, of a soft grayish-brown color, with a moderately large head and a little rounded abdomen, not very unlike the Cynips Kollari, which has already been described. It is a native of India, and the nests which have been mentioned were sent from Bareilly.

THERE is also a nest made by an allied insect belonging to the genus Sphex, and coming, as far as is known, from the same locality. As in the preceding case, the habitation is placed in the hollow of a leaf, which has either curled in the process of withering, or has been bent by the insect and retained in its cylindrical form by silken threads. On inspecting the nest it is seen to consist of a mass of delicate threads that are not unlike those of the common silkworm, which cross and recross each other so as to fill up the hollow of the leaf, and to form a receptacle for the future young.

The insect is a large one, being a full inch in length, and of rather a formidable appearance. In color it is black, with the exception of a few narrow transverse streaks of bright yellow upon the fore part of the thorax, and the abdomen is placed at the end of a long foot-stalk.

In connection with this branch of the subject, I must call the attention of the reader to the curious cell group of Polistes aterrima, a figure of which is given on page 482.

At first sight it looks as if the winged architect had intended to make a cell group like that of the Icaria, and had been brought abruptly to a conclusion. A close inspection, however, shows that the structure is intentional, and not merely the result of accident. The cells are all placed with their mouths downward, and are set in a very peculiar manner.

In the Icaria nests the cells are arranged so as to form a more or less elongated mass, widest in the middle and tapering to the end, the variation in width being caused by the different arrangement of the cells, which are two deep at the commencement, reach three or four deep at the middle, and are reduced at the end to a single row of cells. But in the nest of the Polistes all the cells are two deep, being arranged in two regular rows.

On reference to the illustration, which is exactly one half the length of the original specimen, the reader will see that the cells are of different sizes, and might therefore fancy that the upper or larger cells are those which nurture the larvæ that are destined to be perfect males and females, and that the lower or smaller cells are intended for the workers or neuters. But a careful examination shows that such a supposition would not be correct. It is true that the upper cells are larger than the lower, but this increase of size is simply owing to the fact that they are completed, while the five lower cells are still unfinished.

The attention of the reader is particularly called to the dark bars which are at regular intervals across the cells. These are not merely intended to represent effects of light and shade, nor to show that the little architect has used materials of different colors so as to form alternate bands; but each dark line represents a distinct and well-marked ridge, and each ridge is evidently the result of an addition made to the cell.

Here, then, we have a clew to the manner in which the insect builds its curious home. The cells at the upper part of the nest were originally in the same unfinished state as those at the lower extremity. The insect made a tolerably deep cell, laid an egg therein, and then proceeded to lay the foundation of other cells, placing an egg in each.

When, however, she has formed the third or fourth cell, the eggs in the first and second chambers have been hatched into larvæ, and need to be fed; so the mother insect has now to suspend her architectural labors, and to divide her time between the erection of fresh cells and the feeding of the larvæ in those which were first made. Soon, however, her labors become more onerous and more complicated. The larvæ or grubs which inhabited the first two or three cells have grown so rapidly that they are fast becoming too large for their pendent cradles. Their cells must therefore be enlarged to suit the increased dimensions of the inhabitants. This also is done, each addition being marked by the ridge which has been already mentioned.

Thus, then, the hard-worked mother insect is forced to engage in three distinct labors, namely, the building of new cells, the enlargement of existing cells, and the nurture of the larvæ. A short reference to the illustration will now give the reader a clear idea of the cell group. Above is a series of completed cells, each occupied by a full-grown larva, one of which is being fed by the mother insect, and below is a series of incomplete cells, each of which has received an egg, but neither of which is fit to sustain the weight of the larvæ. If, however, the nest had been allowed to remain in the forest, instead of being carried off to the British Museum, the five lower cells would have been completed like their more perfect predecessors above.

The observant reader will probably have noticed that from the mouths of several cells a scoop-like projection is seen to issue. These projections have been faithfully rendered by the draughtsman, although they denote a certain imperfection in the specimen. They are evidently the result of hard usage, and show that part of a completed cell has been broken away. As is often the case, the fracture has its value, inasmuch as it shows that the normal form of the cell is hexagonal, and that the angles are quite sharp and firm, without needing either pressure or excavation to make them so.

It is much to be regretted that in England there is no representative of this interesting group of insects by which the above-mentioned problem might be solved. We might then know whether males and females belong to the same brood, are nurtured in the same cells, and are of the same size. We might learn whether or not the males are bred in separate establishments, divided from the other sex like diœcious plants. As it is, however, we are in ignorance respecting these points, and no one resident in tropical countries seems to have the energy to conduct a series of experiments which would involve expenditure of time and labor. Travelers and residents in tropical countries are often admirable collectors, but, with few exceptions, are poor observers, except of facts that pass immediately under their observation. They make valuable collections, and record many useful isolated facts; but, unfortunately, they seldom appear able to carry on a series of experiments that would occupy several successive years, and thus we lose much valuable as well as interesting knowledge, and waste much time in trying to discover by inference that which we ought to know from observation.