Insect Architecture

by James Rennie

One of the examples of this occurs in the ghost-moth (Hepialus humuli), which, before it retires into the earth, feeds upon the roots of the hop or the burdock. Like other insects which construct cells under ground, it lines the cemented earthen walls of its cell with a smooth tapestry of silk, as closely woven as the web of the house-spider.

Inaccurate observers have inferred that these earthen structures were formed by a very rude and unskilful process—the caterpillar, according to them, doing nothing more than roll itself round, while the mould adhered to the gluey perspiration with which they describe its body to be covered. This is a process as far from the truth as Aristotle’s account of the spider spinning its web from wool taken from its body. Did the caterpillar do nothing more than roll itself in the earth, the cavity would be a long tube fitted exactly to its body (Fig. c): it is essentially different.

Nests, &c., of an Earth-mason Caterpillar.

It does not indeed require very minute observation to perceive that every grain of earth in the structure is united to the contiguous grains by threads of silk; and that consequently, instead of the whole having been done at once, it must have required very considerable time and labour. This construction is rendered more obvious by throwing one of these earthen cases into water, which dissolves the earth, but does not act on the silk which binds it together. To understand how this is performed, it may not be uninteresting to follow the little mason from the beginning of his task.

When one of those burrowing caterpillars has done feeding, it enters the earth to the depth of several inches, till it finds mould fit for its purpose. Having nowhere to throw the earth which it may dig out, the only means in its power of forming a cavity is to press it with its body; and, by turning round and round for this purpose, an oblong hollow is soon made. But were it left in this state, as Réaumur well remarks, though the vault might endure the requisite time by the viscosity of the earth alone, were no change to take place in its humidity, yet, as a great number are wanted to hold out for six, eight, and ten months, they require to be substantially built; a mere lining of silk, therefore, would not be sufficient, and it becomes necessary to have the walls bound with silk to some thickness.

When a caterpillar cannot find earth sufficiently moist to bear kneading into the requisite consistence, it has the means of moistening it with a fluid which it ejects for the purpose; and as soon as it has thus prepared a small pellet of earth, it fits it into the wall of the vault, and secures it with silk. As the little mason, however, always works on the inside of the building, it does not, at first view, appear in what manner it can procure materials for making one or two additional walls on the inside of the one first built. As the process takes place under ground, it is not easy to discover the particulars, for the caterpillars will not work in glazed boxes. The difficulty was completely overcome by M. Réaumur, in the instance of the caterpillar of the water-betony moth (Cucullia scrophulariæ, Schrank), which he permitted to construct the greater part of its underground building, and then dug it up and broke a portion off from the end, leaving about a third part of the whole to be rebuilt. Those who are unacquainted with the instinct of insects might have supposed that, being disturbed by the demolition of its walls, it would have left off work; but the stimulus of providing for the great change is so powerful, that scarcely any disturbance will interrupt a caterpillar in this species of labour.

Earth-mason Caterpillars’ Nests, with the perfect Moth, &c.

The little builder accordingly was not long in recommencing its task for the purpose of repairing the disorder, which it accomplished in about four hours. At first it protruded its body almost entirely beyond the breach which had been made, to reconnoitre the exterior for building materials. Earth was put within its reach, of the same kind as it had previously used, and it was not long in selecting a grain adapted to its purpose, which it fitted into the wall and secured with silk. It first enlarged the outside of the wall by the larger and coarser grains, and then selected finer for the interior. But before it closed the aperture, it collected a quantity of earth on the inside, wove a pretty thick net-work tapestry of silk over the part which remained open, and into the meshes of this, by pushing and pressing, it thrust grains of earth, securing them with silk till the whole was rendered opaque; and the further operations of the insect could no longer be watched, except that it was observed to keep in motion, finishing, no doubt, the silken tapestry of the interior of its little chamber. When it was completed, M. Réaumur ascertained that the portion of the structure which had been built under his eye was equally thick and compact with the other, which had been done under ground.