Insect Architecture

by James Rennie

The grubs of several of the numerous species of may-fly (Ephemera) excavate burrows for themselves in soft earth, on the banks of rivers and canals, under the level of the water, an operation well described by Scopoli, Swammerdam, and Réaumur. The excavations are always proportioned to the size of the inhabitant; and consequently, when it is young and small, the hole is proportionally small, though, with respect to extent, it is always at least double the length of its body. The hole, being under the level of the river, is always filled with water, so that the grub swims in its native element, and while it is secure from being preyed upon by fishes, it has its own food within easy reach. It feeds, in fact, if we may judge from its egesta, upon the slime or moistened clay with which its hole is lined.

In the bank of the stream at Lee, in Kent, we had occasion to take up an old willow stump, which, previous to its being driven into the bank, had been perforated in numerous places by the caterpillar of the goat-moth (Cossus ligniperda). From having been driven amongst the moist clay, these perforations became filled with it, and the grubs of the ephemeræ found them very suitable for their habitation: for the wood supplied a more secure protection than if their galleries had been excavated in the clay. In these holes of the wood we found several empty, and some in which were full-grown grubs. (J. R.)

The architecture of the grub of a pretty genus of beetles, known to entomologists by the name of Cincindela, is peculiarly interesting. It was first made known by the eminent French naturalists, Geoffroy, Desmarest, and Latreille. This grub, which may be met with during spring, and also in summer and autumn, in sandy places, is long, cylindric, soft whitish, and furnished with six scaly brown feet. The head is of a square form, with six or eight eyes, and very large in proportion to the body. They have strong jaws, and on the eighth joint of the body there are two fleshy tubercles, thickly clothed with reddish hairs, and armed with a recurved horny spine, the whole giving to the grub the form of the letter Z.

Nest of the Grubs of Ephemeræ.
A. The Grub. B. Perforations in a river bank.
C. One laid open to show the parallel structure.

Nests of Ephemeræ in holes of Cossus.

With their jaws and feet they dig into the earth to the depth of eighteen inches, forming a cylindrical cavity of greater diameter than their body, and furnished with a perpendicular entrance. In constructing this, the grub first clears away the particles of earth and sand by placing them on its broad trapezoidal head, and carrying the load in this manner beyond the area of the excavation. When it gets deeper down, it climbs gradually up to the surface with similar loads by means of the tubercles on its back, above described. This process is a work of considerable time and difficulty, and in carrying its loads the insect has often to rest by the way to recover strength for a renewed exertion. Not unfrequently, it finds the soil so ill adapted to its operations, that it abandons the task altogether, and begins anew in another situation. When it has succeeded in forming a complete den, it fixes itself at the entrance by the hooks of its tubercles, which are admirably adapted for the purpose, forming a fulcrum or support, while the broad plate on the top of the head exactly fits the aperture of the excavation, and is on a level with the soil. In this position the grub remains immovable, with jaws expanded, and ready to seize and devour every insect which may wander within its reach, particularly the smaller beetles; and its voracity is so great, that it does not spare even its own species. It precipitates its prey into the excavation, and in case of danger it retires to the bottom of its den, a circumstance which renders it not a little difficult to discover the grub. The method adopted by the French naturalists was to introduce a straw or pliant twig into the hole, while they dug away, by degrees and with great care, the earth around it, and usually found the grub at the bottom of the cell, resting in a zig-zag position like one of the caterpillars of the geometric moths.

When it is about to undergo its transformation into a pupa, it carefully closes the mouth of the den, and retires to the bottom in security.

It does not appear that the grub of the genus Cincindela uses the excavation just described for the purpose of a trap or pitfall, any further than that it can more effectually secure its prey by tumbling them down into it; but there are other species of grubs which construct pitfalls for the express purpose of traps. Among these is the larva of a fly (Rhagio vermileo), not unlike the common flesh maggot. The den which it constructs is in the form of a funnel, the sides of which are composed of sand or loose earth. It forms this pitfall of considerable depth, by throwing out the earth obliquely on all sides; and when its trap is finished, it stretches itself along the bottom, remaining stiff and motionless, like a piece of wood. The last segment of the body is bent at an angle with the rest, so as to form a strong point of support in the struggles which it must often have to encounter with vigorous prey. The instant that an insect tumbles into the pitfall, the grub pounces upon it, writhes itself round it like a serpent, transfixes it with its jaws, and sucks its juices at its ease. Should the prey by any chance escape, the grub hurls up jets of sand and earth, with astonishing rapidity and force, and not unfrequently succeeds in again precipitating it to the bottom of its trap.