Insect Architecture

by James Rennie

Mason-Wasp’s Nest and Cocoons.—About one-third the natural size.

Mason-Wasp (Odynerus murarius).—Natural size.

Another mason-wasp (Odynerus murarius, Latr.), differing little in appearance from the former, may often be seen frequenting sandy banks exposed to the sun, and constructing its singular burrows. The sort of sand-bank which it selects is hard and compact; and though this may be more difficult to penetrate, the walls are not liable to fall down upon the little miner. In such a bank, the mason-wasp bores a tubular gallery two or three inches deep. The sand upon which Réaumur found some of these wasps at work was almost as hard as stone, and yielded with difficulty to his nail; but the wasps dug into it with ease, having recourse, as he ascertained, to the ingenious device of moistening it by letting fall two or three drops of fluid from their mouth, which rendered the mass ductile, and the separation of the grains easy to the double pickaxe of the little pioneers.

Nests, &c., of Mason-Wasps.—About half the natural size.
a, The tower of the nest; b, the entrance after the tower is removed; c, the cell; d, the cell, with a roll of caterpillars prepared for the larva.

When this wasp has detached a few grains of the moistened sand, it kneads them together into a pellet about the size of one of the seeds of a gooseberry. With the first pellet which it detaches, it lays the foundation of a round tower, as an outwork, immediately over the mouth of its nest. Every pellet which it afterwards carries off from the interior is added to the wall of this outer round tower, which advances in height as the hole in the sand increases in depth. Every two or three minutes, however, during these operations, it takes a short excursion, for the purpose, probably, of replenishing its store of fluid wherewith to moisten the sand. Yet so little time is lost, that Réaumur has seen a mason-wasp dig in an hour a hole the length of its body, and at the same time build as much of its round tower. For the greater part of its height this round tower is perpendicular; but towards the summit it bends into a curve, corresponding to the bend of the insect’s body, which in all cases of insect architecture, is the model followed. The pellets which form the walls of the tower are not very nicely joined, and numerous vacuities are left between them, giving it the appearance of filigree-work. That it should be thus slightly built is not surprising, for it is intended as a temporary structure for protecting the insect while it is excavating its hole, and as a pile of materials, well arranged and ready at hand, for the completion of the interior building,—in the same way that workmen make a regular pile of bricks near the spot where they are going to build. This seems, in fact, to be the main design of the tower, which is taken down as expeditiously as it had been reared. Réaumur thinks that, by piling in the sand which has previously been dug out, the wasp intends to guard her progeny for a time from being exposed to the too violent heat of the sun; and he has even sometimes seen that there were not sufficient materials in the tower, in which case the wasp had recourse to the rubbish she had thrown out after the tower was completed. By raising a tower of the materials which she excavates, the wasp produces the same shelter from external heat as a human creature would who chose to inhabit a deep cellar of a high house. She further protects her progeny from the ichneumon-fly, as the engineer constructs an outwork to render more difficult the approach of an enemy to the citadel. Réaumur has seen this indefatigable enemy of the wasp peep into the mouth of the tower, and then retreat, apparently frightened at the depth of the cell which he was anxious to invade.

The mason-wasp does not furnish the cell she has thus constructed with pollen and honey, like the solitary bees, but with living caterpillars, and these always of the same species—being of a green colour, and without feet. She fixes the caterpillars together in a spiral column: they cannot alter their position, although they remain alive. They are an easy prey to their smaller enemy; and when the grub has eaten them all up, it spins a case, and is transformed into a pupa, which afterwards becomes a wasp. The number of caterpillars which is thus found in the lower cavity of the mason-wasp’s nest is ordinarily from ten to twelve. The mother is careful to lay in the exact quantity of provision which is necessary to the growth of the grub before he quits his retreat. He works through his store till his increase in this state is perfected, and he is on the point of undergoing a change into another state, in which he requires no food. The careful purveyor, cruel indeed in her choice of a supply, but not the less directed by an unerring instinct, selects such caterpillars as she is conscious have completed their growth, and will remain thus imprisoned without increase or corruption till their destroyer has gradually satisfied the necessities of his being. “All that the worm of the wasp,” says Réaumur, “has to do in his nest, from his birth to his transformation, is to eat.” There is another species of wasp which does not at once enclose in its nest all the sustenance which its larva will require before transformation, but which from time to time imprisons a living caterpillar, and when that is consumed, opens the nest and introduces another.

[The upper figure in the accompanying illustration exhibits two of the curious towers built by this interesting insect and drawn of their natural size.]

The insect is one of the most plentiful in England, and can be found on sunny days, flitting about sand-banks and making its curious habitations. The length is nearly half an inch, and the colour is black, variegated with five yellow bands upon the abdomen.