Insect Architecture

by James Rennie

Bumps or wurbles produced on cattle by the Ox breeze-fly.

We have not so many examples of galls of this kind as we have of vegetable galls; and when we described the surprising varieties of the latter, we did not perceive that it was essential to the insects inhabiting them to preserve a communication with the external air: in the galls of trees, openings expressly designed or kept free for the admission of air are never observed. Must the grub, then, which inhabits the latter have less need of respiring air than the grub of the breeze-flies in a flesh-gall? Without doubt, not; but the apertures by which the air is admitted to the inhabitants of the woody gall, although they may escape our notice, in consequence of their minuteness, are not, in fact, less real. We know that, however careful we may be in inserting a cork into a glass, the mercury with which it is filled is not sheltered from the action of the air, which weighs upon the cork; we know that the air passes through, and acts upon the mercury in the tube. The air can also, in the same way, penetrate through the obstruction of a gall of wood, though it have no perceptible opening or crack; but the air cannot pass in this manner so readily through the skins and membranes of animals.

In order to see the interior of the cavity of an animal gall, Réaumur opened several, either with a razor or a pair of scissors; the operation, however, cannot fail to be painful to the cow, and consequently renders it impatient under the process. The grub being confined in a tolerably large fistulous ulcer, a part of the cavity must necessarily be filled with pus or matter. The bump is a sort of cautery, which has been opened by the insect, as issues are made by caustic: the grub occupies this issue, and prevents it from closing. If the pus or matter which is in the cavity, and that which is daily added to it, had no means of escaping, each tumor would become a considerable abscess, in which the grub would perish; but the hole of the bump, which admits the entrance of the air, permits the pus or matter to escape; that pus frequently mats the hairs together which are above the small holes, and this drying around the holes acquires a consistency, and forms in the interior of the opening a kind of ring. This matter appears to be the only aliment allowed for the grub, for there is no appearance that it lives, like the grubs of flesh-flies, upon putrescent meat. Mandibles, indeed, similar to those with which other grubs break their food, are altogether wanting. A beast which has thirty, forty, or more of these bumps upon its back, would be in a condition of great pain and suffering, terrible indeed in the extreme, if its flesh were torn and devoured by as many large grubs; but there is every appearance that they do not at all afflict, or only afflict it with little pain. For this reason cattle most covered with bumps are not considered by the farmer as injured by the presence of the fly, which generally selects those in the best condition.

A fly, evidently of the same family with the preceding, is described in Bruce’s ‘Travels,’ under the name of zimb, as burrowing during its grub state in the hides of the elephant, the rhinoceros, the camel, and cattle. “It resembles,” he says, “the gad-fly in England, its motion being more sudden and rapid than that of a bee. There is something peculiar in the sound or buzzing of this insect; it is a jarring noise together with a humming, which as soon as it is heard all the cattle forsake their food and run wildly about the plain till they die, worn out with fatigue, fright, and hunger. I have found,” he adds, “some of these tubercles upon almost every elephant and rhinoceros that I have seen, and attribute them to this cause. When the camel is attacked by this fly, his body, head, and legs break out into large bosses, which swell, break, and putrefy, to the certain destruction of the creature.”[GU] That camels die under such symptoms, we do not doubt; but we should not, without more minutely-accurate observation, trace all this to the breeze-fly.

MM. Humboldt and Bonpland discovered, in South America, a species, probably of the same genus, which attacks man himself. The perfect insect is about the size of our common house-fly (Musca domestica), and the bump formed by the grub, which is usually on the belly, is similar to that caused by the ox breeze-fly. It requires six months to come to maturity; and if it is irritated it eats deeper into the flesh, sometimes causing fatal inflammations.

Grub Parasite in the Snail.

During the summer of 1829, we discovered in the hole of a garden-post, at Blackheath, one of the larger grey snail shells (Helix aspersa, Muller), with three white soft-bodied grubs burrowing in the body of the snail. They evidently, from their appearance, belonged to some species of beetle, and we carefully preserved them in order to watch their economy. It appeared to us that they had attacked the snail in its stronghold while it was laid up torpid for the winter; for more than half of the body was already devoured. They constructed for themselves little cells attached to the inside of the shell, and composed of a sort of fibrous matter, having no distant resemblance to shag tobacco, both in form and smell, and which could be nothing else than the remains of the snail’s body. Soon after we took them, appearing to have devoured all that remained of the poor snail, we furnished them with another, which they devoured in the same manner. They formed a cocoon of the same fibrous materials during the autumn, and in the end of October appeared in their perfect form, turning out to be Drilus flavescens, the grub of which was first discovered in France in 1824. The time of their appearance, it may be remarked, coincides with the period when snails become torpid. (J. R.)

In the following autumn, we found a shell of the same species with a small pupa-shaped egg deposited on the lid. From this a caterpillar was hatched, which subsequently devoured the snail, spun a cocoon within the shell, and was transformed into a small moth (of which we have not ascertained the species) in the spring of 1830.

[Before concluding the account of the parasite insects, it will be necessary to mention two of our British Ichneumonidæ, which not only deposit their eggs in the larvæ of other insects, but make for themselves cells of very beautiful structure. In the accompanying illustration are shown the cells of one of our commonest and most useful ichneumonidæ (Microgaster glomeratus), together with the insect itself. At Fig. 1a ([p. 438]) is shown the little insect of the natural size, and the same is given at 1 much magnified.