Insect Architecture

by James Rennie

Artichoke Gall of the Oak-bud, with Gall-fly (Cynips quercus gemmæ), natural size, and its ovipositor (a) magnified.

From the very nature of the process of forming willow-galls, bedeguar, and the artichoke of the oak, whatever theory be adopted, it will be obvious that their growth must be rapid; for the thickening of the exuded sap, which is quickly effected by evaporation, will soon obstruct and finally close the orifice of the puncture made by the parent insect. It is accordingly asserted by Réaumur and other observers, that all the species of galls soon reach their full growth.

Leafy Gall of Dyer’s Broom, produced by Cynips genistæ? A, gall, natural size; B, a leaflet magnified.

A very minute reddish-coloured grub feeds upon dyer’s broom (Genista), producing a sort of gall, frequently globular, but always studded with bristles, arising from the amorphous leaves. The stem of the shrub passes through this ball, which is composed of a great number of leaves, shorter and broader than natural, and each rolled into the form of a horn, the point of which ends in a bristle. In the interior we find a thick fleshy substance, serving to sustain the leaves, and also for the nourishment of the grubs, some of which are within and some between the leaves. They are in prodigious numbers,—hundreds being assembled in the small gall, and so minute as scarcely to be perceived without the aid of a magnifying glass. The bud of the plant attacked by those grubs, instead of forming a shoot, pushes out nothing but leaves, and these are all rolled, and turned round the stem. Some shrubs have several of these galls, which are of various sizes, from that of a filbert to that of a walnut.

A similar but still more beautiful production is found upon one of the commonest of our indigenous willows (Salix purpurea), which takes the name of rose-willow, more probably from this circumstance than from the red colour of its twigs. The older botanists, not being aware of the cause of such excrescences, considered the plants so affected as distinct species; and old Gerard accordingly figures and describes the rose-willow as “not only making a gallant show, but also yielding a most cooling air in the heat of summer, being set up in houses for decking the same.” The production in question, however, is nothing more than the effect produced by a species of gall-fly (Cynips salicis) depositing its eggs in the terminal shoot of a twig, and, like the bedeguar and the oak artichoke, causing leaves to spring out, of a shape totally different from the other leaves of the tree, and arranged very much like the petals of a rose. Decandolle says it is found chiefly on the Salix helix, S. alba, and S. riparia.[GF]

A production very like that of the rose-willow may be commonly met with on the young shoots of the hawthorn, the growth of the shoot affected being stopped, and a crowded bunch of leaves formed at the termination. These leaves, besides being smaller than natural, are studded with short bristly prickles, from the sap (we may suppose) of the hawthorn being prevented from rising into a fresh shoot, and thrown out of its usual course in the formation of the arms. These bristles appear indiscriminately on both sides of the leaves, some of which are bent inwards, while others diverge in their natural manner.

This is not caused by the egg or grub of a true gall-fly, but by the small white tapering grub of some dipterous insect, of which we have not ascertained the species, but which is, probably, a cecidomyia. Each terminal shoot is inhabited by a number of these—not lodged in cells, however, but burrowing indiscriminately among the half-withered brown leaves which occupy the centre of the production. (J. R.)

A more remarkable species of gall than any of the above we discovered, in June, 1829, on the twig of an oak in the grounds of Mr. Perkins, at Lee, in Kent. When we first saw it, we imagined that the twig was beset with some species of the lanigerous aphides, similar to what is vulgarly called the American or white blight (Aphis lanata); but on closer examination we discarded this notion. The twig was indeed thickly beset with a white downy, or rather woolly, substance around the stem at the origin of the leaves, which did not appear to be affected in their growth, being well formed, healthy, and luxuriant. We could not doubt that the woolly substance was caused by some insect; but though we cut out a portion of it, we could not detect any egg or grub, and we therefore threw the branch into a drawer, intending to keep it as a specimen, whose history we might complete at some subsequent period.