Insect Architecture

by James Rennie

Currant Gall of the catkins of the Oak, produced by Cynips quercus pedunculi?

The gall of the oak, which forms an important dye-stuff, and is used in making writing-ink, is also produced by a Cynips, and has been described in the ‘Library of Entertaining Knowledge’ (Vegetable Substances, p. 16). The employment of the Cynips psenes for ripening figs is described in the same volume, p. 244.

Gall of the Hawthorn Weevil, drawn from specimen. a, Opened to show the grub.

Gall of a Hawthorn Weevil.

In May, 1829, we found on a hawthorn at Lee, in Kent, the leaves at the extremity of a branch neatly folded up in a bundle, but not quite so closely as is usual in the case of leaf-rolling caterpillars. On opening them, there was no caterpillar to be seen, the centre being occupied with a roundish, brown-coloured, woody substance, similar to some excrescences made by gall-insects (Cynips). Had we been aware of its real nature, we should have put it immediately under a glass or in a box, till the contained insect had developed itself; but instead of this, we opened the ball, where we found a small yellowish grub coiled up, and feeding on the exuding juices of the tree. As we could not replace the grub in its cell, part of the walls of which we had unfortunately broken, we put it in a small pasteboard box with a fresh shoot of hawthorn, expecting that it might construct a fresh cell. This, however, it was probably incompetent to perform: it did not at least make the attempt, and neither did it seem to feed on the fresh branch, keeping in preference to the ruins of its former cell. To our great surprise, although it was thus exposed to the air, and deprived of a considerable portion of its nourishment, both from the part of the cell having been broken off, and from the juices of the branch having been dried up, the insect went through its regular changes, and appeared in the form of a small greyish-brown beetle of the weevil family. The most remarkable circumstance in the case in question, was the apparent inability of the grub to construct a fresh cell after the first was injured,—proving, we think, beyond a doubt, that it is the puncture made by the parent insect when the egg is deposited that causes the exudation and subsequent concretion of the juices forming the gall. These galls were very abundant during the summer of 1830. (J. R.)

A few other instances of beetles producing galls are recorded by naturalists. Kirby and Spence have ascertained, for example, that the bumps formed on the roots of kedlock or charlock (Sinapis arvensis) are inhabited by the larvæ of a weevil (Curculio contractus, Marsham; and Rhynchœnus assimilis, Fabr.); and it may be reasonably supposed that either the same or similar insects cause the clubbing of the roots of cabbages, and the knob-like galls on turnips, called in some places the anbury. We have found them also infesting the roots of the hollyhock (Alcea rosea). They are evidently beetles of an allied genus which form the woody galls sometimes met with on the leaves of the guelder-rose (Viburnum), the lime-tree (Tilia Europæa), and the beech (Fagus sylvatica).

There are also some two-winged flies which produce woody galls on various plants, such as the thistle-fly (Tephritis cardui, Latr.). The grubs of this pretty fly produce on the leaf-stalks of thistles an oblong woody knob. On the common white briony (Bryonia dioica) of our hedges may be found a very pretty fly of this genus, of a yellowish-brown colour, with pellucid wings, waved much like those of the thistle-fly with yellowish brown. This fly lays its eggs near a joint of the stem, and the grubs live upon its substance. The joint swells out into an oval form, furrowed in several places, and the fly is subsequently disclosed. In its perfect state, it feeds on the blossom of the briony. (J. R.) Flies of another minute family, the gall-gnats (Cecidomyiæ, Latr.), pass the first stage of their existence in the small globular cottony galls which abound on germander speedwell (Veronica chamædrys), wild thyme (Thymus serpyllum), and ground-ivy (Glechoma hederacea). The latter is by no means uncommon, and may be readily recognised.

Certain species of plant-lice (Aphides), whose complete history would require a volume, produce excrescences upon plants which may with some propriety be termed galls, or semi-galls. Some of these are without any aperture, whilst others are in form of an inflated vesicle, with a narrow opening on the under side of a leaf, and expanding (for the most part irregularly) into a rounded knob on its upper surface. The mountain-ash (Pyrus aucuparia) has its leaves and young shoots frequently affected in this way, and sometimes exhibits galls larger than a walnut or even than a man’s fist; at other times they do not grow larger than a filbert. Upon opening one of these, they are found to be filled with the aphides sorbi. If taken at an early stage of their growth, they are found open on the under side of the leaf, and inhabited only by a single female aphis, pregnant with a numerous family of young. In a short time the aperture becomes closed, in consequence of the insect making repeated punctures round its edge, from which sap is exuded and forms an additional portion of the walls of the cell.