One of the most common instances of what we mean occurs on the leaves of the currant-bush, which may often be observed raised up into irregular bulgings, of a reddish-brown colour. On examining the under side of such a leaf there will be seen a crowd of small insects, some with and some without wings, which are the Aphides ribis in their different stages, feeding securely and socially on the juices of the leaf.
The most remarkable instance of this, however, which we have seen, occurs on the leaves of the elm, and is caused by the Aphis ulmi. The edge of an elm-leaf inhabited by those aphides is rolled up in an elegant convoluted form, very much like a spiral shell; and in the embowered chamber thus formed the insects are secure from rain, wind, and partially from the depredations of carnivorous insects. One of their greatest enemies, the lady-bird (Coccinella), seldom ventures, as we have remarked, into concealed corners except in cold weather, and contrives to find food enough among the aphides which feed openly and unprotected, such as the zebra aphides of the alder (Aphides sambuci). The grubs, however, of the lady-bird, and also those of the aphidivorous flies (Syrphi), may be found prying into the most secret recesses of a leaf to prey upon the inhabitants, whose slow movements disqualify them from effecting an escape. (J. R.)
The effect of the puncture of aphides on growing plants is strikingly illustrated in the shoots of the lime-tree and several other plants, which become bent and contorted on the side attacked by the insects, in the same way that a shoot might warp by the loss of its juices on the side exposed to a brisk fire. The curvings thus effected become very advantageous to the insects, for the leaves sprouting from the twig, which naturally grow at a distance from each other, are brought close together in a bunch, forming a kind of nosegay, that conceals all the colour of the sprig, as well as the insects which are embowered under it, protecting them against the rain and the sun, and at the same time hiding them from observation. It is only requisite, however, where they have formed bowers of this description, to raise the leaves, in order to see the little colony of the aphides,—or the remains of those habitations which they have abandoned. We have sometimes observed sprigs of the lime-tree, of a thumb’s thickness, portions of which resembled spiral screws; but we could not certainly have assigned the true cause for this twisting, had we not been acquainted with the manner in which aphides contort the young shoots of this tree.[GG] The shoots of the gooseberry and the willow are sometimes contorted in the same way, but not so strikingly as the shoots of the lime.
Shoot of the Lime-tree contorted by the punctures of the Aphis tiliæ.
Pseudo-Galls.
It may not be out of place to mention here certain anomalous excrescences upon trees and other plants, which, though they much resemble galls, are not so distinctly traceable to the operations of any insect. In our researches after galls, we have not unfrequently met with excrescences which so very much resemble them, that before dissection we should not hesitate to consider them as such, and predict that they formed the nidus of some species of insects. In more instances than one we have felt so strongly assured of this, that we have kept several specimens for some months, in nurse-boxes, expecting that in due time the perfect insect would be disclosed.
One of these pseudo-galls occurs on the common bramble (Rubus fruticosus), and bears some resemblance to the bedeguar of the rose when old and changed by weather. It clusters round the branches in the form of irregular granules, about the size of a pea, very much crowded, the whole excrescence being rather larger than a walnut. We expected to find this excrescence full of grubs, and were much surprised to discover, upon dissection, that it was only a diseased growth of the plant, caused (it might be) by the puncture of an insect, but not for the purpose of a nidus or habitation. (J. R.)
