Chapter XIX Structures of Gall-Flies and Aphides
Many of the processes which we have detailed bear some resemblance to our own operations of building with materials cemented together; but we shall now turn our attention to a class of insect-architects, who cannot, so far as we know, be matched in prospective skill by any of the higher orders of animals. We refer to the numerous family which have received the name of gall-flies,—a family which, as yet, is very imperfectly understood, their economy being no less difficult to trace than their species is to arrange in the established systems of classification; though the latter has been recently much improved by Mr. Westwood.
One of the most simple and very common instances of the nests constructed by gall-insects, may be found in abundance during the summer, on the leaves of the rose-tree, the oak, the poplar, the willow (Salix viminalis), and many other trees, in the globular form of a berry, about the size of a currant, and usually of a green colour, tinged with red, like a ripe Alban or Baltimore apple.
When this psuedo-apple in miniature is cut into, it is found to be fresh, firm, juicy, and hollow in the centre, where there is either an egg or a grub safely lodged, and protected from all ordinary accidents. Within this hollow ball the egg is hatched, and the grub feeds securely on its substance, till it prepares for its winter sleep, before changing into a gall-fly (Cynips) in the ensuing summer. There is a mystery as to the manner in which this gall-fly contrives to produce the hollow miniature apples, each enclosing one of her eggs; and the doubts attendant upon the subject cannot, so far as our present knowledge extends, be solved, except by plausible conjecture. Our earlier naturalists were of opinion that it was the grub which produced the galls, by eating, when newly hatched, through the cuticle of the leaf, and remaining till the juices flowing from the wound enveloped it, and acquired consistence by exposure to the air. This opinion, however, plausible as it appeared to be, was at once disproved by finding unhatched eggs on opening the galls.
There can be no doubt, indeed, that the mother gall-fly makes a hole in the plant for the purpose of depositing her eggs. She is furnished with an admirable ovipositor for that express purpose, and Swammerdam actually saw a gall-fly thus depositing her eggs, and we have recently witnessed the same in several instances. In some of these insects the ovipositor is conspicuously long, even when the insect is at rest; but in others, not above a line or two of it is visible, till the belly of the insect be gently pressed. When this is done to the fly that produces the currant-gall of the oak, the ovipositor may be seen issuing from a sheath in form of a small curved needle, of a chestnut-brown colour, and of a horny substance, and three times as long as it at first appeared.
What is most remarkable in this ovipositor is, that it is much longer than the whole body of the insect, in whose belly it is lodged in a sheath, and, from its horny nature, it cannot be either shortened or lengthened. It is on this account that it is bent into the same curve as the body of the insect. The mechanism by which this is effected is similar to that of the tongue of the woodpeckers (Picidæ), which, though rather short, can be darted out far beyond the beak, by means of a forked bone at the root of the tongue, which is thin and rolled up like the spring of a watch. The base of the ovipositor of the gall-fly is, in a similar way, placed near the anus, runs along the curvature of the back, makes a turn at the breast, and then, following the curve of the belly, appears again near where it originates. We copy from Réaumur his accurate sketch of this remarkable structure.
With this instrument the mother gall-fly pierces the part of a plant which she selects, and, according to our older naturalists, “ejects into the cavity a drop of her corroding liquor, and immediately lays an egg or more there; the circulation of the sap being thus interrupted, and thrown, by the poison, into a fermentation that burns the contiguous parts and changes the natural colour. The sap, turned from its proper channel, extravasates and flows round the eggs, while its surface is dried by the external air, and hardens into a vaulted form.”GA Kirby and Spence tell us, that the parent fly introduces her egg “into a puncture made by her curious spiral sting, and in a few hours it becomes surrounded with a fleshy chamber.”GB M. Virey says, the gall tubercle is produced by irritation, in the same way as an inflamed tumor in an animal body, by the swelling of the cellular tissue and the flow of liquid matter, which changes the organization, and alters the natural external form.GC This seems to be the received doctrine at present in France.GD
Sprengel, speaking of the rose-willow, says, the insect in spring deposits its eggs in the leaf buds. “The new stimulus attracts the sap,—the type of the part becomes changed, and from the prevailing acidity of the animal juice, it happens, that in the rose and stock-shaped leaves which are pushed out, a red instead of a green colour is evolved.”GE
Without pretending positively to state facts which are, perhaps, beyond human penetration, we may view the process in a rather different light. (J. R.) Following the analogy of what is known to occur in the case of the saw-flies, after the gall-fly has made a puncture and pushed her egg into the hole, we may suppose that she covers it over with some adhesive gluten or gum, or the egg itself, as is usual among moths, &c., may be coated over with such a gluten. In either of these two cases, the gluten will prevent the sap that flows through the puncture from being scattered over the leaf and wasted; and the sap, being thus confined to the space occupied by the eggs, will expand and force outwards the pellicle of gluten that confines it, till becoming thickened by evaporation and exposure to the air, it at length shuts up the puncture, stops the further escape of the sap, and the process is completed. This explanation will completely account for the globular form of the galls alluded to; that is, supposing the egg of the gall-fly to be globular, and covered or coated with a pellicle of gluten of uniform thickness, and consequently opposing uniform resistance, or rather uniform expansibility, to the sap pressing from within. It will also account for the remarkable uniformity in the size of the gall apples; for the punctures and the eggs being uniform in size, and the gluten, by supposition, uniform in quantity, no more than the same quantity of sap can escape in such circumstances.
But though this explanation appears to be plausible, it is confessedly conjectural; for though Swammerdam detected a gall-fly in the act of depositing her eggs, he did not attend to this circumstance; and in the instances which we have observed, some unlucky accident always prevented us from following up our observations. The indefatigable Réaumur, on one occasion, thought he would make sure of tracing the steps of the process in the case of the gall-fly which produces the substance called bedeguar on the wild rose-tree, and to which we shall presently advert. His plan was to enclose in a box, in which a brood of flies had just been produced from a bedeguar, a living branch from a wild rose-tree; but, to his great disappointment, no eggs were laid, and no bedeguar formed. Upon further investigation, he discovered that the brood of flies produced from the bedeguar were not the genuine bedeguar insects at all, but one of the parasite ichneumons (Callimone bedeguaris, Stephens), which had surreptitiously deposited their eggs there, in order to supply their young with the bedeguar grubs, all of which they appeared to have devoured. It may prove interesting to look into the remarkable structure of the bedeguar itself, which is very different from the globular galls above described.
The gall-fly of the willow (Cynips viminalis) deposits, as we have just seen, only a single egg on one spot; but the bedeguar insect lays a large cluster of eggs on the extremity of a growing branch of the wild rose-tree, making, probably, a proportionate number of punctures to procure materials for the future habitation of her young progeny. As in the former case, also, each of these eggs becomes (as we may suppose) surrounded with the sap of the rose, enclosed in a pellicle of gluten. The gluten, however, of the bedeguar insect is not, it would appear, sufficiently tenacious to confine the flowing sap within the dimensions of any of the little clustered globes containing the eggs, for it oozes out from numerous cracks or pores in the pellicle; which cracks or pores, however, are not large enough to admit a human hair. But this, so far from being a defect in the glutinous pellicle of the bedeguar fly, is, as we shall presently see, of great utility. The sap which issues from each of these pores, instead of being evaporated and lost, shoots out into a reddish-coloured, fibrous bristle.
It is about half an inch long, and, from the natural tendency of the sap of the rose-tree to form prickles, these are all over studded with weak pricklets. The bedeguar, accordingly, when fully formed, has some resemblance, at a little distance, to a tuft of reddish-brown hair or moss stuck upon the branch. Sometimes this tuft is as large as a small apple, and of a rounded but irregular shape; at other times it is smaller, and in one instance mentioned by Réaumur, only a single egg had been laid on a rose-leaf, and, consequently, only one tuft was produced. Each member of the congeries is furnished with its own tuft of bristles, arising from the little hollow globe in which the egg or the grub is lodged.
The prospective wisdom of this curious structure is admirable. The bedeguar grubs live in their cells through the winter, and as their domicile is usually on one of the highest branches, it must be exposed to every severity of the weather. But the close, non-conducting, warm, mossy collection of bristles, with which it is surrounded, forms for the soft, tender grubs a snug protection against the winter’s cold, till, through the influence of the warmth of the succeeding summer, they undergo their final change into the winged state; preparatory to which they eat their way with their sharp mandibles through the walls of their little cells, which are now so hard as to be cut with difficulty by a knife. (J. R.)
Another structure, similar in principle, though different in appearance, is very common upon oak-trees, the termination of a branch being selected as best suited for the purpose. This structure is rather larger than a filbert, and is composed of concentric leaves diverging from the base, and expanding upwards, somewhat like an artichoke. Whether this leafy structure is caused by a superinduced disease, as the French think, or by the form of the pores in the pellicle of gluten surrounding the eggs, or rather by the tendency of the exuding sap of the oak to form leaves, has not been ascertained; but that it is intended, as in the case of the bedeguar, to afford an efficient protection against the weather to the included eggs or grubs, there can be no doubt.
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.
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.
A few weeks afterwards, on opening this drawer, we were surprised to see a brood of several dozens of a species of gall-fly (Cynips), similar in form and size to that whose eggs cause the bedeguar of the rose, and differing only in being of a lighter colour, tending to a yellowish brown. We have since met with a figure and description of this gall in Swammerdam. We may remark that the above is not the first instance which has occurred in our researches, of gall insects outliving the withering of the branch or leaf from which they obtain their nourishment.
The woolly substance on the branch of the oak which we have described was similarly constituted with the bedeguar of the rose, with this difference, that instead of the individual cells being diffused irregularly through the mass, they were all arranged at the off-goings of the leaf-stalks, each cell being surrounded with a covering of the vegetable wool, which the stimulus of the parent egg, or its gluten, had caused to grow, and from each cell a perfect fly had issued. We also remarked that there were several small groups of individual cells, each of which groups was contained in a species of calyx or cup of leaf-scales, as occurs also in the well-known gall called the oak-apple.
We were anxious to watch the proceedings of these flies in the deposition of their eggs, and the subsequent developments of the gall-growths; and endeavoured for that purpose to procure a small oak plant in a garden-pot; but we did not succeed in this: and though they alighted on rose and sweet-briar trees, which we placed in their way, we never observed that they deposited any eggs upon them. In a week or two the whole brood died, or disappeared. (J. R.)
There are some galls, formed on low-growing plants, which are covered with down, hair, or wool, though by no means so copiously as the one which we have just described. Among the plants so affected are the germander speedwell, wild thyme, ground-ivy, and others to which we shall afterwards advert.
The well-known oak-apple is a very pretty example of the galls formed by insects; and this, when compared with other galls which form on the oak, shows the remarkable difference produced on the same plant by the punctures of insects of different species. The oak-apple is commonly as large as a walnut or small apple, rounded, but not quite spherical, the surface being irregularly depressed in various places. The skin is smooth, and tinged with red and yellow, like a ripe apple; and at the base there is, in the earlier part of the summer, a calyx or cup of five or six small brown scaly leaves; but these fall off as the season advances. If an oak-apple be cut transversely, there is brought into view a number of oval granules, each containing a grub, and embedded in a fruit-looking fleshy substance, having fibres running through it. As these fibres, however, run in the direction of the stem, they are best exhibited by a vertical section of the gall; and this also shows the remarkable peculiarity of each fibre terminating in one of the granules, like a footstalk, or rather like a vessel for carrying nourishment. Réaumur, indeed, is of opinion that these fibres are the diverted nervures of the leaves, which would have sprung from the bud in which the gall-fly had inserted her eggs, and actually do carry sap-vessels throughout the substance of the gall.
Réaumur says the perfect insects (Cynips quercus) issued from his galls in June and the beginning of July, and were of a reddish-amber colour. We have procured insects, agreeing with Réaumur’s description, from galls formed on the bark or wood of the oak, at the line of junction between the root and the stem. These galls are precisely similar in structure to the oak-apple, and are probably formed at a season when the fly perceives, instinctively, that the buds of the young branches are unfit for the purpose of nidification.
There is another oak-gall, differing little in size and appearance from the oak-apple, but which is very different in structure, as, instead of giving protection and nourishment to a number of grubs, it is only inhabited by one. This sort of gall, besides, is hard and woody on the outside, resembling a little wooden ball of a yellowish colour, but internally of a soft, spongy texture. The latter substance, however, encloses a small hard gall, which is the immediate residence of the included insect. Galls of this description are often found in clusters of from two to seven, near the extremity of a branch, not incorporated, however, but distinctly separate.
We have obtained a fly very similar to this from a very common gall, which is formed on the branches of the willow. Like the one-celled galls just described, this is of a hard, ligneous structure, and forms an irregular protuberance, sometimes at the extremity, and sometimes on the body, of a branch. But instead of one, this has a considerable number of cells, irregularly distributed through its substance. The structure is somewhat spongy, but fibrous; and externally the bark is smoother than that of the branch upon which it grows. (J. R.)
The currant-galls (as the French call them) of the oak are exactly similar, when formed on the leaves, to those which we have first described as produced on the leaves of the willow and other trees. But the name of currant-gall seems still more appropriate to an excrescence which grows on the catkins of the oak, giving them very much the appearance of a straggling branch of currants or bird-cherries. The galls resemble currants which have fallen from the tree before being ripe. These galls do not seem to differ from those formed on the leaves of the oak; and are probably the production of the same insect, which selects the catkin in preference, by the same instinct that the oak-apple gall-fly, as we have seen, sometimes deposits its eggs in the bark of the oak near the root.
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.
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.
In this early stage of its growth, however, the gall does not, like the galls of the cynips, increase very much in dimensions. It is after the increase of the inhabitants by the young brood that it grows with considerable rapidity; for each additional insect, in order to procure food, has to puncture the wall of the chamber and suck the juices, and from the punctures thus made the sap exudes, and enlarges the walls. As those galls are closed all round in the more advanced state, it does not appear how the insects can ever effect an exit from their imprisonment.
A much more common production, allied to the one just described, may be found on the poplar in June and July. Most of our readers may have observed, about midsummer, a small snow-white tuft of downy-looking substance floating about on the wind, as if animated. Those tufts of snow-white down are never seen in numbers at the same time, but generally single, though some dozens of them may be observed in the course of one day. This singular object is a four-winged fly (Eriosoma populi, Leach), whose body is thickly covered with long down—a covering which seems to impede its flight, and make it appear more like an inanimate substance floating about on the wind, than impelled by the volition of a living animal. This pretty fly feeds upon the fresh juices of the black poplar, preferring that of the leaves and leaf-stalks, which it punctures for this purpose with its beak. It fixes itself with this design to a suitable place upon the principal nervure of the leaf, or upon the leaf-stalk, and remains in the same spot till the sap, exuding through the punctures, and thickening by contact with the air, surrounds it with a thick fleshy wall of living vegetable substance, intermediate in texture between the wood and the leaf, being softer than the former and harder than the latter. In this snug little chamber, secure from the intrusion of lady-birds and the grubs of aphidivorous flies (Syrphi), she brings forth her numerous brood of young ones, who immediately assist in enlarging the extent of their dwelling, by puncturing the walls. In one respect, however, the galls thus formed differ from those of the mountain-ash just described,—those of the poplar having always an opening left into some part of the cell, and usually in that portion of it which is elongated into an obtuse beak. From this opening the young, when arrived at the winged state, make their exit, to form new colonies; and, during their migrations, attract the attention of the most incurious by the singularity of their appearance. (J. R.)
On the black poplar there may be found, later in the season than the preceding, a gall of a very different form, though, like the other, it is for the most part on the leaf-stalk. The latter sort of galls are of a spiral form; and though they are closed, they open upon slight pressure, and appear to be formed of two laminæ, twisted so as to unite. It is at this opening that an aperture is formed spontaneously for the exit of the insects, when arrived at a perfect state. In galls of this kind we find aphides, but of a different species from the lanigerous ones, which form the horn-shaped galls above described.
Leaf-Rolling Aphides.
It may not be improper to introduce here a brief sketch of some other effects, of a somewhat similar kind, produced on leaves by other species of the same family (Aphidæ). In all the instances of this kind which we have examined, the form which the leaf takes serves as a protection to the insects, both from the weather and from depredators. That there is design in it appears from the circumstance of the aphides crowding into the embowering vault which they have formed; and we are not quite certain whether they do not puncture certain parts of the leaf for the very purpose of making it arch over them; at least, in many cases, such as that of the hop-fly (Aphis humuli), though the insects are in countless numbers, no arching of the leaves follows. The rose-plant louse, again (Aphis rosæ), sometimes arches the leaves, but more frequently gets under the protecting folds of the half-expanded leaf-buds. (J. R.)
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.
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.)
Another sort of excrescence is not uncommon on the terminal shoots of the hawthorn. This is in general irregularly oblong, and the bark which covers it is of an iron colour, similar to the scoriæ of a blacksmith’s forge. When dissected, we find no traces of insects, but a hard, ligneous, and rather porous texture. It is not improbable that this excrescence may originate in the natural growth of a shoot being checked by the punctures of aphides, or of those grubs which we have described.
Many of these excrescences, however, are probably altogether unconnected with insects, and are simply hypertrophic diseases, produced by too much nourishment, like the wens produced on animals. Instances of this may be seen at the roots of the hollyhock (Althea rosea) of three or four years’ standing; on the stems of the elm and other trees, immediately above the root; and on the upper branches of the birch, where a crowded cluster of twigs sometimes grows, bearing no distant resemblance to a rook’s nest in miniature, and provincially called witch-knots.
One of the prettiest of these pseudo-galls with which we are acquainted, is produced on the Scotch fir (Pinus sylvestris), by the Aphis pini, which is one of the largest species of our indigenous aphides. The production we allude to may be found, during the summer months, on the terminal shoots of this tree, in the form of a small cone, much like the fruit of the tree in miniature, but with this difference, that the fruit terminates in a point, whereas the pseudo-gall is nearly globular. Its colour also, instead of being green, is reddish; but it exhibits the tiled scales of the fruit cone.
We have mentioned this the more willingly that it seems to confirm the theory which we have hazarded respecting the formation of the bedeguar of the rose and other true galls—by which we ascribed to the sap, diverted from its natural course by insects, a tendency to form leaves, &c., like those of the plant from which it is made to exude.