CHAPTER X.

APHIS, OR PLANT-LOUSE.

Every Tree, every part of a Tree, has its peculiar SpeciesSuck Vegetable Juices-Shelter themselves from bad Weather in the concave parts of Leaves.

AMONG the most curious of those animals which are parasitical on plants, is the aphis or plant-louse. It is an insect common enough in our fields and gardens, and there is scarcely a tree or shrub which is not attacked by one or more species peculiar to itself. Some are winged, and others have no wings; and some are black, green, brown, in short, all colours; but however they may vary in these non-essentials, their habits and instincts are similar. They live in society, and attach themselves to the stems, the leaves, and roots of shrubs and vegetables, the juices of which they suck by means of a tube, with which they are furnished for the purpose; and they frequently cling in such numbers to the sustaining plant, as to give it a most unsightly appearance.

If the rose-tree, or any other plant, be carefully examined, some portion of it will be found covered with little transparent insects of a green colour. They appear to be in a state of perfect repose: they are, however, in reality, diligently occupied in pumping out the juices of the plant. The following is a magnified figure of the insect. The length of the sucking pump or trunk,

in some species, extends beyond the body; when they walk, it is folded under the belly: in the generality, however, the ordinary length of this member is about one-third that of the insect. While employed in using this instrument, they form at times two layers, one over the other; the second or upper layer walk freely over the first, and not being able to suck, are diligently employed in bringing forth young.

These immense societies of suckers must of course drain the leaf, and exhaust the juices of the plant to which they are attached: this is the fact, and, by Leeuwenhoek, they have been truly termed the pests of the garden. However, the effect is occasionally curious enough; for, instead of withering, the parts to which the aphides are attached enlarge or twist, and by so doing furnish shelter to their enemies. The insect chooses the concavity of a shoot, for example, and this, through loss of juice, being diverted from its straight direction, assumes the shape of a corkscrew; in the concave folds of this diverted shoot it is that the aphis shelters itself from the weather.

Reaumur says, that this curve takes place on the side from which the insects suck the juices, for the same reason that a piece of wood, soaked in water,

and exposed to the action of the fire, is bent to the side acted upon by that element. Another effect of the curling of the shoot is, that the leaves, which, if extended, would stand far apart, are so drawn together as to form a complete covering, and thus the insects are at once defended from wind and rain, and concealed from the view of their natural enemies.

If shoots can be bent by insects which attach themselves to this portion of a plant, the leaves must of course be more easily susceptible of this operation.

Generally, they seek the under surface of the leaves, probably as that part which affords them most

shelter. If the upper surface of a gooseberry, currant, or apple leaf be examined, it will be found studded with pale, reddish, or citron-coloured eminences; and on the under surface, cavities will be discovered answering to these eminences, and peopled with aphides. These portions of the leaf are thicker than the rest.

A still more remarkable change of form is caused in the plant by the development of galls formed by the plant-louse. When opened, they will be found hollow, and filled with a colony of these creatures. In size they vary from that of a nut to that of the human fist.

It has been already stated, that the formation of the true gall-nut is owing to the deposition of an egg in some part of a plant. But in the formation of the protuberances inhabited by the aphides there is this remarkable difference; that the parent, instead of burying her offspring, buries herself, and then, as the walls of her wonderful mansion rise up around and inclose her, she begins to people her abode. When Reaumur examined the smallest of these at its first formation, he found it tenanted by one old aphis only:-when he examined a larger gall-nut he found, in addition to the old aphis, one or two young ones-and in a protuberance of a larger size still, he

discovered a more abundant population. The mode by which the insect is at last thoroughly enclosed is thus described by him:- "Let us imagine that the mother-aphis, still young, pricks the leaf; the punctured spot swells all around the insect, and consequently it becomes enclosed within a little cavity. If it continue to prick it at the lowest part of this cavity, this place will go on swelling in length, so as to become oblong or cylindrical. Let us conceive that the insect always continues to puncture it forwards; as soon as the gall has risen to a certain height above the superior surface of the leaf, the insect is no longer in its original position; viz., on the plane of the inferior surface of the leaf. Here then it is that there is a small opening into the incipient bladder; this aperture is only an indentation in the leaf. As soon as the insect removes from the aperture towards the other end of the bladder, nothing tends to hinder the bent sides from meeting soonest at the narrowest part, and so at last closing it up entirely. Here then we have the insect shut up oblong sack or bladder. It brings forth young, these prick the gall, and suck the juice on all sides, so that being thus irritated in every direction, it grows in every direction, and consequently a globular form results. And, as the punctures are always in a direction farthest from the original aperture, that part enlarges least; and consequently the gall appears to be attached to the leaf by a pedicle or foot-stalk."

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Another class of aphides, which inhabit the poplar, instead of forming galls, contrive to double the leaf so nicely on the nerve which runs through the middle, as to bring the two edges exactly together, and so con