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The Aphis: Richardson

The history of aphides has also been very copiously treated upon by Dr. Richardson, in a paper printed in the 41st vol. of the Philosophical Transactions, and by the late ingenious Mr. Curtis, in the 6th vol. of the Transactions of the Linnæan Society. The tenor of Dr. Richardson's remarks is briefly this: The great variety of species which occur in the insects now under consideration, may render an inquiry into their particular natures not a little perplexing; but by reducing them under their proper genus, the difficulty is considerably diminished. We may reasonably suppose all the insects comprehended under any distinct genus, to partake of one general nature; and by diligently examining any particular species, we may thence gain some insight into the nature of all the rest. With this view, Dr. Richardson chose out of the various sorts of aphides, the largest of those found on the rose-tree; not only as its size makes it more conspicuous, but as there are few of so long duration. This sort appears early in the spring, and continues late in autumn; while several are limited to a much shorter term, in conformity to the different trees and plants whence they draw their nourishment.

If, at the beginning of February, the weather happens to be so warm as to make the buds of the rose-tree swell and appear green, small aphides are frequently to be found on them, though not larger than the young ones in summer, when first produced. It will be found, that those aphides which appear only in spring, proceed from small black oval eggs, which were deposited on the last year's shoot; though it happens that, when the insects make too early an appearance, the greater part suffer from the sharp weather that usually succeeds, by which means the rose-trees are some years freed from them. The same kind of animal is then at one time of the year viviparous, and at another oviparous. Those aphides which withstand the severity of the weather, seldom come to their full growth before the month of April, at which time they usually begin to breed, after twice casting off their exuvia, or outward covering.

When they first come from the parent, they are enveloped in a thin membrane, having the appearance of an oval egg; these egg-like appearances adhere by one extremity to the mother, while the young ones contained in them extend to the other, and by that means gradually draw the ruptured membrane over the head and body to the hind-feet. Being thus suspooded in the air, the insect soon frees itself from the me< ane in which it was confined, and, after its limbs are THE APH ১. a little strengthened, is set down on some tender shoots, and left to provide for itself. In the spring months there appear on the rose-trees but two generations of aphides, including those which proceed immediately from the last year's eggs; the warmth of the summer adds so much to their fertilit that no less than five generations succeed each other in the interval. One is produced in May, which casts off their covering; while the months of June and July each supply two more, which cast off their coverings three or four times, according to the different warmth of the season. This frequent change of their outward coat is the more extraordinary, because it is repeated more often when the insects come the soonest to their growth, which sometimes happens in ten days, when they have had plenty of warmth and nourishment. Early in the month of June, some of the third generation, which were produced about the middle of May, after casting off the last covering, discover four erect wings, much longer than their bodies; and the same is observable in all the succeeding generations which are produced during the summer months, but, like all the others, without any diversity of sex: for some time before the aphides come to their full growth, it is easy to distinguish which will have wings, by a remarkable fulness of the breast, which in the others is hardly to be distinguished from the body. When the last covering is ejected, the wings, which were before folded up in a very narrow compass, are gradually extended in a surprising manner, till their dimensions are at last very considerable. The increase of these insects in the summer time is so very great, that by wounding and exhausting the tender shoots, they would frequently suppress all vegetation, had they not many enemies to restrain. them.

Notwithstanding these insects have a numerous tribe of enemies, they are not without their friends, if those may be considered as such, who are officious in their attendance, for the good things they expect to reap thereby. The ant and bee are of this kind, collecting the honey in which the aphides abound, but with this difference, that the ants are constant visitors, the bee only when flowers are scarce; the ants will suck in the honey while the aphides are in the act of discharging it; the bees only collect it from the leaves on which it has fallen. In the autumn, three more generations of aphides are produced, two of which generally make their appearance in the month of August, and the third before the middle of September. The two first differ in no respect from those which are found in summer, but the third differs greatly from all the rest. Though all the aphides which have hitherto appeared were female, in this generation several male insects are found, but not by ary means so numerous as the females. The females have, at first, the same appearance as those of the former generations, but in a few days their colour changes from a green to a yellow, which is gradually converted into an orange before they come to their full growth; they differ also, in another respect, from those which occur in summer, for all these yellow females are without wings. The male insects are, however, still more remarkable, their outward appearance readily distinguishing them from this and all other generations. When first produced, they are not of a green colour like the rest, but of a reddish brown, and have afterwards a dark line along the back; they come to their full growth in about three weeks, and then cast off their last covering, the whole insect being, after this, of a bright yellow colour, the wings only excepted, but after this change they become a deeper yellow, and, in a very few hours, of a dark brown, if we except the body, which is something lighter coloured, and has a reddish cast. Where there are a number crowded together, they of course interfere with each other, in which case they will frequently deposit their eggs on other parts of the branches. It is highly probable that the aphides derive considerable advantages by living in society: the reiterated punctures of a great number of them may attract a larger quantity of nutritious juices to that part of the tree or plant where they have taken up their abode.