[81] Reaumur Mem. des Insectes, tom. 4, p. 415.
OF THE APHIDES OR PUCERONS.
These are a species of insects that have opened new views of the œconomy of animated beings; they belong to the hemiptera class. The rostrum is inflected, the antennæ are longer than the thorax; some have four erect wings, others are entirety without them. Towards the end of the abdomen there are two tubes ejecting that most delicate juice called honey-dew. Various names have been applied to them, the proper one is aphis, that by which they are most generally known, is puceron; they are also frequently called vine-fretters or plant-lice: many among the genera are both oviparous and viviparous, bringing forth their young alive in summer, but in autumn depositing their eggs upon the branches and bark of trees. The different aphides are very curious objects for the microscope: they are a very numerous genus, Linnæus has enumerated thirty-three different species, whose trivial names are taken from the plant which they inhabit, though it is probable the number is much larger, as the same plant is often found to support two or three different sorts of them. Their habits are very singular: an aphis or puceron, brought up in the most perfect solitude from the very moment of its birth, in a few days will be found in the midst of a numerous family; repeat the experiment on one of the individuals of this family, and you will find this second generation will multiply like its parent; and this you may pursue through many generations.
M. Bonnet had repeated experiments of this kind, as far as the sixth generation, which all uniformly presented the observer with fruitful virgins, when he was engaged in a series of new and tedious experiments, from a suspicion imparted by M. Trembley in a letter to him, who thus expresses himself: “I have formed the design of rearing several generations of solitary pucerons, in order to see if they would all equally bring forth young. In cases so remote from usual circumstances, it is allowed to try all sorts of means; and I argued with myself, Who knows but that one copulation might serve for several generations?” This “WHO KNOWS” persuaded M. Bonnet that he had not sufficiently pursued his investigations. He therefore now reared to the tenth generation his solitary aphides, having the patience to keep an exact account of the days and hours of the birth of each generation. The result of this pursuit was, his discovering both males and females among them, whose amours were not in the least equivocal; the males are produced only in the tenth generation, and are but few in number; these soon arriving at their full growth, copulate with the females, and the virtue of this copulation serves for ten successive generations; all these generations, except the first from fecundated eggs, are produced viviparous, and all the individuals are females, except those of the last generation, among whom some males appear, to lay the foundation of a fresh series.
In order to give a further insight into the nature of these insects, I shall insert an extract of a description of their different generations, by Dr. Richardson, as published in the Philosophical Transactions for the year 1771.
The great variety of species which occur in the insects now under consideration, may make an inquiry into their particular natures seem not a little perplexing, but by reducing them under their proper genera, 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, 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 the more conspicuous, but as there are few others of so long a duration. This sort appears early in the spring, and continues late in the autumn; while several are limited to a much shorter term, in conformity to the different trees and plants from whence they draw their nourishment.
If at the beginning of February the weather happen 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 shoots; though when it happens that the insects make too early an appearance, the greater part suffers from the sharp weather that usually succeeds; by which means the rose-trees are some years in a manner 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. It appears that they are all females, which produce each of them a numerous progeny, and that without having intercourse with any male insect; they are viviparous, and what is equally singular, the young ones all come into the world backwards. When they first come from the parent, they are enveloped by a thin membrane, having in this situation 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 the other, by that means gradually drawing the ruptured membrane over the head and body to the hind feet. During this operation, and for some time after, the fore part of the head adheres, by means of something glutinous, to the vent of the parent. Being thus suspended in the air, it soon frees itself from the membrane in which it was confined; and after its limbs are 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 fertility, that no less than five generations succeed one another in the interval. One is produced in May, which casts off its 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, where warmth and plenty of nourishment conspired.
Early in the month of June, some of the third generation, which were produced about the middle of May, after casting off their 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 still 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 fullness of the breast, which in the others is hardly to be distinguished from the body. When the last covering is rejected, the wings, which were before folded up in a very narrow compass, are gradually extended in a surprizing 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 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 the 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.