APHIS, the Plant-Louse, or Paccron, an extensive genus of insects belonging to the order Homoptera. They are interesting to naturalists on account of their very peculiar economy, and uo less so to gardeners and farmers, on whose crops many species commit most destructive depredations. As instances of the latter we may refer to the Hop Fly (A. Hamuli), and the Bean-Dolphin (A. Fabaa), whilst all our garden flowers, such as the Rose, the China-Aster, the Chrysanthemum, and others, suffer from their attacks. During the summer of 1833, the cabbage and turnip crops in Kent were much injured and often destroyed by countless swarms of A. Brassiere.
These insects are characterised by a soft oval body, a small head, entire and semi-globular eyes, antenme of seven joints longer than the body, often setaceous, sometimes thickened towards the top, the two joints at the base very short, the next very long and cylindrical. The beak (haustdlum) arises from the under part of the head between the fore legs, and descends almost perpendicularly. The wings, when developed, are four in number, but some naturalists represent the upper wings rather as wing-cases (elytra), from their difference of texture. The legs are very long and slender, in consequence of which they walk awkwardly.
At the extremity of the abdomen most species are furnished with a pair of projecting tubes, through which they eject a sweet viscid fluid, well known under the name of honey-dew, erroneously supposed to be an exudation from the leaves on which it is found. It is also said that the Aphides feed on this, which is impossible from the structure of their mouths. Ants however and bees are very fond of it.
In sketching the history of these singular insects, it will be most convenient to begin it at the close of autumn, when many of the species, such as A. Quercus, A. Rosa', &c., are numerous, some winged and some without wings, of both sexes, so that while the first may fly to a distance, the second are confined to their native plant or its vicinity.
After pairing, the mother Aphis deposits what have been by some naturalists termed eggs, in a place suitable for their passing tho winter ; but different places are chosen by different species. Some choose the oak, and place the eggs on an exposed twig high on the tree, others in the sheltered crevices of bark, or even under ground. Bonnet seems to be of opinion that the Aphides are always viviparous and never lay eggs, what are commonly called eggs produced in autumn being a sort of cocoon, consisting of the young Aphis inclosed in an envelope. From our own observations on those of
the oak, we are convinced that this is the fact ; but we cannot upon negative evidence, that none of the species lay real eggs.
The cocoons or eggs, whichever they may be, remain torpid during the winter (the parents having died after producing them), and are called into life with the return of genial weather in the spring. The number of insects produced must of course correspond to the number of cocoons or eggs laid the preceding autumn, but being all ushered into active life at the same time, their simultaneous appearance has led to the popular but erroneous notion, that they are generated by the air. Blighting weather, as it is termed, is also accused of spreading the destructive swarms over hop-grounds or bean-fields, but their rapid increase is wholly caused by their wonderful powers of multiplying.
All the Aphides, it has been well ascertained, which appear in spring are exclusively females, no males being found till the autumn ; and these females are endowed with a fecundity almost incredible. M. Latreille says that one female during the summer mouths will produce about 25 a day, and M. Ildaumur calculated that one Aphis may be the progenitor, during its life, of the enormous number of 5,904,900,000 descendants. It is not necessary for tho young female Aphides pro duced during summer to pair with a male, which indeed would bo impossible, as no males are then to be found; yet these females go on producing each their 25 a day of living young ones, all of which become in a short time as fertile as their parent The following calculation of the fecundity of a species of Aphis from Professor Owen's lectures on 'Comparative Anatomy,' will afford some explanation of the extraordinary numbers in which these creatures sometimes occur. "The Aphis lanigera produces each year 10 viviparous broods, and one which is oviparous, and each generation averages 100 individuals : 1st generation I Aphis produces 2d 100 one hundred 3d 10,000 ten thousand 4th 1,000,000 one million 5th ,, 100,000,000 one hundred millions 6th . . . . 10,000,000,000 ten billions 7th . . . 1,000,000,000,000 one trillion 8th . . 100,000,000,000,000 one hundred trillions 9th . 10,000,000,000,000,000 ten quatrillions IOth 1,000,000,000,000,000,000 one quintillion.