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The life-history of the rose-aphis, small and familiar as is the insect itself, forms one of the most marvellous and extraordinary chapters in all the fairy tales of modern science. Nobody need wonder why the blight attacks his roses so persistently when once he has learnt the unusual provision for exceptional fertility in the reproduction of these insect plagues. The whole story is too long to give at full length, but here is a brief recapitulation of a year’s generations of common aphides.

In the spring, the eggs of last year’s crop, which have been laid by the mothers in nooks and crannies out of reach of the frost, are quickened into life by the first return of warm weather, and hatch out their brood of insects. All this brood consists of imperfect females, without a single male among them; and they all fasten at once upon the young buds of their native bush, where they pass a sluggish and uneventful existence in sucking up the juice from the veins on the one hand, and secreting honey-dew upon the other. Four times they moult their skins, these moults being in some respects analogous to the metamorphosis of the caterpillar into chrysalis and butterfly. After the fourth moult, the young aphides attain maturity; and then they give origin, parthenogenetically, to a second brood, also of imperfect females, all produced without any fathers. This second brood brings forth in like manner a third generation, asexual, as before; and the same process is repeated without intermission as long as the warm weather lasts. In each case, the young simply bud out from the ovaries of the mothers, exactly as new crops of leaves bud out from the rose-branch on which they grow. Eleven generations have thus been observed to follow one another rapidly in a single summer; and indeed, by keeping the aphides in a warm room, one may even make them continue their reproduction in this purely vegetative fashion for as many as four years running. But as soon as the cold weather begins to set in, perfect male and female insects are produced by the last swarm of parthenogenetic mothers; and these true females, after being fertilised, lay the eggs which remain through the winter, and from which the next summer’s broods have to begin afresh the wonderful cycle. Thus, only one generation of aphides, out of ten or eleven, consists of true males and females: all the rest are false females, producing young by a process of budding.

Setting aside for the present certain special modifications of this strange cycle which have been lately described by M. Jules Lichtenstein, let us consider for a moment what can be the origin and meaning of such an unusual and curious mode of reproduction.

The aphides are on the whole the most purely inactive and vegetative of all insects, unless indeed we except a few very debased and degraded parasites. They fasten themselves early in life on to a particular shoot of a particular plant; they drink in its juices, digest them, grow, and undergo their incomplete metamorphoses; they produce new generations with extraordinary rapidity; and they vegetate, in fact, almost as much as the plant itself upon which they are living. Their existence is duller than that of the very dullest cathedral city. They are thus essentially degenerate creatures: they have found the conditions of life too easy for them, and they have reverted to something so low and simple that they are almost plant-like in some of their habits and peculiarities.

The ancestors of the aphides were free winged insects; and, in certain stages of their existence, most living species of aphides possess at least some winged members. On the rose-bush, you can generally pick off a few such larger winged forms, side by side with the wee green wingless insects. But creatures which have taken to passing most of their life upon a single spot on a single plant hardly need the luxury of wings; and so, in nine cases out of ten, natural selection has dispensed with those needless encumbrances. Even the legs are comparatively little wanted by our modern aphides, which only require them to walk away in a stately sleepy manner when rudely disturbed by man, lady-birds, or other enemies; and indeed the legs are now very weak and feeble, and incapable of walking for more than a short distance at a time under exceptional provocation. The eyes remain, it is true; but only the big ones: the little ocelli at the top of the head, found amongst so many of their allies, are quite wanting in all the aphides. In short, the plant-lice have degenerated into mere mouths and sacks for sucking and storing food from the tissues of plants, provided with large honey-tubes for getting rid of the waste sugar.