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.

Now, the greater the amount of food any animal gets, and the less the amount of expenditure it performs in muscular action, the greater will be the surplus it has left over for the purposes of reproduction.  Eggs or young, in fact, represent the amount thus left over after all the wants of the body have been provided for.  But in the rose-aphis the wants of the body, when once the insect has reached its full growth, are absolutely nothing; and it therefore then begins to bud out new generations in rapid succession as fast as ever it can produce them.  This is strictly analogous to what we see every day taking place in all the plants around us.  New leaves are produced one after another, as fast as material can be supplied for their nutrition, and each of these new leaves is known to be a separate individual, just as much as the individual aphis.  At last, however, a time comes when the reproductive power of the plant begins to fail, and then it produces flowers, that is to say stamens (male) and pistils (female), whose union results in fertilisation and the subsequent outgrowth of fruit and seeds.  Thus a year’s cycle of the plant-lice exactly answers to the life-history of an ordinary annual.  The eggs correspond to the seeds; the various generations of aphides budding out from one another by parthenogenesis correspond to the leaves budded out by one another throughout the summer; and the final brood of perfect males and females answers to the flower with its stamen and pistils, producing the seeds, as they produce the eggs, for setting up afresh the next year’s cycle.