Aphides, then, are essentially degraded insects, which have become almost vegetative in their habits, and even in their mode of reproduction, but which still retain a few marks of their original descent from higher and more locomotive ancestors. Their wings, especially, are useful to the perfect forms in finding one another, and to the imperfect ones in migrating from one plant to its nearest neighbours, where they soon become the parents of fresh hordes in rapid succession. Hence various kinds of aphides are among the most dreaded plagues of agriculturists. The 'fly,' which Kentish farmers know so well on hops, is an aphis specialised for that particular bine; and, when once it appears in the gardens, it spreads with startling rapidity from one end of the long rows to the other. The phylloxera which has spoilt the French vineyards is a root-feeding form that attacks the vine, and kills or maims the plant terribly, by sucking the vital juices on their way up into the fresh-forming foliage. The 'American blight' on apple trees is yet another member of the same family, a wee creeping cottony creature that hides among the fissures of the bark, and drives its very long beak far down into the green sappy layer underlying the dead outer covering. In fact, almost all the best-known 'blights' and bladder-forming insects are aphides of one kind or another, affecting leaves, or stalks, or roots, or branches. It is one of the most remarkable examples of the limitation of human powers that while we can easily exterminate large animals like the wolf and the bear in England, or the puma and the wolverine in the settled States of America, we should be so comparatively weak against the Colorado beetle or the fourteen-year locust, and so absolutely powerless against the hop-fly, the turnip-fly, and the phylloxera. The smaller and the more insignificant our enemy, viewed individually, the more difficult is he to cope with in the mass. All the elephants in the world could have been hunted down and annihilated, in all probability, with far less labour than has been expended upon one single little all but microscopic parasite in France alone. The enormous rapidity of reproduction in the family of aphides is the true cause of our helplessness before them. It has been calculated that a single aphis may during its own lifetime become the progenitor of 5,904,900,000 descendants. Each imperfect female produces about ninety young ones, and lives long enough to see its children's children to the fifth generation. Now, ninety multiplied by ninety four times over gives the number above stated. Of course, this makes no allowance for casualties which must be pretty frequent: but even so, the sum-total of aphides produced within a small garden in a single summer must be something very extraordinary.