Many other insects besides ants, however, are fond of the sweet secretions of the aphides, and it is probable that the honey-dew thus acts to some extent as a preservative of the species, by diverting possible foes from the insects themselves, to the sugary liquid which they distil from their food-plants.  Having more than enough and to spare for all their own needs, and the needs of their offspring, the plant-lice can afford to employ a little of their nutriment as a bribe to secure them from the attacks of possible enemies.  Such compensatory bribes are common enough in the economy of nature.  Thus our common English vetch secretes a little honey on the stipules or wing-like leaflets on the stem, and so distracts thieving ants from committing their depredations upon the nectaries in the flowers, which are intended for the attraction of the fertilising bees; and a South American acacia, as Mr. Belt has shown, bears hollow thorns and produces honey from a gland in each leaflet, in order to allure myriads of small ants which nest in the thorns, eat the honey, and repay the plant by driving away their leaf-cutting congeners.  Indeed, as they sting violently, and issue forth in enormous swarms whenever the plant is attacked, they are even able to frighten off browsing cattle from their own peculiar acacia.

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.