APHIDES (pl. of aphis), minute insects, often known as plant-lice, blight, or greenfly, belonging to the family Aphididae of the order Hemiptera (q.v.). They are characterized by long antennae and legs, two-jointed tarsi with paired claws and a long rostrum ensheathing the mouth-parts. Both winged and wingless forms occur, the wings being transparent with few veins and the hind pair much the smaller. There is usually a pair of abdominal tubes or cornicles through which a waxy secretion is exuded; these insects also secrete "honey-dew," a sweet substance that is voided through the anus and much sought after by ants. Aphides generally pass their life on young shoots and foliage, more rarely on roots, and feed by sucking up the sap through their needle-like mouth-parts. Their life-cycle and reproduction are of a remarkable nature which, briefly, is as follows : The winter is passed as eggs which are laid by the fertilized females; they hatch in spring into wingless females which reproduce by parthenogenesis and bring forth living young. Winged females soon appear which behave likewise and, during the whole summer generation after generation of aphides are produced, all of which are females reproducing living young without the intervention of males. It is only towards the end of the season that males and egg-laying females occur and fertilization takes place. The winged forms are migrants which seek out fresh plants, thus dispersing the species. In some aphides the whole life-cycle is spent on one species of host plant ; but in others definite migration occurs towards autumn, when the aphides leave their summer, or inter mediate, host and pass to the winter host upon which the eggs are laid. The bean aphis (Aphis rumicis) , for example, utilizes the spindle tree as its chief winter host, while in May and June it flies to broad beans, dock, sugar beet, etc., which serve as its intermediate hosts. See also PARTHENOGENESIS and CYTOLOGY.
Considering their sap-sucking habit, capacity for migration and enormous powers of reproduction, it is scarcely remarkable that aphides rank as serious plant pests. Thus, the woolly aphis or American blight, Eriosoma lanigerum, is an enemy of apple in many parts of the world, producing gall-like growths on the branches and roots; Aphis gossypii is a widely spread pest of cotton and of plants of the cucumber and melon tribe; the hop damson aphis, Phorodon liumuli, attacks the hop in summer and the plum and damson in autumn and spring; while the grain aphis, Toxoptera graminum, is a severe pest of various cereals in both Europe and North America. Wherever possible, aphides are best controlled by sprays containing nicotine and soft soap, which need to be applied when these insects first appear (see ENTOMOLOGY : Economic).
The group Pliylloxerinae differs from true aphides in that egg laying is not confined to the fertilized females but is the rule in the parthenogenetic generations also. The Phylloxera (q.v.) of the vine and species of Chermes (Adelges), with their allies, are well known examples. In Chermes the life-cycle is passed on conifers and there are two parallel series of forms differing in habits. One series infests spruce and its cycle is completed in a year, while the other series extends over two years and occurs partly on spruce and partly on larch, pine or fir. (A. D. I.)