Aphides, the plant lice, a family of Hymenoptera (q.v.). They are minute in size, but occur in such enormous numbers as to do serious injury to the plants on which they live; their numbers are kept in check mainly by the lady-birds. They secrete a milky juice, to obtain which they are kept captive and milked by ants. They are hemimetabolic, i.e. they undergo only a partial metamor phosis. They are of interest biologically as one of the type cases of that alternation of sexual and asexual methods of reproduction known as parthenogenesis; during the summer the Aphides are sexless and they reproduce asexually, but in the autumn a generation of males and females is produced; these copulate and lay eggs, which in the following spring are hatched into the asexual forms. The type genus is Aphis.