Cecidomyia (Gr. kekidion, 'a gall-nut,' and myia, 'a fly' or 'gnat'), a genus of dipterous (two-winged) insects in the Tipularia (gnat and mosquito) division. They have beautiful, delicate, downy wings, which have three nervures, and are horizontal when at rest; antennae as long as the body, with bead-like joints, and whorls of hairs at the joints; long legs, and the first joint of the tarsi very short. The species are numerous; nearly thirty in Britain, and sixty in Europe. All are of small size, but some of them are very important on account of the ravages which their minute maggots effect in grain-crops. C. cerealis, sometimes called the Barley Midge, a brownish-red fly with silvery wings, of which the maggot is vermillion coloured, is often very destructive to crops of barley and spelt in Germany. The little maggots live in families between the stalk and the sheath of the leaf, abstracting the juice of the plant.—The Wheat-fly (q.v.) and the Hessian Fly (q.v.) belong to this genus. Some of the species of Cecidomyia deposit their eggs on the young buds of trees, which the larvæ transform into galls.
While forms like the Hessian fly are of great economic importance, another Cecidomyia is, on account of its extraordinary mode of reproduction, of great scientific interest. According to Wagner, the female lays her eggs under tree-bark or the like; these develop in winter into larvæ. The larvæ, still immature, become reproductive and parthenogenetic. The ovaries rupture, the eggs fall into the body-cavity, where the stimulus of fertilisation is somehow replaced, for the ova develop into larvæ. These eat their parent larva, and after finishing the viscera, leave the empty skin. The nemesis of reproduction overtakes them also, for within them again, though likewise only larvæ, a fresh batch of larvæ develops in similar fashion. After several generations of this immature and fatal reproduction, the final set of larvæ metamorphose in summer into sexual winged insects. See REPRODUCTION.