May Flies, a family of Neuroptera (q.v.), known as the Ephemeridce. They differ from other members of this order by the absence or imperfect development of the bind wings. Their development is somewhat abnormal, as the pupa gives rise to a "subimago," which flies about for some time and then moults; after this it is in the form of the fully-matured adult or imago. This introduction of an extra-developmental stage is known as hypermetamorphism. Thelarva livesfor two or threeyears, but the mouth of the adult or imago is so imperfectly developed that it cannot take food, and therefore can only live for a day. Owing to this, they are often known as "Dayflies." The larva? breathe by "tracheal gills." Ephemera vulgata (Linn.), which is used by anglers under the name of the Brown or Green Drake, is the best-known English species.