Insect Order Ephemeroptera comprise the Mayflies. The earliest certain mayfly nymphs in the fossil record are from the late Carboniferous. The difficulty of placing fossil nymphs in an insect order, however, make this in dispute, with some authors claiming appearance as early as the Devonian is possible. Most fossil mayflies are larval fossils, ostensibly due to the very short time they have in the adult stage. After living as aquatic nymphs for one to two years, they live as adults only for a few days, an ephemeral existence indeed. When you only have a few days as an adult, finding a member of the opposite sex to perpetuate the species is of paramount importance; and any advantage would be a valuable trait. Both young and adult mayflies are an important part of the food chain, especially for stream fish.

The wings of Mayflies have been hypothesized by some scientists to be highly adapted (tracheal) gills, leading to conjecture that their gill blades and insect wings have a common origin, and research continues into this question. Interestingly, extant mayfly nymphs do not have gills until after their first moult. This trait could be shared by the common ancestor of all flying insects.