Engelsk definition
An Order of aquatic INSECTS comprising about 2,500 species worldwide. It is the only insect group with two flying stages, each with relatively large forewings. The immature stage called "naiad" or "nymph" lasts one year in fresh water, and the adult stage is shortlived, lasting from a few minutes to a few days, depending on the species. The Ephemeroptera includes mayflies (also called shadflies)