Ephemeroptera (mayflies; subclass Pterygota, infraclass Palaeoptera) Order of insects in which the adults are short-lived, surviving from a few hours to a few days (see EPHEMERAL). The order is somewhat primitive, its members having two pairs of membranous wings with a simple network of veins, resembling the venation of dragonflies. The eyes are large; the antennae are short; the mouth-parts are unsclerotized and vestigial (the adults do not feed); the thorax is strongly developed for flight; and the abdomen possesses three long tail filaments. The adults are small to medium-sized, soft-bodied insects, with large, triangular fore wings, and small, rounded, hind wings. The wings are held rigidly upright above the body at rest. The nymphs are aquatic and herbivorous, although some have sharp, modified mandibles and are partly predacious. The nymphal stage may last several years, but the majority are univoltine. The metamorphosis is simple and there is no pupal stage, the first winged stage being known as the subimago, or dun. The sub-imago resembles the adult, but is often a dull-brown colour. The sub-imaginal stage may last as little as five minutes before the skin is shed and the brightly coloured adult (spinner) emerges. The males fly in swarms with a dancing, vertical flight, and have several mating adaptations (e.g. reversible foreleg tarsal joints) and modified eyes. Ephemeroptera are unique in that they undergo a moult after wings have become full-sized and functional. The 2100 species are an important part of the diet of freshwater fish.