The stage is called the subimago, or dun to anglers, and the fully mature stage is the imago, or spinner. Ephemeroptera is the only insect order in which a winged stage moults, and the discarded skin, wing membranes and all, can be found intact on waterside plants.
The subimago is duller, its wings often faintly opaque and fringed with fine hairs, and its legs and tail filaments are shorter than the adult's. The imago that emerges has clearer wings, longer forelegs used by males to grasp females in flight, and mature reproductive organs.
Neither winged stage can eat: the mouthparts are vestigial and the gut is largely an air-filled sac that helps keep the insect buoyant in flight. Everything is paid for by the nymph, which may spend a year or more feeding on algae and detritus in the streambed before the few hours of flight it supports.