Which order of insects do mayflies belong to?

The immature stages of mayflies are aquatic. They generally live in unpolluted habitats with fresh water. Some species are active swimmers, others are flattened and cling to the underside of stones, a few are burrowers who dig tunnels in the sand or mud. Most species are herbivorous. Their diet consists primarily of algae and other aquatic plant life scavenged from surrounding habitat. Some species mature quickly, in as little as four weeks, while others develop more slowly (one to four years per generation).

Once a mayfly completes development as a naiad, it leaves the aquatic environment, rising to the water surface in a bubble of air. It quickly molts to a winged form (the subimago) and flies to a nearby leaf. The subimago is a brief transitional stage that molts again into a sexually mature adult (imago). The imago usually has transparent wings and a shiny exoskeleton in contrast to the cloudy wings and dull body of the subimago. Mayflies are the only living insects that molt after they have wings.

Most adults are delicate insects with a very short lifespan. They do not feed (mouthparts are vestigial), and some species emerge, reproduce, and die in a single day. Males generally fly in swarms, and females fly into the swarm and are quickly grabbed by a male. Copulation takes place in flight, and the female usually lays her clutch of eggs within minutes or hours. Males die shortly after mating; females usually die soon after oviposition.