The scientific name of mayflies, Ephemeroptera (εφήμερος = for a day, πτερον = wing), refers to the short lifespan of adults in this group. Once they have completed their final molt, mayflies live only long enough to mate and reproduce, from a few hours in some species to a few days in others. Immature mayflies, called nymphs or naiads, live from a few weeks to a year or two under water in ponds and streams. Mayflies are unique among living insect orders in having a winged, immature stage, the subimago, which emerges from the water and then molts again after a brief period into the final reproductive adult or imago. Since adult mayflies live for such a short time, the emergence of males and females must be closely synchronized which can lead to huge swarms of freshly emerged imagos in some areas.
Mayfly nymphs are important members of aquatic food webs. Most feed on algae and detritus, although a few species are predaceous. Many fishes and other freshwater animals depend on mayfly nymphs as a major food source. Since mayflies are also very sensitive to changes in water chemistry, they are often used as indicators of water quality.