The \mayflies are an order (Ephemeroptera) of insects that grow up in fresh water, and live very briefly as adults, as little as a few hours but more typically a day or two. About 2,500 species in 23 families are known.
The nymphss live on the bottom of lakes and streams, usually under rocks, clinging to vegetation or digging in silt or mud. Most species are vegetarian, with some types being predators. The nymph stage may last from several months to as much as several years, with a number of molts along the way. Mayfly nymphs are distinctive in having external pairs of gills along the abdomen, as well as 2-3 long cerci at the end, giving them a bit of a frilly appearance. In the last aquatic stage, small wings are visible, a feature unique to this order.
The adult's one purpose is to reproduce; the mouthparts are useless, and the digestive system filled with air. The wings are large and shiny, with the forewings much larger than the hind wings. The males' eyes are usually large, and the front legs long, used to grasp females and often held in front when resting. In some species, all legs aside from the males' front legs are useless, and adults' entire lives are spent in flight.
It often happens that all the mayflies in a population mature at once, and for a day or two in the springtime, mayflies will be everywhere, dancing around each other in large groups, or resting on every available surface.
Both immature and adult mayflies are an important part of the food chain, particularly for carnivorous fish like trout.