mayfly, any insect of the order Ephemeroptera, so named because the adults live for a short time, often only a single day, during which they molt twice, mate, and lay their eggs in freshwater. The adults are medium to large, shiny, slender insects with two pairs of fragile, transparent, many-veined wings, and two or three long threadlike tails. The long forelegs of the male are used to clasp the female during the mating flight. Mayflies, also called June bugs, shad flies, and salmon flies, emerge by the thousands from streams, ponds, and lakes at twilight in the early spring; the males form large mating swarms and when a female flies into the swarm she is seized by a male and the two depart to mate. Mayflies lack fully developed mouthparts and do not feed. The insect undergoes incomplete metamorphosis, the egg hatching directly into an aquatic naiad, or nymph, with chewing mouthparts, which passes through some 20 nymphal stages over a period of two years or more, feeding on algae and diatoms and breathing oxygen taken directly from water by gills. It emerges from the water to transform into a subadult phase known as the subimago, unique among insects, in which it has wings and can fly but has immature legs, tail, and reproductive system. Adult mayflies are an important food source for many animals; several fishing flies are modeled after them. Mayflies are classified in the phylum Arthropoda, class Insecta, order Ephemeroptera.
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