There are hundreds of species in North America.
Various families in the order Ephemeroptera (mayflies)

Adults are slender, soft-bodied, with four membranous, extensively veined wings held upright and together (like a butterfly). The forewings are much longer and often overlap the hindwings. When perching, the front pair of legs are often held outward. They have short antennae and large compound eyes. There are 2 long, threadlike cerci (antenna-like appendages extending from the tip of the abdomen). The naiads (nymphs) somewhat resemble the adults, though they lack wings, have a series of leaflike external gills attached below the abdomen, have smaller eyes and often have a flattened head that helps them to adhere to rocks in fast-flowing water. Nymphs possess 3 long cerci (sometimes 2) extending from the tip of the abdomen.

Length: to 1¼ inches.
Habitat and conservation: 
Usually found near water. The name of the order, Ephemeroptera, is from Greek words for “short-lived” (as in “ephemeral”) and “wing” (the “-ptera” part). It’s a good name, because as winged adults, mayflies only live a few days. The adults’ only function is to reproduce. Their swarms often provoke feeding frenzies among fish. The nymphs (or naiads) play important roles in aquatic ecosystems, eating algae and other small items and being eaten by larger animals.
Different species of mayflies eat different things in their aquatic immature stages, but most creep around on rocks in lakes, ponds, streams or at river edges, eating algae and other small plants. Once they float up to the surface and molt into a winged adult, they have only vestigial mouthparts and cannot eat or drink.
Distribution in Missouri: 
Members of this order of insects are common throughout the state near ponds and streams. Three species of mayflies are listed as Missouri Species of Conservation Concern and thus are vulnerable to extirpation from our state: one is these is Baetisca obesa, which has no common name; the other two are Frison's serratellan mayfly (Serratella frisoni) and a heptageniid mayfly (Stenonema bednariki).
Life cycle: 
Mayflies are the only insect to have two “adult” molts. They begin life as eggs laid on the surface of the water that sink to the bottom. The aquatic nymphs of mayflies are called naiads, and creep around rocks and vegetation. After months or years (depending on the species), they float to the surface and molt to a winged but sexually immature subadult. Often within hours, another molt occurs and the final stage emerges—the reproductive adults which live for only days or hours.
Human connections: 
In places where mayflies synchronize their maturation and mating cycles, appearing in swarms, they usually cause fish to swarm, too, and anglers create fishing flies to resemble adult mayflies. Where swarms are especially large, they can be a nuisance and cause problems for motorists.
Ecosystem connections: 
Both immature stages and winged adult stages are favorite foods for fish such as trout, bass and many others, as well as smaller aquatic predators in the food chain. Adult mayflies are also eaten by terrestrial predators such as birds.
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