Giant Michigan Mayfly

They are aquatic insects whose immature stage (called "naiad" or, colloquially, "nymph") usually lasts one year in freshwater. The adults are short-lived, from a few minutes to a few days depending on the species. About 2,500 species are known worldwide, including about 630 species in North America. The lifespan of an adult mayfly can vary from just 30 minutes to one day depending on the species. The primary function of the adult is reproduction; the mouthparts are vestigial, and the digestive system is filled with air.
The wings are membranous, with extensive venation, and are held upright like those of a butterfly. The hindwings are much smaller than the forewings, and may be vestigial or absent. The second segment of the thorax, which bears the forewings, is enlarged, holding the main flight muscles.