The order Ephemeroptera, or mayflies, is an important part of Iowa's aquatic insect fauna. Species from this group of insects are valuable in determining water quality. Mayflies spend most of their life as aquatic larvae or nymphs. Usually they can be found on bottom substrates of rivers, streams and lakes around the state. They are distinguished from other aquatic insects by having wing pads on the thorax, three pairs of segmented legs, gills occurring on the abdomen and an abdomen that terminates with three (sometimes two) thin tail filaments. Adults have membranous wings that are held together above the body when not flying. Their front wings are larger than their hind wings, and two or three long tail filaments are at the end of the abdomen. The adult mayfly is terrestrial and is only alive for a few hours or days. The word Ephemeroptera comes from this reference to the short life span of the adult with the Greek word "Ephemeros," meaning short-lived and "ptera," meaning wing. There are about 2,500 species worldwide and nearly 630 species within North America. Currently, 109 species are reported from Iowa (McCafferty et al. 2006, McCafferty et al. 2003).