Mayflies belong to an order of insects called Ephemeroptera, with 21 families and 676 species in North America. All go through a series of life stages: The first stage of the mayfly life cycle is the egg. After leaving the egg, a mayfly spends almost a full year underwater as a larvae (“nymph”, to a fly-fisherman). Mayfly larvae can be categorized by body shape and habitat-of-choice into six groups: swimmers, crawlers, clingers, climbers, sprawlers and burrowers. Larvae of most species feed on algae growing on rocks, shedding their skins up to twenty times as they grow. To reach the next life stage – biologists call it a subimago, fly-fishermen call it a dun — the larvae will either crawl out of the water to hatch on a rock, or emerge on the water surface where they often become food for trout. A mayfly dun resembles a fully formed adult, but the wings are cloudy in color, and the insect is not fully sexually mature. Less than 24 hours later, the dun will undergo the final molt. Biologists call the final life stage of the mayfly the imago, fly-fishermen call it the spinner. This phase also lasts only about 24 hours – just long enough for the mayfly to mate, lay eggs, and die.
As nymphs, mayflies live underwater and are somewhat difficult to identify. They are generally less than an inch long, and can often be found clinging to the bottom of rocks on the stream-bottom. With a magnifying glass, you can see that they have tiny gills on the sides of their abdomens, and two or three tails. As adults, all mayflies are similar in shape. All mayflies have two or three tails, and when at rest, adult mayflies hold their wings upright like the sail on a sailboat.
Hughes, Dave. 2005. Handbook of Hatches. Mechanicsburg, VA: Stackpole Books.
Voshell, J.R. 2002. A Guide to Common Freshwater Invertebrates of North America. Blacksburg, VA: McDonald & Woodward Publishing Company.