Mayflies live as nymphs for 3 months to 2 years, depending on the species. When they're mature, the entire generation emerges from the stream as adults, often within the span of a couple weeks. They emerge as "duns," an immature winged adult stage, and molt within a day or two into "spinners," the mature adults.
The duns most often emerge during an hour or two each day for a couple weeks sometime in the spring or summer, though there's great variation between species. These events are barely noticeable in some species and reach nightmarish proportions in others.
There are three ways mayfly nymphs emerge into duns. Most often, the nymph swims to the water's surface and splits open its exoskeleton above the thorax. The dun wriggles out onto the surface, and many species float along on the surface for a while as their wings dry.
In a some species, the winged dun emerges from its nymphal case several feet underwater and swims to the surface, wings and all. Other species emerge by crawling out onto streamside rocks or logs as nymphs.
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 (similar to a house fly's wings but with many more veins) and are held upright like those of a butterfly. The forewings are much larger than the hind wings. In most species, the males' eyes are usually large and the front legs unusually long, for use in locating and grasping females during mid-air mating. In some species, all legs aside from the males' front legs are useless.It often happens that all the mayflies in a population mature at once (the hatch), and for a day or two in the spring or fall, mayflies will be everywhere, dancing around each other in large groups, or resting on every available surface.