The Ephemeroptera are thought to have evolved towards the end of the Carboniferous.

The nymphs are mainly herbivorous feeding off algae and plant debris. They breathe though a series of tracheal gills, usually seven pairs, growing out of the side of their abdomen.

Development to adult state can take as long as two years in colder regions, but only a few months in warmer areas. During this time the nymph can go through as many as 45 moults (Stenacron interpunctatum canadense)!

During the final instar the nymph will stop feeding for a while, then climb out of the water or float to the surface. Then within a few seconds the skin splits and the insect emerges in its final nymphal stage and flies off. Mayflies are the only insects that have fully functional wings before they reach adulthood. Within a few hours, or in some species minutes, this nymphal skin is also moulted and the insect emerges as a full adult.

Adult mayflies do not live very long, some just for a few hours while others can hang around for a few days. Dolania americana lives for less than five minutes after her final molt. During the five minutes she chooses a mate, mates, and lays her eggs. They are weak fliers, so rarely stray far from water unless the wind carries them. Their whole purpose as adults is to mate. Some of them have no mouthparts, and even those that do do not feed. Usually a large number of adults emerge in a synchronized hatch.

Mating. Mayflies mate on the wing. After mating the female drops eggs into the water, though some species actually go underwater to lay the eggs on plants. After this the adults usually flop into the water where they are the beloved food of fishes, something well known to anglers.

Mayfly nymphs (see the drawing of a nymph in the Caenidae family left and below ) spend their lives in water, usually preferring clean water, so are rarely found in polluted waterways. So their presence is usually an indicator of relatively unpolluted water. Acid rain has led to the loss of many populations in Northern Europe and America. The nymphs live in the mud or among water plants. Ephemerella sp. (left) are usually found creeping along the mud or on stream and river beds, and occasionally cover themselves with debris.