A brightly coloured, freshwater insect (order Ephemeroptera) with long, membranous wings and tail, that lives as an adult only long enough to mate and lay eggs, no more than a few days and in most cases for less than 24 hours. The insect spends up to several years as an aquatic, herbivorous larva, called a nymph or naiad, moulting several times before developing wings, as a sub-imago or alate. The sub-imago lasts for only a few minutes before moulting for a last time to emerge as the imago (adult). Only one generation of nymphs matures each year and all the imagos emerge together, not necessarily in May, so briefly there are large swarms of them. There are about 2500 species found on all continents except Antarctica.