Carboniferous mayflies include gigantic species with wing spans of atleast 45 cm. Modern Ephemeroptera number about 2000 species in 19 families , most of them associated with running water. The adults are less diverse in form than the nymphs. They are among the most primitive aquatic insects and probably arose in the Carboniferous period, 280-350 million years ago. The taxonomy of the immature stages is poorly known because the nymphs of many species have not yet been associated with adult forms.
Mayflies are unique insects in that they have two adult stages.
Beneath the subimaginal cuticle the adult eyes, legs a nd genitalia are clearly visible. In contrast to the imagos, mayfly nymphs exhibit considerable diversity in appearance although many stream species are dorso-ventrally flattened. Mayfly nymphs go through a large number of moults as they grow, with most species having 15-25 instars. Es timates for some species reach as high as 50 and variations occur within single species. Ephemeroptera has a cosmopolitan distribution but is absent from the Arctic and antartic.