Mayflies are delicate, quite weak flying insects most often seen with two or three long trailing tails (cerci). The front legs of the mayfly are very long, especially in the male of the species. Mayflies have two pairs of wings with complex vein patterns, the hind pair is always much smaller than the fore.

Mayflies have very poorly developed mouthparts, meaning they cannot feed and so have a short life span of about one day. The larvae of the mayfly can be found in a wide range of freshwater habitats, many have quite exacting requirements for dissolved oxygen content and flow levels. It is for this reason that they can be quite a useful indicator of freshwater quality, and are often used as one of the key indicators in biological monitoring of water quality.

There are approximately 2500 species of mayfly, 2000 can be found in Europe, and 80 are present in the United Kingdom.