The mayflies is a small, yet well-known order of primitive, winged insects.

The nymphs, which are called naiads, live in fresh water and feed primarily on plants and organic debris. The winged imagines do not feed, and only live a short time. They often swarm in great numbers, and are an important part of the diet of many fish, birds, bats and dragonflies. Both nymphs and imagines are often used as models for the artificial flies used by anglers. The mayflies are assumed to be the most basal order among the winged insects, and they possess many traits that are primitive for these. Among the primitive traits are the resting position of the wings, which cannot be folded flat over the abdomen and have to be held erect over the body when at rest, and two or three long, multisegmented caudal filaments. Mayflies have incomplete metamorphosis, and a unique feature of the order is that there are two winged instars; subimago and imago.