1. (noun) mayfly - an insect widespread in Aotearoa/New Zealand rivers and streams. Adults are short-lived and at rest its wings point straight up like sails. The nymph stage is found on the undersides of river stones, they are easy to identify by their three long tail filaments. They drift downstream in the current, and fish feed on them. The typical length including the tail is 15 millimetres. Behind the legs, along the sides of the body, are feathered gills for breathing underwater. The bodies are often flattened to form a low profile to the current, which helps prevent them being swept away. There are about 40 endemic species of mayfly.