Depending on the species, mayflies range from 2 to 18 mm in length, excluding the antennae and caudal filaments. They are often whitish or yellowish in colour.
Mayflies have vestigial mouthparts and are therefore unable to feed. The membranous and extensively veined wings are held vertically when at rest.
Mayflies have two to three long filaments at the tip of the abdomen. These have a tactile role, stabilize the insect’s flight and, together with the wings, help prevent joined mating couples from falling to the ground.