Ephemeroptera are delicate insects with two pairs (rarely just one pair) of triangular shaped wings with many veins – the front pair are large and the hind pair are small. They have long front legs, that are often directed forward. The antennae are very short and there are usually three (less commonly only 2) long, tail-like appendages. The adults have non-functional mouthparts and do not feed.
Immature mayflies have elongate bodies with long legs, short antennae and usually three tails (some only have two). They have leaf like gills on the sides of the abdomen. Nymphs are found in aquatic habitats, are often found on rocks or other substrates. Both the immatures and adults are an important fish food.
Adults are common around water, especially in spring, when they may emerge in large numbers. Adult mayflies live only for one or two days. They do not feed during their adult life but mate and lay eggs during their short adult life. Mayflies are the only insect group that molts after the wings are fully developed. The first winged stage is called the subimago and this stage typically has cloudy wings. Mayflies have incomplete metamorphosis. They can be up to about 1 inch long.