Animated coloured scanning electron micrograph (SEM) of Aquatic nymph stage of the mayfly (Drunella doddsi). Mayflies are insects which belong to the Order Ephemeroptera. They have been placed into an ancient group of insects termed the Paleoptera, which also contains the dragonflies and damselflies. They are aquatic insects whose immature stage (called naiad or nymph) usually lasts for a few months to as long as several years in fresh water. The naiad lives primarily in streams under rocks, decaying vegetation, or in the sediment. Most species feed on algae or diatoms, but there are a few predatory species. Mayfly naiads are distinctive in that most have seven pairs of gills on the dorsum of the abdomen. In addition, most possess three long cerci or tails at the end of their bodies. The adults are short-lived, from a few hours to a few days depending on the species.