Mayfly Nymph

Mayfly nymphs are the aquatic immature stage of insects in the order Ephemeroptera, one of the oldest living groups of winged insects. The order's name, meaning "short-lived wing," refers to the fleeting adult stage, which in many species lasts only a day or two, in stark contrast to the nymphal stage that can persist underwater for months or even a year or more.

Nymphs are recognized by the rows of feathery or plate-like gills running along the sides of their abdomen, along with three (occasionally two) long, thread-like tails at the rear of the body. Most species graze on algae and organic debris rather than actively hunting prey, though a few predatory mayfly nymphs do exist.

Because mayfly nymphs are highly sensitive to pollution and low oxygen levels, their presence or absence in a stream is widely used by biologists as an indicator of water quality, with diverse mayfly populations generally signaling clean, well-oxygenated freshwater.

• Nymphs are 5-25 mm long, with a flattened or streamlined body depending on the habitat they occupy (fast riffles versus slow pools).
• Rows of feathery, leaf-like, or plate-shaped gills line the sides of the abdomen, used for extracting oxygen from the water.
• Three long, thin tail filaments (cerci) extend from the rear of the abdomen, though some species have only two.
• Six legs end in a single claw, and body coloring is typically brown, tan, or olive to blend with stream substrate.
• Lookalikes include stonefly nymphs, which lack abdominal gills (their gills, if present, are near the legs) and typically have two tails rather than three.

Mayfly nymphs are found in freshwater habitats worldwide, with the greatest diversity in clean, well-oxygenated streams and rivers, though some species also live in lakes. Different species are adapted to specific microhabitats, including clinging to rocks in fast riffles, burrowing into soft sediment, climbing among vegetation, or swimming freely in open water. Because of their sensitivity to pollution, mayfly nymphs are most abundant and diverse in unpolluted waterways.

Most mayfly nymphs are grazers or collectors, feeding on algae, diatoms, and fine organic detritus that accumulates on rocks and sediment, though a smaller number of predatory species feed on other small invertebrates. Their gill movements create a current that helps maintain oxygen flow across the gill surfaces, particularly important in slower-moving water. Nymphs are a major food source for fish, including many trout and other stream fish, and their presence supports entire freshwater and riparian food webs, since emerging adults are also eaten in large numbers by birds, bats, and other predators. Adult mayflies do not feed at all, living only long enough to mate and lay eggs before dying.

Eggs are laid in water, often scattered directly onto the surface or attached to submerged objects. Nymphs hatch and undergo gradual metamorphosis, molting numerous times, sometimes over 20 or more instars, over a period ranging from a few weeks to about a year depending on species and water temperature. Uniquely among insects, mayflies pass through a winged subadult stage called a subimago before molting one final time into the reproductive adult, or imago. Adults typically emerge in large synchronized swarms, mate, lay eggs, and die within a day or two, completing one of the shortest adult life spans in the insect world.