Mayfly

Mayflies belong to the order Ephemeroptera, one of the oldest lineages of winged insects, with a fossil record stretching back hundreds of millions of years. They are unique among insects in passing through a winged, non-reproductive subimago stage before molting once more into the sexually mature adult, a trait found in no other insect order.

Adult mayflies lack functional mouthparts entirely and cannot feed; their brief time in the air, sometimes just a few hours, is devoted entirely to mating and egg-laying. Because of this fleeting adult stage, mass emergences can produce enormous, short-lived swarms along lakeshores and riverbanks, occasionally dense enough to be detected on weather radar.

Beneath the water, mayfly nymphs spend far longer developing, often a year or more, and are highly sensitive to pollution, making their presence a widely used indicator of clean, well-oxygenated water.

• Slender, soft body ranging from about 5 to 25 mm depending on species
• Large, triangular forewings held upright together over the back like a sail
• Small or absent hindwings
• Two or three long, thread-like tail filaments (cerci), often longer than the body itself
• Large compound eyes, especially prominent in males
• Lookalikes: stoneflies hold their wings flat or roof-like rather than upright and have shorter tails; caddisflies have hairy, moth-like wings rather than clear triangular ones

Mayflies are found on every continent except Antarctica. Nymphs live underwater in clean, well-oxygenated lakes, rivers, and streams, and are considered sensitive indicators of good water quality, often absent from polluted waterways. Winged adults are found near these water bodies, sometimes forming massive emergence swarms along shorelines during their brief flight period.

Aquatic nymphs feed on algae, diatoms, and organic detritus, and serve as an essential food source for fish and other aquatic predators throughout their long development. Winged adults do not feed at all, since their mouthparts are non-functional, and exist solely to mate and lay eggs during their short adult lifespan. Emergences are often highly synchronized, with huge numbers of nymphs transforming into winged forms within the same day or two, producing dense mating swarms over water at dusk.

Eggs are laid on or in the water and sink to settle on the bottom. Nymphs, sometimes called naiads, live underwater for months to more than a year, molting numerous times as they grow. A mature nymph rises to the water's surface and molts into a winged but sexually immature subimago, a stage unique to mayflies among all insects, which then molts once more within a day into the fully mature imago. Adults mate, lay eggs, and typically die within a day or two, with most temperate species completing one generation per year, though timing varies by species and climate.