Invertebrates
Mayfly
Ephemeroptera · also called mayflies, shadflies
A mayfly spends most of its life as a patient, unglamorous grub on the bottom of a clean stream, sometimes for two or three years. Then it hauls itself into the air, and everything speeds up: it cannot eat, it has hours or at most a couple of days left, and its entire remaining purpose is to find a mate before it dies. It is also the only insect known to moult again after its wings have already grown in.
Overview
Mayflies are the order Ephemeroptera, one of the oldest lineages of winged insects, with a fossil record stretching back some 300 million years. Almost the whole life is spent as an aquatic nymph, grazing algae and organic debris on a stream or lake bed and breathing through external gills, often for months to a few years.
A day in the air
The winged adult has no functional mouthparts and does not eat, living entirely off reserves from its nymphal life, and typically survives only hours to a couple of days — just long enough to mate and, for the female, lay eggs. Uniquely among insects, a mayfly moults once more after gaining wings, from a dull subimago (an angler’s “dun”) into the mature imago.
Swarms and sentinels
Mayflies often emerge in enormous synchronised swarms, sometimes visible on weather radar, a strategy that overwhelms predators with sheer numbers so enough survive to breed. Because the nymphs are so sensitive to pollution and low oxygen, mayflies are one of the standard biological indicators of clean water, and they are a major food source underpinning fly fishing.
Status
Least Concern for most widespread species, but mayflies as a group are declining across North America and Europe from agricultural runoff, artificial light disrupting swarms, and the loss of clean, well-oxygenated water — their disappearance is often an early warning of wider water-quality problems.
Mayfly: common questions
Do mayflies really only live for one day?
The adult, winged stage does typically live only a day or two, and in some species just a few hours, but that is only the final, brief chapter of a much longer life. Before it ever grows wings, a mayfly spends the great majority of its life, commonly many months and in some species up to two or three years, as an aquatic nymph living on a stream or lake bottom. So while the popular idea that mayflies live for just a day is true of the adult phase people actually notice swarming near water, it dramatically understates the insect's total lifespan.
Why don't adult mayflies eat?
Because their mouthparts are reduced and non-functional by the time they reach the winged adult stage, and their entire digestive system is simplified or shut down, since the adult's sole biological purpose is to mate and, for females, to lay eggs, tasks it can complete using energy reserves stored up during its long nymphal life underwater. Not needing to feed lets the adult mayfly put all its remaining resources into reproduction during its very short time in the air, and it is also why swarms of emerging mayflies pose no threat to crops or gardens despite sometimes appearing in enormous numbers.
Why do so many mayflies emerge at exactly the same time?
Synchronised mass emergence is a defence strategy called predator swamping. If mayflies emerged in a steady trickle throughout the season, fish, birds, and other predators could simply eat a large share of them as they appeared. By instead emerging in a single, enormous, tightly timed swarm, far more mayflies appear at once than predators can possibly consume, so even though many individuals are still eaten, a large fraction survive long enough to mate. These mass emergences can be so large and dense along major rivers and lakes that they are detected on weather radar and can coat nearby roads and buildings.
Why are mayflies used to judge water quality?
Mayfly nymphs are particularly sensitive to pollution, low dissolved oxygen, and excess sediment, more so than many other aquatic invertebrates, so a stream or lake with a healthy, diverse population of mayfly nymphs is generally a strong sign of clean, well-oxygenated water, while their absence or decline often signals pollution, warming, or habitat degradation. Because of this reliable sensitivity, biologists and water-quality monitoring programmes routinely survey mayfly nymphs, along with a few similarly sensitive insect groups, as one of the standard, cost-effective ways to assess the ecological condition of rivers and lakes without relying solely on chemical testing.