# Mayfly — Facts, Threats & Conservation

*Order Ephemeroptera — about 3,000 described species worldwide*

> Mayflies (order Ephemeroptera) are freshwater insects whose nymphs live underwater for months to years and whose winged adults typically survive only hours to a few days; they are the only insects that moult after gaining wings and are indicators of clean rivers.

**IUCN status:** Not Evaluated as a group — sensitive to water quality; adult stage lasts hours to days  ·  **WARN range:** Freshwater worldwide, Rivers and lakes with clean, well-oxygenated water

## Quick facts
| Fact | Value |
| --- | --- |
| Adult lifespan | Often hours to two days |
| Nymph habitat | Clean freshwater rivers and lakes |
| Wings | Held upright at rest; two pairs |
| Tail | Two or three long filaments (cerci) |
| Indicator value | Sensitive to pollution and low oxygen |
| Unique moult | Only insects that shed skin after winged |
| IUCN status | Not Evaluated for most species |

## Scientific classification
- **Kingdom:** Animalia
- **Phylum:** Arthropoda
- **Class:** Insecta
- **Order:** Ephemeroptera
- **Unique trait:** Moult after wings (subimago → imago)
- **Described species:** About 3,000 worldwide
- **Aquatic stage:** Nymph — months to years

## Conservation status
- **Status:** Not Evaluated
- **Population:** Not applicable at order level
- **Trend:** Declining locally with water pollution and hydrological alteration
- **Assessed:** Most species not assessed
- **CITES:** Not listed
- Mayfly loss from a river is an early warning. Restoration of clean, flowing water brings them back when pollution is reduced.

## Key facts: Mayfly
- Mayfly nymphs are aquatic for months to years; winged adults often live only hours to a couple of days.
- Adults undergo a final moult from subimago to imago after wings develop — unique among insects.
- Many adult mayflies do not feed; mouthparts are vestigial and the stage exists for reproduction.
- Upright wings and two or three long tail filaments (cerci) help identification.
- Nymphs are sensitive to pollution, low oxygen and sediment — their absence signals river degradation.
- Mass emergences synchronise mating and overwhelm fish predators — a key event for anglers and ecologists.

## Two lives in one animal
Mayfly ecology splits sharply between nymph and adult. Nymphs have gills and live on stones, vegetation or burrowed in silt, feeding on algae and detritus or filtering particles. They moult repeatedly underwater until ready to emerge.

The winged subimago climbs out, flies weakly, then moults again into the sexually mature imago with clearer wings and fuller colour. That post-wing moult is unknown elsewhere in insects. Adult mayflies swarm over water, mate in flight or on the surface, females lay eggs, and both sexes die — often within 24 hours for many species.

## Why rivers without mayflies are sick
Ephemeroptera nymphs need oxygen-rich water and stable substrates. Agricultural runoff, sewage, dam fluctuation and fine sediment smother gills and food. When mayflies disappear from a reach that once had them, the change usually predates visible algal blooms or fish kills.

Biomonitoring programmes index mayfly abundance alongside stoneflies and caddisflies as clean-water signals. Restoration that brings mayflies back — reduced pollution, natural flow regimes — is measurable success.

## The angler's hatch
Fly fishers name mayfly species and stages precisely because trout rise to emergences predictably. The dun (subimago) and spinner (imago) stages have different silhouettes on the water. Mass hatches can bring every fish in a pool to the surface simultaneously.

That cultural visibility belies fragility. The same river prized for fishing may lose hatches silently when water quality slips. Mayflies connect sport, ecology and water policy more directly than most insects.

## Status and assessment gaps
Most mayfly species are Not Evaluated individually on the IUCN Red List. Extinction risk concentrates on endemic species tied to single streams or lakes — isolated spring runs, mountain torrents — rather than on common river species still abundant where water is clean.

Climate change alters flow timing, which can desynchronise emergence from fish feeding peaks. Dams block drift and alter temperature downstream. The order-level story is water quality and hydrology, not a single species trade.

## What WARN does
WARN publishes free education for indicator species whose disappearance signals environmental damage before most people notice. Mayflies are standard river-health witnesses — when they vanish, the problem is already in the water.

If this guide helps you understand wildlife and the pressures it faces, a gift to WARN supports habitat protection and free public education in our partner countries.

## Frequently asked questions: Mayfly
### What do mayflies eat?
Nymphs feed on algae, detritus and organic material underwater. Many adult mayflies do not eat at all — mouthparts are reduced and the adult stage is devoted to mating.

### Where do mayflies live?
Clean rivers, streams and lakes worldwide. Nymphs live underwater on stones, plants or in sediment; adults swarm over water surfaces during emergence.

### Are mayflies dangerous?
No. They cannot bite or sting people meaningfully. Swarms can be a nuisance when attracted to lights near rivers, but they pose no health risk.

### How long do mayflies live?
Nymphs live months to years underwater. Winged adults of many species survive only hours to about two days — among the shortest adult lives of any insect.

### How big are mayflies?
Most adults are small to medium insects — roughly 5–30 mm body length depending on species, plus long tail filaments and wings held upright like a sail.

### Why do mayflies die so quickly?
Evolution has compressed the adult stage into a mating window. They emerge, reproduce and die; extended adult feeding is unnecessary for many species.

### Are mayflies endangered?
Most species are not individually assessed. Pollution and habitat alteration remove them locally; endemic stream species face higher extinction risk than widespread river mayflies.

## Sources
- [Buglife — Mayflies](https://www.buglife.org.uk/bugs/bug-directory/mayflies/)
- [Environment Agency — River fly monitoring](https://www.gov.uk/government/publications/river-fly-monitoring)
- [Animal Diversity Web — Ephemeroptera](https://animaldiversity.org/accounts/Ephemeroptera/)

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Full guide: https://worldanimalrescuenetwork.org/wildlife-guides/mayfly