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Mayfly

Mayflies are aquatic insects in the order Ephemeroptera, known for their short-lived winged adult stage and their importance as indicators of freshwater ecosystem health.

By Ethan Anderson| Reviewed by Olivia Bennett, Standards & Fact-Checking Lead| Updated July 27, 2026|2 min read| Fact-checked

Mayflies are aquatic insects in the order Ephemeroptera, known for their short-lived winged adult stage and their importance as indicators of freshwater ecosystem health. More than 2,500 species have been described worldwide, and the group includes some of the oldest living winged insect lineages. Adults typically do not feed, while the immature stages live in water.

Classification and appearance

Mayflies are soft-bodied insects in the order Ephemeroptera. Adults typically have membranous wings held upright at rest, reduced hind wings, and long bristlelike tails.

Life cycle

Mayflies have an unusual life cycle with four stages: egg, nymph, subimago, and imago. They are notable for being the only insects known to molt after developing functional wings.

Habitat and ecology

The immature stages are aquatic and occur in freshwater habitats, often in clean, oxygen-rich water. Nymphs feed on algae and plant material and are an important food source for fish, birds, and other animals.

Distribution and diversity

Mayflies occur worldwide except in Antarctica. More than 2,500 species have been described, with hundreds recorded in Canada and the United States.

Key facts

  • Mayflies belong to the insect order Ephemeroptera and are not true flies.
  • They spend most of their lives as aquatic nymphs before emerging as winged adults.
  • Adults usually have nonfunctional mouthparts and do not feed.
  • Mayflies are widely used as bioindicators because many species are sensitive to polluted water.
  • The group is among the oldest surviving lineages of winged insects.
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Canadian context

Mayflies are common in Canada, with hundreds of species recorded in North America. Canadian environmental agencies also use mayflies as indicators of freshwater ecosystem health.

Frequently asked questions

Are mayflies flies?
No. Mayflies belong to the order Ephemeroptera and are not true flies in the order Diptera.
Why are they called mayflies?
The name refers to the brief appearance of the winged adults, though many species emerge in late spring or summer rather than only in May.
How long do adult mayflies live?
Usually only a short time, often around a day, though lifespan varies by species.
What do mayflies eat?
Nymphs feed on algae and vegetation. Adults generally do not feed because their mouthparts are nonfunctional.
Why are mayflies important to scientists?
Their larvae are widely used as bioindicators of water quality because many species require clean, well-oxygenated freshwater.

References

  1. Encyclopaedia Britannicahttps://www.britannica.com/animal/mayfly
    Supports: Classification, global species count, life cycle, morphology, and distribution.
  2. National Wildlife Federationhttps://www.nwf.org/Educational-Resources/Wildlife-Guide/Invertebrates/Mayflies
    Supports: Habitat, adult lifespan, nymph ecology, and bioindicator role.
  3. Government of Northwest Territorieshttps://www.gov.nt.ca/ecc/en/services/biodiversity/mayflies
    Supports: Two winged stages and general mayfly biology.
  4. Woodland Trusthttps://www.woodlandtrust.org.uk/trees-woods-and-wildlife/animals/flies/mayfly/
    Supports: Habitat preferences, feeding, and ecological relationships.
  5. WebMDhttps://www.webmd.com/a-to-z-guides/what-to-know-about-mayflies
    Supports: Species count, ecological importance, and adult feeding limitations.