A medium-sized mayfly that emerges in spectacular synchronized swarms from the Great Lakes and Mississippi River — sometimes dense enough to show up on weather radar.

Also called: Burrowing Mayfly, Hex (anglers' name)

Native status: Native to North America

§ Quick facts

SizeAdults 2.0–3.5 cm body; nymphs 2.5–4 cm
Weight~0.1 g
LifespanAdults 1–3 days; aquatic larvae 1–2 years
OrderEphemeroptera
FamilyEphemeridae

§ Encyclopedia

Hexagenia limbata, the giant mayfly, is a species of mayfly in the family Ephemeridae. It is native to North America where it is distributed widely near lakes and slow-moving rivers. The larvae, known as nymphs, are aquatic and burrow in mud and the adult insects have brief lives. They are often referred to as fish flies around the Great Lakes as they tend to cause the areas around water to smell like rotten fish.

Source: Wikipedia — Hexagenia limbata Licensed under CC BY-SA 4.0

§ Identification

Field marks. Large mayfly with yellowish body and translucent wings; long pair of "tails" (cerci) extending behind the body; nymphs with prominent burrow-digging legs and feathery gills.

Similar species. Other Ephemera species (different patterns). Other mayfly families (different sizes, shapes).

Voice. N/A

§ Diet & behavior

Diet. Adults: do not feed. Larvae: detritus, algae filtered from stream and lake bottom sediments.

Behavior. Among the largest mayflies in North America. Famous synchronized emergences ("hatches") on the Mississippi River, Great Lakes, and other major waterways can be so massive that they show up on weather radar — millions of mayflies emerging within hours of each other, mating in the air, then dying in piles thick enough to require snowplows on bridges. Sensitive to pollution; presence is a biological indicator of clean water.

§ Range

Northern US and Canada

§ Habitat

Clean lakes and rivers with soft mud or silt bottoms

§ Threats

Pollution destroying lake/river larvae, light pollution disorienting swarms, dam construction.

§ Conservation status

According to the IUCN Red List, the eastern burrowing mayfly is currently classified as Least Concern.

§ What helps

Support clean water laws. Reduce shoreline lighting during emergence. Tolerate swarms (they're brief).

Support our work More ways to help

§ Further reading

§ Sources cited

  • Aquatic Insect Ecology
  • In-Fisherman Magazine
  • iNaturalist