Order Ephemeroptera - Mayflies

in angling or flyfishing terminology, an adult mayfly is called a spinner; the winged pre-adult is called a dun

from the Greek "ephemeros" (of or for a day; short-lived) + "pteron" (wing; feather) refering to the short time that adults are on the wing ["ephemeros" comes from "epi" (upon) + "hemera" (day)]

MAYFLY: adults normally appear in large numbers beginning in May

611 spp. in 59 genera of 21 families north of Mexico(1); worldwide, over 3000 spp. in over 400 genera of at least 42 families(2)

adult body length 2-32 mm (How to Know the Insects; Roger Bland)

Adult (imago): body delicate or "flimsy", varying from almost transparent to white, yellow, orange, green, brown, or black; thorax and abdomen bare, often shiny; legs slender, solid color; front legs often held forward and sometimes upward in front of head when at rest; forewings large, triangular, with many cross veins; hindwings much smaller than forewings (hindwings absent in some species); both wings usually transparent but sometimes patterned, held vertically and together above thorax when at rest

Pre-adult (subimago): wings cloudy in appearance, and body dull and pubescent but otherwise similar to adult; pre-adults molt a final time to become adults

Nymph: body elongate, flattened or cylindrical, usually greenish or brownish but color varies according to the type of food eaten; legs long; antennae short; abdomen with lateral plate-like gills and usually three long thin tail projections (cerci); some species have only two cerci

throughout North America

most nymphs develop in streams and rivers that are well-oxygenated and relatively free of pollution; some species develop in lakes or ponds, usually in shallow water where the oxygen content is highest

adults may be found on vegetation near water, and are also attracted to light

some adults emerge in late April (or even earlier in the south) but the greatest numbers first appear in May, and adults may be seen until September in the north; later, and perhaps all year in the far south

nymphs feed on pieces of organic matter such as plant material or algae and debris that accumulates on rocks or other substrates in flowing water

adults do not feed; they have no functional mouthparts

Mating and oviposition. Adult males gather in mid-air swarms, usually 5-15 metres above the ground; females fly into the swarm, and mating occurs in flight. Females deposit eggs while flying low over the water, or by dipping the abdomen into the water; some species submerge themselves and lay eggs underwater.

Development. Mayflies are hemimetabolous, that is, they undergo incomplete metamorphosis, a form of simple metamorphosis. Nymphs (or naiads) develop through several stages (instars) by molting. The number of molts varies depending on species, temperature, and water conditions. Mature nymphs swim to water surface or crawl onto rocks or plants, then molt into winged subadults (subimagos) which fly to nearby plants and molt again into adults (called imagos). Adult lifespan ranges from 1.5 hours to one or two weeks; most adults live only two or three days.

The only insect group whose members molt in winged condition.

Adults and nymphs are an important source of food for fish and other aquatic wildlife. Anglers often use mayflies as bait, or tie "flies" that are made to resemble adult mayflies (spinners) or pre-adults (dun).