This family is distinguished by the large, 10-30 mm, size of the adults and larvae that live in streams and lakes. The family is widespread on all continents except Australia; it is present in New Zealand and Madagascar.

Giant mayfly, fishfly, Green Bay fly, Hexagenia limbata, H. bilineata Adults are 18-30 mm long, excluding tails. The body is yellowish brown, and the head and thorax have brown markings. The wings are pale yellow and the margin of the hind wing is brown. Nymphs are 17-33 mm long, excluding tails; they have brown markings on a yellowish-brown body. The larval gills are large and feathered. Hexagenia larvae burrow in silt deposits oflakes, ponds, streams, and rivers throughout North America.

Eggs are laid as females rest on the surface of the water with their wings spread; rarely females fly above the water and ovipositas their abdomens touch the surface; fecundity is about 4000. Hatching depends on temperature, but usually occurs in 14 days, but eggs may remain dormant for long periods. Nymphs are in the soft bottom oflakes and streams; they live in U-shaped burrows at a depth of about 12 cm. In the northern part of their distribution, development is completed in 3-6 weeks. Emergence of adults is in late afternoon or at dusk. The subimago stage lasts 24-48 h. Larvae of these species are often sold as fish bait. Massive emergence along waterways occurs during warm months. Hexagenia species are known as the Green Bay fly because of the large number of adults that once emerged from Green Bay of Lake Michigan. At one time,

Hexagenia adults would annually invade the city of Green Bay, Wisconsin. However, pollution of the lake has greatly reduced their populations.

Coffinfly, Ephemera guttulata Adults are 18-23 mm long, excluding tails, and are pale white, with brown forelegs. Wings are clear to amber with numerous dark veins. This species lives in shallow lakes and along rivers with sandy or silty bottoms. Subimagos emerge from May to August. Sometimes large numbers emerge at one time and collect around street lights. It occurs in eastern USA and eastern Canada.

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