(Say, 1824) 
Baetis bilineata Say, 1824
Palingenia bilineata (Say, 1824)
Hexagenia bilineata is a species of mayfly in the family Ephemeridae. It is native to North America where it is found in the Upper Mississippi Valley. Sometimes adults of this mayfly are so abundant as to cause a nuisance because of their enormous numbers. The larvae are aquatic and burrow in mud and the adult insects have brief lives.
2 Distribution and habitat
4 Human impact
When the adults are ready to emerge, the mayfly nymphs (larvae) swim to the surface of the water during the night. Their skin splits and winged subimagos struggle free, usually in less than a minute, and fly to nearby trees to rest. They are a dull gray color and have short, coarse legs, bristly cerci and cloudy, grayish wings. Some eight to eighteen hours later, these subimagos moult into mature adults (imagos). These are altogether more delicate in appearance than the subimagos; the wings are transparent, the legs are longer and more slender, the cerci lack bristles, the eyes are larger and the body is patterned in brown and cream. The females are much larger than the males.
Distribution and habitat
Hexagenia bilineata occurs in the Upper Mississippi Valley. Adults appear in summer and are found near still and slow-moving water. The nymphs are found burrowing in the mud and silt in shallow lakes and slow-moving streams and rivers. This mayfly is generally more abundant than the closely related Hexagenia limbata, but that species becomes more plentiful from Keokuk, Iowa northwards. When a lock was drained near Keokuk in July 1958, 344 nymphs of H. bilineata were found in 10.5 square feet (1 m2) of sediment.
Around dusk, male imagos congregate in swarms. They often aggregate in the lee of structures such as buildings or trees, or if there is no wind, all round such a structure. Each male occupies a fixed position facing into the wind, about one foot (30 cm) from the next one, stabilizing its position with its cerci. They do not bob up and down as do males of H. limbata. Any female which flies into a swarm is pursued by males until copulation