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.

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.

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.

[2] On the evening of July 23, 2014 in the La Crosse, Wisconsin area, the insects were airborne in such large numbers that they were detectable on weather radar, the enormous swarm resembling a rainstorm.