Giant Mayfly

The mature adult stage of Hexagenia limbata can be as much as 27 mm (1.1 in) long. The females tend to be slightly larger than the males but have smaller eyes. The forewings are long and membranous while the hind wings are much smaller and have dark margins. The tip of the abdomen bears two long cerci or tails. When the insect is at rest, the wings are folded vertically above the back. The color and body markings are somewhat variable but this insect is usually some shade of yellow, pale brown or white.

Hexagenia limbata is the most widely distributed mayfly in North America. It is known from most of Canada and all the states in the United States except Alaska and Arizona. It is at its most common in the Great Lakes region. Its habitat is lakes and slow-moving rivers with muddy bottoms in which the nymphs can burrow. Adults can be found congregating in swarms near the water or resting in vegetation. Due to lake pollution, H. limbata was not observed on the U.S. side of Lake Erie between 1953 and the early 1990s.

The female lays eggs in the water which sink to the bottom. They may hatch fairly soon, but many overwinter as eggs and hatch in the spring. The newly hatched nymph is about 1 mm (0.04 in) long. It has a long, roughly cylindrical body and passes through up to thirty molts during its development. It has tusks on its mandibles with which it can dig, three pairs of legs, developing wingpads in later growth stages, and three elongated tails. It lives in a U-shaped burrow which it excavates in the mud, and has seven pairs of feathery gills on its abdomen with which it wafts water through its burrow to increase aeration. In cold northern lakes, it may take two years to develop, but in canals in the south of the United States, this may be reduced to as little as seventeen weeks. When the fully developed nymph rises to the surface to emerge as an adult, it first enters a subimago stage. This is similar in appearance to the adult, but has cloudy rather than clear wings and lacks functional genitalia. …