The Tomah mayfly has sometimes been referred to as a living fossil. Nymphs have greatly expanded, wing-like flanges on the abdomen, which are reminiscent of characteristics of fossil mayflies from the Carboniferous era. These large abdominal flanges, as well as small bumps on the thorax (midsection) of both nymphs and adults, distinguish the Tomah mayfly from all other mayflies. It is also an unusually large mayfly, measuring nearly an inch in length. This species is the only representative of its genus in the world.
Tomah mayflies complete their life cycles in a single year. Eggs are laid in the stream channel during June, and the larvae, or nymphs, hatch the following November or December. The immature mayflies grow slowly beneath the ice, feeding on decomposing vegetation and algae. After snowmelt in March or April, the nymphs migrate from the stream channel to the adjacent inundated floodplain. Here they become predaceous and feed on other species of mayfly nymphs. This predatory behavior is highly unusual for mayflies, most of which feed on dead plant material. In the floodplain, the nymphs grow rapidly.
During the last two weeks of May, the nymphs molt to the final stage of larval development. Finally, in late May and early June, they crawl out of the water onto an upright stem or leaf and molt to the winged subadult form. This "hatching" period occurs mainly during the late morning and early afternoon hours, and the population emerges over a period of about 10 days. The newly emerged subadults then fly to the forest canopy along the stream, and in about 3-4 days molt to the final adult stage. The adults live from 1-9 days, during which mating and egg laying take place over the stream in the early evenings. They do not feed as adults. Female Tomah mayflies have the ability to reproduce parthenogenetically - that is, they do not require a male to fertilize their eggs. Young produced in this manner are identical genetic copies of their mother.