Call them what you will: Fish fly, May fly, Caddis, Canadian soldier, Shad fly, Sand fly. They are about to invade Vermilion in the millions. May flies generally hang around for three weeks, but can be with us six weeks.
Genus ephemera - the larger, lighter-colored ones, not long ago were rare. As water quality improves, they have become abundant, possibly as plentiful as their smaller, browner cousins, the genus hexagenia.
May flies are aquatic insects whose immature stage usually lasts one year in fresh water. The adults are short-lived, from a few hours to a few days depending on the species. About 2,500 species are known worldwide, including about 630 species in North America.
The may fly are pollution–sensitive animals. This means if mayflies are in or around the water, the water should be good quality, perhaps even good enough to drink without distilling or boiling.
It often happens that all the may flies in a population mature at once (the hatch), and for a day or two in the spring or fall, may flies will be everywhere, dancing around each other in large groups, or resting on every available surface.
Both immature and adult may flies are an important part of the food web, particularly for carnivorous fish such as trout in cold water streams or bass and catfish in warm water streams. But they are also an industrial nuisance, as the large population of dead adults can clog the intakes of air and water supply systems.