In some areas, succession occurs by different species. For example, in Utah Epeorus longimanus is followed by E. deceptivus . Some mayfly nymphs are quite sensitive to pollution and are used to evaluate water pollution and stream health. Mating occurs in a swarm, and at these times there may be such dense clouds of mayflies in the air near streams that driving becomes impossible. Because their wings are so fragile, the imagos need calm weather in order to mate.
Males usually swarm very near the water, though swarm formations vary from species to species.
Within the swarm, the insects are always changing positions. When a female enters the swarm, males try to mate with her. Some float to the ground while mating, and others continue flying. Once a male has successfully mated, he will guard the female to make sure that no other male mates with her. The female then flies to water to lay her eggs.