This makes them good biotic indicators of water pollution. About 700 species can be found in North America. Adults do not feed. Adult mayflies tend to be weak flyers. During the spring and summer, female adults fly above the water surface repeatedly touching the water with their abdomens to deposit eggs.
Some species crawl under rocks in the stream to deposit their eggs.
Egg development usually takes a few weeks, although egg hatching may be delayed several months. The length of the nymph stage varies with temperature but is commonly 3 to 6 months. Mayflies typically overwinter as nymphs and then crawl, swim, or float to the surface to emerge. Adults often emerge from the stream simultaneously in an event called an emergence or hatch. Strongly attracted to light, mayflies have been known to completely cover the fronts of cars parked next to bridges or to nearly smother campfires.