Mayflies hatch from eggs  deposited into the sediment in bodies of fresh water, then spend 1 to 3 years as underwater nymphs, burrowing in sediment and eating algae or other organic matter. Once enough time has passed and tempurature conditions are right these nymphs transform into sub-imagoes and move toward the water’s surface. The sub-imago, (aka: dun) is not an adult but has now gained wings. The dun rests on the surface of the water until its wings are dry before it is able to fly.

The adults live less than 24 hours, and their singular purpose are to find mates before death. Adult mayflys of many  species don’t have mouths nor digestive systems, an intersting trait, as they don’t live long enough as adults to need food.

Mayflies swarm, and mate in flight. The males dip up and down, while the females fly straight through the swarms until they are “caught” by a male. After mating, the female lays her (up to 8000 eggs) in the water, which sink to the bottom and soon hatch into nymphs. After mating and laying eggs, the adults die and fall in the water, becoming food for aquatic species.

Since the Industrial Revolution, Pittsburgh has had a reputation as a dirty, polluted steel town.  And until the 1970’s, this was largely true. The city has made major efforts to clean up its environment, including the three major rivers that converge downtown. The return of the mayflies to urban Pittsburgh is one measure that tremendous progress has been made in the restoration project.

A similar re-emergence of mayflies occurred on Lake Erie in 1999. Lake Erie had not seen large numbers of mayflies since the 1950’s, when industry began to discharge massive amounts of organic pollutants into the lake. Mayflies can be important indicators of not only water quality, but also for bottom sediments, where pollutants often settle and can remain for several years.