The adult mayflies normally live for only a day or two. They have only vestigial mouth parts and, therefore, do not eat. Larvae live underwater feeding on plants, organic debris or, in some cases, small aquatic prey. At least one species that breeds in the Susquehanna River emerges from the water in such numbers that they create clouds when disturbed from their perches. The old bridge between Wrightsville and Columbia from York to Lancaster County has large ornamental lights that attract the mayflies in such numbers that when the adults die on the bridge they can cause slippery and hazardous driving conditions. In recent years the towns have decided to turn the lights off during the emergence to try to cut down on the numbers that die on the bridge.

Susquehanna River
York County, PA