Summer is mayfly season on the Upper Mississippi River!
Mayfly nymphs are aquatic insects that swim up from the river bottom and emerge at the surface as their first winged form, called a subimago, during the nighttime. They generally molt into their final adult form, called an imago, the next day. Adult mayflies swarm at dusk to mate and lay their eggs over water. The adults die the following day.
Mayflies are an important food source for many animals. Their abundance is an indicator of water quality. Mayfly swarms can be very large, sometimes even large and dense enough to be detected on weather radar!
These two photographs of adult mayflies were taken the morning of June 5th. The yellow ones, Hexagenia limbata, were found in De Soto, WI. The dusky brown one, Hexagenia bilineata, was found in French Island, WI.
Photo credit: Katie Lieder/USFWS
via: Midwest Fisheries Center - USFWS - Wisconsin