Mayflies are an important living indicator of clean streams and rivers. Most mayfly adults lay their eggs immediately after mating, after which the eggs take anywhere from ten days to many months to hatch. Cloeon cognatum is an exception because it is ovoviviparous, which means that a mated female holds her eggs internally until embryonic development is complete (about 18 days). She then lays up to 2,300 eggs, which hatch as soon as they hit the water.
In this video, Stroud Water Research Center Entomologist David Funk captured the moment a C. cognatum female laid a brood of eggs that hatched immediately into hundreds of nymphs.