Mayflies are an indicator species for clean streams and rivers. Most mayflies lay their eggs immediately after mating and the eggs then take anywhere from 10 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), after which she lays them in water and they hatch immediately. This female was dropped onto the water surface moments before the video started. Video credit: David H. Funk