Mayflies live under the water as nymphs for one to three years. Then, they come out in droves to find a mate. The clock is ticking, because they only have 24 hours as an adult before they die. The adults do not have mouths and their digestive systems are nonfunctional at this stage, because they do not have time to use them.
Once they have mated, a female will lay as many as 8000 eggs in the sediment before dying.