After mating, adult females, also called spinners, lay their eggs on the surface of streams or lakes; they die afterward. The eggs sink and stay in the bottom until hatching. This period varies from a few days to several months, according to the species and, more often, the water temperature. Females can lay up to 8,000 eggs at a time.
The naiads or nymphs live in the water under rocks or vegetation after they hatch. They construct burrows in the sediments where they shelter. The naiad stage is long, lasting from one to two years. The presence of mayfly naiads in a lake or stream indicates good water quality because this insect is sensitive to pollution. During their growth, naiads shed their exoskeleton several times.
The naiad lives in the burrow and emerges at dusk. The exoskeleton breaks and a winged insect, called subimago or dun, appears. This stage is short, lasting from a few minutes to about two days. During this time, the juvenile reaches its sexually maturity and becomes an adult. In this stage, mayflies become the main diet of many fish species.
Adults have short lives, which last from less than a hour to about three days. The adults have only one purpose: to reproduce. The mating of mayflies always happens at dusk, when males gather in large swarms over the water surface or near the river or lake banks. Adult mayflies are not biting insects.