Mayflies are aquatic insects belonging to the order Ephemeroptera, which also contains dragonflies and damselflies
The immature stages are freshwater loving "nymphs". These nymphs must have clean water to survive and the presence of them indicate an unpolluted environment.
Young mayflies live in the water their whole lives and can live for years in the Nymph stage.
When the young mayflies become adults, they will shed their exoskeletons in the water.
June-July is the time of year when they hatch, and they can be very noticeable. A swarm of young mayflies is a common occurrence in Tennessee.
This is how trout anglers came up with the term 'match the hatch' when fly fishing. The fisherman will use a swarm of mayflies to disguise his lure.
Adult mayflies do not bite, sting or carry disease-causing organisms and only live a day or two.