We had what appears to have been a Mayfly hatching swarm near our pond last week. These primitive insects have been behaving this way since before there were dinosaurs. There are thousands of species of Mayflies, but only a few hatch in May or other Spring months.
When Mayflies do hatch and swarm, the newly-formed adults arise, mate, lay eggs in the water, and die – all within hours. These adults don’t eat anything (they don’t even have functional mouths). Their presence is a good sign: their eggs and resulting larvae live (sometimes years) on algae and only in non-polluted water, where they are part of the food chain. (Brooklin, Maine)