A mayfly is a small, delicate insect that lives near rivers, streams, and lakes. The adults often live only a day or two — sometimes just a few hours — which is why people also use mayfly, a little poetically, for something that does not last. Though actually, the young live in the water much longer; they come out, grow wings, fly, mate, and die in a rush. They can appear in great clouds in late spring, around May, which is how they got the name, but you may see them in other warm months too. People who fish with a rod know them well, because fish rise to eat them. Fly here is the insect, as in dragonfly and butterfly, not the verb to fly.

蜉蝣（壽命極短的水邊昆蟲）