Adult mayflies typically live for only a day or two; they live fast and go out with a bang. Often all the mayflies within a population will hatch at the same time, blanketing the skies in superswarms. The briefness of their lives requires that they emerge in concert so that they can breed.
Of the roughly 2,500 known species of mayfly, about 630 of them can be found in North America. Since the insects are harmless, without stings or even mouths for biting or eating, mayfly swarms are understandably more welcome than many other insect swarms, even though some swarms are so thick that they can become road hazards.