Why do mayflies emerge in huge swarms?

Mayflies emerge in massive swarms primarily to facilitate reproduction within their extremely short adult life spans. Because adult mayflies possess nonfunctional mouthparts and cannot feed, their brief existence—often lasting only a few hours or days—is dedicated entirely to mating and egg deposition.1

To ensure successful mating, male mayflies perform a "mating flight" or dance over water, which serves as a visual attraction for females. By swarming in such incredible numbers, these insects increase the likelihood that males and females will locate one another quickly before their life cycle concludes. Once mating is completed on the wing, the female deposits her eggs in the water and dies shortly thereafter.12