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Why do mayflies sometimes swarm over blacktopped roads?
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mayfly
An adult mayfly (Ephemera vulgata) in Europe.
Mayflies sometimes swarm over blacktopped roads because they mistake the dark, reflective surfaces for bodies of water, such as streams or ponds. During their mating and egg-laying flights, female mayflies are driven by instinct to deposit their eggs on water surfaces. When they encounter a blacktopped road, the visual cues can lead them to believe they have found a suitable site, causing them to swarm over the road and attempt to drop their eggs onto the pavement.1