In a scene straight out of a movie, mayflies took over the outside of a convenience store in the United States. These flies regularly amass by the millions in towns in United States and Europe. Every year a swarm of flies takes over a bridge over the Ebro river in Spain, mistaking the sheen of asphalt under lights for water where they lay their eggs.

Mayflies live underwater as larvae for a year, and the emergence of the swarms gives scientists useful information about the health of the river ecosystem, where they serve as food for fish, birds, and small mammals.

https://www.youtube.com/watch?v=9I2A9N_C_Z8

To see different swarm behavior in action, watch a huge swarm of sardines as they evade underwater predators.