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.

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