Millions, if not billions, of mayflies hatching along the Mississippi River near La Crosse, Wisconsin, gathered in a swarm large enough to appear on local radar systems early last week. The swarming insects have been blamed for a three-car accident that sent one man the hospital.

Though the epicenter of the swarm was in La Crosse, the expanse of mayflies stretched from Red Wing, Minnesota, to Prairie du Chien, Wisconsin. Zack Taylor, a meteorologist with the National Weather Service, told the Minnesota Star Tribune the mayfly swarm looked the same on the radar as does a minor rain squall.

The Mayflies appeared as 40 dbz reflectivity on radar, but not a drop of rain fell. http://t.co/EKMYSOG0Wn pic.twitter.com/53dBprwvte— Capital Weather Gang (@capitalweather) July 24, 2014

Mayflies rise from the surface of the Mississippi River to mate a handful of times every summer. After mating, the females return to the river to lay their eggs and die. The males fly about for another few weeks before puttering out.

"What made this unique was the massive number of insects that were involved," Taylor said. "The signature on the radar was pretty impressive."

The frenzy lasted a few hours, covering nearby buildings and roads. The swarm was responsible for a three-car accident, when one car lost control as the road became slick as a sheet of insects was crushed under tires. The swarm also made visibility exceedingly difficult. The car spun out of its lane and hit two cars traveling in the opposite direction. One man was injured and taken to the hospital, but was expected to make a full recovery.

Massive amount of mayflies emerge in Wisconsin http://t.co/GGfx2aHRIu pic.twitter.com/R9ddbb79Hu— FOX 32 News (@fox32news) July 23, 2014

Despite the momentary dangers they may cause on roads, the mayflies are mostly a welcome sight. Not only are they a nutritious snack for fish and a vital part of the aquatic food chain, they're also evidence the river's water is relatively healthy, free of toxins that might otherwise stunt the flies' development.