Once a year, the bugs emerge — millions of them. Every summer, they swarm en masse around the banks of the Mississippi River. It's mating season for mayflies.

There are so many of them, in fact, that they can show up on weather radar.

Check out this weather radar GIF from the evening of July 20, which shows clouds of flies leaving the Upper Mississippi River in Wisconsin and taking to the air to breed.

Though they live underwater for a few years, depending on the species, they only last a week or so after leaving the water — just enough time to breed.

After taking flight, they swarmed the towns of La Crosse, La Crescent, and Stoddard, traveling generally to the north.

Though it happens every year, this swarm is particularly large. The National Weather Service compares this to the 2012 hatching event, earning the "massive emergence" tag in their records.

In 2012, there were so many that officials used snowplows to remove them from the roads.

Though the swarm may appear terrifying, and causes car accidents, it's actually a good sign for the health of the river. The flies are sensitive to pollution and don't survive in dirty water. And they provide a veritable banquet for birds, bats, and other predators.

After finding a mate and breeding, the females return to the river to drop their eggs, and then die.