Swarm Of Flies Shows Up On Weather Radar

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.
This radar picture from the evening of July 20 shows clouds of mayflies leaving the Upper Mississippi River in Wisconsin and taking to the air to breed.