At about 8:45 p.m. Sunday the National Weather Service picked up this rather beautiful radar event, in which what registers as "light-moderate rain" seems to emanate from the Mississippi River between Wisconsin and Iowa and into Minnesota. But rain it was not. It was a swarm of mayflies. Gobs of mayflies. Piles and piles of mayflies.
The swarm lasted for a few hours and by the time it was over many a windshield and wall was caked in slimy bug carcasses. The swarm was blamed for a three-car pileup in Wisconsin that left one person hospitalized.
Scientists weren't taken off guard by the event—it happens from time to time (a very similar "massive emergence" happened in June 2012) and is actually a sign of the health of the Mississippi. Mayflies gestate under water but once they mutate into winged creatures and rise from the depths they have one job and one job only—to make babies. The swarm seen in the radar above seems to move north because, like a weather system, it is carried that way in the wind.
The event, and others like it, amount to a feast for animals that feed on the mayfly orgy, making it a good time of year to be a bird or a fish—or the owner of a carwash, for that matter.