La Crosse NWS

Close your mouth while looking at these photos, or a bug might fly in.

When people monitor the weather radar, they often have to make judgment calls about whether to head inside. But anybody who saw what exploded on the Doppler Sunday evening over La Crosse, Wisconsin, better have skedaddled for shelter—or be covered by thousands upon thousands of tiny, horny flies:

This storm-cloud-looking apparition is actually a massive swarm of mayflies, an "emergence" of them, to get all Lovecraftian. Mayfly nymphs spend a year or two in the water (in this case, the Mississippi River) munching on organic decay. Then, when summer arrives, they take flight en masse and proceed to make sweet bug love before immediately going back to the water to lay eggs and die.

This year's emergence was so large people were comparing it to the last Biblical-style, one in 2012. If you don't recall that stunning example of life's majesty, here's a picture (close your mouth before viewing, or a bug might fly in):

And because somebody had to clean up this mess, here is a shot of the aftermath:

The 2012 horde of flies was so dense it may have triggered a multivehicle car wreck, reports WBTW-TV:

The mayfly hatch was a problem up and down the Mississippi. Police in the town of Trenton say a large hatch of mayflies may have triggered a three-vehicle crash on a Wisconsin road.

The St. Paul Pioneer Press reports the road had become slick from the mayflies Sunday evening, causing at least one of the drivers involved in the crash to lose control of her vehicle. They also said visibility was limited at the time of the crash due to the massive cloud of mayflies in the air.

Here are a few photos that the National Weather Service provided from Sunday night. Let's hope somebody burned that gas station to the ground and sealed it in a cement sarcophagus:

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