Mayfly Season Is Here, And It's The Stuff Of Nightmares

An Illinois bridge resembled a scene from a horror movie this weekend when swarms of inch-long mayflies greased the road and blanketed cars in a creepy reminder of the insects' mating season.

The unnerving scene along the Illinois River was showcased in two photos shared by the Havana Police Department Monday morning.

One photo shows a police cruiser caked with the winged hitchhikers’ bodies, leaving one to wonder just how the officers were able to get inside. The other photo shows a road glistening from the mayflies' smashed yellow bodies.

“At one point they had piled 6 inches high and when ran over, became very slick,” the department stated in its Facebook post, which issued a warning to drivers.

Northeast of Illinois, residents in Michigan have already been busy combing through their own mayfly invasions over Lake St. Clair and Lake Erie. Local weather radars have picked up mayfly swarms that resemble storm patterns.

The infestation is an annual event in the region. Last July in Sabula, Iowa, the state's Department of Transportation had to shut down a bridge over the Mississippi River until a snowplow could clear the piles of insects off the roads. Their invasion was described as knee-deep.

In southeast Pennsylvania last June, a bridge had to be closed for two straight nights after multiple motorcyclists skidded across the bodies of the insects and crashed, The Associated Press reported at the time.

But the mayflies’ presence isn't entirely a bad thing. In fact, the real horror would be if they ever stopped appearing, biologists say.

That's because the insects spend most of their lives at the bottom of rivers and lakes and need clean water to survive. When they appear in large numbers, therefore, it indicates not only a healthy population but a healthy environment, said Kent Johnson, a member of the Twin Cities Metropolitan Council who supervises environmental quality for the greater Minneapolis-St. Paul area.

"They're indicators of excellent water quality," Johnson told CBS News during last year's invasion. "As scientists, we can only spot check it once in a while and make some assumptions based on that."

Fortunately, if you're in a mayfly-heavy area and you can't wait to be rid of them, you're in luck: The bugs typically live for only 24 to 72 hours after emerging from the water. But then there's the cleanup.