>> they're like a tornado. >> reporter: a tornado of flies? >> yeah. >> reporter: and like a tornado, the swarms are so large they can be tracked on radar. though they don't bite or sting, mayflies can still elicit terror. some river towns even turn off their street lamps to keep the light-loving bugs at bay. that's because mayflies drop dead after an exhausting 24-hour mating frenzy, leaving piles that can make driving treacherous. as overwhelming as these mayflies can be, they actually bring some welcome news in a messy package. >> it's kind of a celebration for biologists. >> reporter: kent johnson supervises environmental quality for the greater minneapolis-st. paul area. he says the flies spend most of their lives at the river bottom and need clean water to survive. >> they're indicators of excellent water quality. we can only spot check it once in a while and make some assumptions. >> reporter: so, these may flies are like a cheat sheet?