In this photo provided by the U.S. Fish and Wildlife Service a female "Hexagenia limbata" mayfly. The National Weather Service captured a massive swarm on radar Sunday, July 20, 2014 as thousands of ... more 
In this photo provided by the U.S. Fish and Wildlife Service a female "Hexagenia limbata" mayfly. The National Weather Service captured a massive swarm on radar Sunday, July 20, 2014 as thousands of the flies, commonly found in tributaries of the Upper Mississippi River, came out of the river and drifted north on the wind. (AP Photo/U.S. Fish and Wildlife Service) less 
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Associated Press | Photo By U.S. Fish and Wildlife Service
Fri, Jul 25, 2014 5:09 PM EDT