mayfly swarm - Google Image Search
http://images.google.com/images?btnG=Search&hl=en&imgsz=xxlarge&lr=&q=mayfly+swarm&svnum=10
Includes weather radar images of Michigan, near where I grew up in Algonac.
JS Online:Radar image has some bugs in it, literally
http://www.jsonline.com/story/index.aspx?id=457479
"The bugs were so thick that they showed up as a rainstorm of mayflies on National Weather Service radar."