I drove into town today and noticed clouds rising from the flowers in the ditches. They were clearly visible from 30 or 4o feet away.
There must have been thousands of individuals. The clouds shifted and changed shape slowly.
I ran my hand through the cloud and was able to capture a few individuals. Some were injured by my actions, but most flew off as soon as I opened my hand. I was able to get this photograph of one. This fly is very small, less than 2 mm long.
I attended a talk once by a scientist who works for the NOAA weather radar station. He talked about seeing animal migrations with the weather radar, and mentioned that often there would be blips on the radar daily from wetland areas–he assumed they were swarms of insects, but had not had time to totally investigate.
This seemed to be a single species swarm, but there seem to be a number of species which have similar behaviors.