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Obviously this isn't a great photo, but it is the best I could do under the circumstances. This swarm of very small flies had maybe 50 individuals in it, flying around in a big circle.

Despite the lack of detail and sharpness, can anyone name the family?

I wish I could have found one perched on a leaf, but I couldn't.

Location: Near the Ohio River, West Virginia, USA. Date: 3 June 2007. Size of each fly: very small.