Just a supposition here, but, I'm thinking these flies are practicing for their big game this weekend against the reigning champion fleas. It seems to me to involve too many in the backfield -- it must be some kind of flea-flicker play.
The water-lily in the nearby pond gives them a feel for playing in a dome, I suspect. Notice how big their front line is. How would you like to be a flea and face those heavyweight hovers?