I noticed these flies the other day in our prairie. They are very small–I think they are a type of fruit fly. Each fly is about the size of the head of a housefly.
I watched them doing a little dance. It was obviously a visual display of some kind. The fly would spread its wings and wiggle them, and wiggle its body as well.
Sometimes the fly would show its black butt as well.
Usually when I saw a fly dance I could find another one that was one to two inches away–presumably the intended audience of the fly. Often that fly would dance as well.
The flies pretty much stayed on ragweed and partridge pea leaves, or on the stems of some species of aster.
This dance on a ragweed leaf seems to involve some kind of vacuuming motion with the fly’s mouthparts.