Ommatius robber flies are members of a genus of robber flies. They are medium-sized robber flies with distinctively branching, or slightly feathery antennae (look close, or take a picture and zoom in).
In North America north of Mexico, there are about 13 species in genus Ommatius, and about four or five of these might be found in Missouri. One of the most common species is O. gemma, which is widespread in the southeastern United States. Distinguishing between the species involves looking closely at the body bristles, leg coloration, configuration of reproductive structures, and other details.
As with other robber flies, they can inflict a painful bite if mishandled.
Learn more about these and other robber flies on their group page.