All Robber Flies (Family Asilidae) are predators. They capture prey and inject neurotoxins to immobilize. After also injecting digestive enzymes, they suck out the liquidized innards of their prey, leaving little remaining except for an exoskeleton.
Members of genus Ommatius are easily identified because they are the only North American species with feathery antennae, although these are difficult to see without binoculars or in a photo. You can see the fuzzy moth-like branching of the antennae in the enlarged versions of these photos.
Species identication within this genus is much more difficult, especially for females like the one I photographed, because Ommatius is currently separated into species by characteristics of the male genitalia. On his website, Norm Lavers does note that black on the hind femur takes up more than 25% of the length in O. ouachitensis. The ID I received for this specimen on BugGuide was tentative as are most of the other IDs I found online. Therefore, I will say that this Robber Fly is definitely a member of genus Ommatius and probably an O. ouachitensis.
I photographed this specimen during the late afternoon in a small clearing behind our garden. Becoming most active as dusk approaches seems typical for this species. They perch in open locations waiting to ambush a passing insect. Typical perches are bare twigs, fence wire or, in this particular case, the straw in my mug of water. I've found O. ouachitensis relatively easy to photograph because they tend to remain perched and are not easily disturbed. I've even watched through my camera's viewfinder as one of these Robber Flies suddenly left and then returned to the same location with an insect a second or so later.