There were quite a few of these resting on blades of grass, some in what appeared to be threesomes. I could never get a photo of these, but I was able to capture one of these clusters. It appeared to be two live flies and one dead one. A mating pair of dance flies with a nuptial gift? I had misidentified these as robber flies, but just read Loarie's and Kueda's comments to curiousgeorge and reconsidered my identification.