Looks like this Robber is being attended by Milichiid Flies. See this post and comments.

Fascinating links.

Thanks for the comments, ID help, and link. I checked through the Milichiid Flies in the Guide. They certainly live on the edge! Someplace recently I saw some reference to critters that are below the size threshold of certain predators, thus safe from them.

I forgot to mention how slowly they approach. It is 17 seconds between first & second photos.
Less than 10mm. in 17 seconds is barely perceptible. A reference says that most of the 900+ speciies of NA Robber Flies take only flying prey. There are only a few that take stationary objects.

Pentatomid perhaps? Any photos from that side? Should we move these to Milichidae?

The ID to family seems good, thanks to Stephan and previous submissions. As I mentioned above, it was when I tried to get a shot from the side that I spooked them, so no info on prey. Robber Fly prey in this particular location in the park is 90% Diptera, with the rest Hymenoptera, because that's what's here. Also small bugs they don't seem to bother with. Other locations in the park differ.

Yeah, the Robber Fly is probably too preoccupied to contend with the Milichiids. Besides, it's not economical at all to start catching them when you're already sating yourself over a much bigger meal!