This morning, there was a comment on the post: "Your fly is a wasp. ... Pompilidae, or spider-hunting wasps."
Of course, I looked up the Pompilidae. I found them on BugGuide, and studied the description. Among other characteristics I couldn't verify from my photos, there was this,
... the Pompilidae have the pronotum extending back to the tegulae, the pronotum thus appearing triangular when viewed from the side and horseshoe-shaped when viewed from above.
And sure enough, in my three-quarters view I could see the triangle. In the other, the horseshoe shape. I went on to read the section on food:
Larvae feed on spiders. In some groups the females sting and paralyze their prey and then transport it to a specially constructed nest before laying an egg; in others, leave the paralyzed spider in its nest and lay an egg upon it.
So this was why I found the wasp under a rock inhabited by two large spiders; it's either a new-hatched wasp, or a female looking for a spider to lay an egg in. (And the ones that needed to escape were the spiders, not the wasp, as I had supposed.)
There's more! I browsed through BugGuide's image pages, looking for this particular wasp; spider wasp with orange abdomen, black antennae. I found several, in Texas and Florida, probably not the same species as this one. And then, after a dozen or so pages, I started running into photos of clay or mud cases, the "nest" where a wasp would lay one egg, with a paralyzed spider for larva food.
|They look like this one.|
I found what appears to be the same species as mine, Priocnemis oregona, from Oregon to our south, and a match from Port Alberni, on Vancouver Island, northwest of here, classified as simply Prioncemis. There are no samples of egg cases for this species in the BugGuide pages, but these, from Tennessee are typical, although bigger than most. Some are closed, some open;, one even has a larva inside it.
So there we have it; a correction leads to the solution of a mystery.
Oh, and here's the spider that's potentially the next batch of larva food:
|A bit bigger than the wasp, but stealth, speed and venom may best size.|