So I went back to check on the torpid bee flies of the previous post later that afternoon... It had "warmed up" to all of 54°F but was still overcast, windy and damp. The fly on the thorn stem hadn't moved, but the one on the tall grass had moved around 180° to the other side of the stem. (I did not merely flip the photo, honest!)
The chance to take more photographs allowed me to notice that what I had believed were barbs on the fly's proboscis were really its antennae!
Same fly, nearly the same photo, but with natural light. I don't like using flash, but sometimes macro photography demands it. (Especially when the photographer is still learning how to work her camera out of "automatic everything" mode...)
I have a guess as to why just this one was encouraged to move; note the two new residents that had appeared at the top of the grass stem... Goodness knows, ticks make me wiggle.