Hah! Every once in a while you get lucky...
It turns out that the beast shown above was getting ready to do its molt from dun to spinner, becoming fully adult.
Here is what it looked like a few minutes later.
Single shot, f/16 at 1:1 using Sigma 105 mm macro lens. After the molt, this thing is way too active to shoot a stack with my current equipment.
Comparing this picture with the earlier one, you can see that the row of "large" hairs along the trailing edge of the wings has apparently disappeared during the molt. I took a couple of higher resolution shots with the Oly on bellows, to see if this was completely true. Yep, sure enough, no hairs after the molt.
--Rik