It was one of those pleasant mornings when I actually found what I was looking for -- and I did not have to look very long. This is the common stonefly (Perlid), genus Neoperla, a genus which, to date, I have only found in Powells Creek in Crozet. While most of our Perlids are close to hatching, or have already hatched, the Neoperlas tend to hatch in late summer. So, look for them throughout the summer, and I'd look in "Eccoptura/salamander" type streams, i.e. small streams in which you find a lot of Eccoptura common stones and the salamanders who seem to prefer the same type of conditions.
Neoperla nymphs are unique in two different ways. 1) This is the only Perlid in the U.S. that has only two ocelli -- that's the two black dots at the back of the head. All other genera have a third ocellus in front of these two -- an "anteromedial ocellus" (front and middle). 2) This is one of the few Perlids that is univoltine, i.e. it has a one year life cycle, so you never see more than one generation in a stream at a time.
(See Steven Beaty's "The Plecoptera of North Carolina," pp. 17-18.)
Here are more photos of two of the nymphs that I found this morning: one was clearly less mature than the other, the wing pads were less developed.
Neoperla nymphs have an even row of setae at the back of the head (occipital ridge), which I have pointed out on the following photo.
While I was content to take these photos and leave -- I had accomplished my goal for the day -- I did continue to sort through some leaves and lift up some rocks, and I'm happy to say that the small minnow mayflies are now showing up in good numbers. In the Powells I found both Baetis intercalaris and Baetis pluto.
Baetis intercalaris ("parentheses" marks on the abdominal tergites):
Baetis pluto (tergite 5 is pale; 6 and 7 are dark):
Finally, I did find something I was not expecting to see -- a freeliving caddisfly larva (Rhyacophila fuscula). I should have thought these had all pupated and hatched by now.