I've been told by the experts that there are streams in Virginia where the "patterned" and "brown" forms of Acroneuria abnormis co-occur (see the entry of 9/23) -- so I'm looking. This morning my friends and I checked out the Moormans in Sugar Hollow but no luck. Still, I found two beautiful Perlids, A. carolinensis (in the photo above) and A. abnormis. I thought I'd post a few photos and note how we tell them apart.
In most cases, as we see here, color is good clue: carolinensis nymphs are lighter in color, more yellow. But there are two anatomical features that seal the deal. 1) The tergal banding is reversed. On carolinensis nymphs the posterior margins are dark, on abnormis nymphs they're light. And 2) there are dark bars on the leading edges of the femora on abnormis: the femora on carolinensis are entirely pale save for dark spots both distal and proximal.
The carolinensis nymph flipped on its back for one of my photos and gave me a good look at its terminalia. The size -- thickness -- of the terminalia is the thing we use to determine the gender. Those of the female are larger than those of the male. I'd guess that this was a male -- but don't hold me to that! (For an illustration of the terminalia of Acroneuria nymphs, see Stewart and Stark, Nymphs of North American Stonefly Genera, p. 315.)