It's a stunning insect and it's uncommon: the Perlodid stonefly, Isogenoides hansoni. The Rapidan River in Madison County is the only stream in which I have found it.
The genus is defined by anatomical features. It has "submental gills at least twice as long as [their] greatest width," and the "median ridge of [the] mesosternum extends anteriorly beyond [the] fork of [the] Y to [the] transverse ridge." (Barbara Peckarsky, et.al., Freshwater Macroinvertebrates of Northeastern North America, p. 71.) Naturally, we need microscope photos for a look at these features (these are photos of a previous nymph; I did not keep the nymph that I found today.)
For the species ID, let's turn to Beaty. "I. hansoni -- nymphs 16-24 mm; large denticles on the ventral mandibular tooth; conspicuous, sharply delineated M-shaped pale mark anterior to median ocellus; ocellar triangle bordered by dark but with pale central spot; dark transverse bands on anterior third to half of terga 1-9 and a dark, transverse band along each posterior margin. Relatively rare. Recorded from GSMNP." ("The Plecoptera of North Carolina," p. 22)
I can't be sure of the length of our nymph, but my guess would be ~ 20mm, and I have not looked at the "ventral mandibular tooth". The other features are easy to see in the following photo. (Note that I marked the "median ocellus" as the "anterior ocellus".)
Even better:
(For a more detailed description of Isogenoides, see Stewart and Stark, Nymphs of North American Stonefly Genera, pp. 403-406.)
Beaty notes two other things of interest about Isogenoides: "Primarily collected in small streams to small rivers in the Mountains from late September through April" and "Nymphs have been documented as inhabiting steams that support trout populations." (pp. 22-23) A good description of the Rapidan River.
I've been fortunate to see at least one of these nymphs at the Rapidan every year since 2011. Always makes my day.
Other photos.
1. Large winter stonefly, Taeniopteryx burksi/maura. Fully mature (black wing pads, black prontum.)
2. Large winter stonefly, Taenionema atlanticum -- of which I got a very nice photo.
3. Pronggilled mayfly, Paraleptophlebia (mollis?). The leaf packs were loaded with these, along with brushlegged mayflies and Isoperla montana/sp. Perlodid stoneflies.
4. One of the many Isoperla montana/sp. Perlodids I had in my bowl -- and returned to the stream.
5. And naturally I picked up a few Uenoids (little northern case-makers). They turned out to be Neophylax consimilis. Loved the case on this one.
This one too.
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And these are the riffles inhabited by I. hansoni -- though I always find them in leaf packs. (Photo taken last spring.)
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