The Isonychia nymphs that I found at Buck Mt. Creek, I'm afraid, were something other than I. georgiae. I made a basic mistake. What I thought were single filament forecoxal gills were, in fact, the labial and maxillary palps.
These are the forecoxal gills,
and as you can see they are not single filaments, what we find is a "cluster of filaments." That eliminates the following species from consideration for the species ID: georgiae, obscura, serrata, hoffmani, and similis. With the cluster of filments, it could be Isonychia sayi were there no "marginal spines on the distal margins" of the abdominal gills, but clearly such spines are present.
That being the case, our key -- Larvae of the Southeastern USA: Mayfly, Stonefly, and Caddisfly Species -- gives us the following options: 1) "Gils 6-7 (gills on abdominal segments 6-7) with row of minute robust setae along entire length of median sclerotized ridge," or 2) "Gills 6-7 without distinct row of minute setae on median sclerotized ridge." (Larvae, p. 128) This is the ridge:
and I cannot see what is or is not on that ridge without greater magnification than I can get with my microscope.
So, back to the drawing board. I cannot determine the species. Were I to speculate, I'd go with Isonychia bicolor. That is based on the fact that the unusual abdominal pattern we see on this nymph --
closely resembles Fig. 397 in our key (p. 129) which is an "I. bicolor variant," a species that is "widespread in the Southeast." (p. 128). Best we can do at the moment.