splits them by way of presence/absence of lateral fringes on the pronotum OR that the lateral fringe hairs are not arranged in a single row, + meso- and metanotum without fringes or with sparse fringes near posterolateral corners.

I didn't get a good shot of the posterolateral corners of the meso/metanotum, but all the same it seems like a tough split to be sure about.

I finally found my series of Ostrocerca foersteri nymphs and compared them to a series of Nemoura spiniloba nymphs, both from California. The Ostrocerca were collected in a small seep in Trinity County, while the Nemoura were collected in an ephemeral stream here in Butte County. No other nemourid species were collected at either site.

First impressions: Nemoura is very robust compared to the daintier Ostrocerca. The lateral fringe of spines on the pronotum is regularly spaced and nearly equal in size for Nemoura, but irregularly spaced and varying in size for Ostrocerca. The spines on the foretibia seem to be a little better organized in Ostrocerca, but not by much. It's really easy to see the differences when the two taxa are side by side, but not so much when looking at either one by itself.

In the new Merritt & Cummins & Berg, the key has added a qualifier for the pronotal spine fringe character "or of unequal lengths", which would send one past towards Ostrocerca instead of Nemoura. This seems to be a good quick and dirty character.

Some useful literature

Stewart, K. W., and N. H. Anderson. 2008. The nymphs of three Nemouridae species (Plecoptera) from Oregon temporary headwater streams. Transactions of the American Entomological Society 134(1+2):173-183. -- Ostrocerca dimicki nymph description

Stewart, K. W., and B. P. Stark. 2002. Nymphs of North American stonefly genera (Plecoptera). The Caddis Press, Columbus, Ohio. xi + 510 p.

Stewart, K. W., and B. P. Stark. 2008. Chapter 14: Plecoptera. [pp. 311-384]. In: R. W. Merritt, K. W. Cummins and M. B. Berg (editors), An introduction to the aquatic insects of North America, fourth edition, xvi + 1158 pp. + 39 color plates. Kendall/Hunt Publishing Company, Dubuque, Iowa.