A friend of mine contacted me yesterday, asking how I had determined that the Roach-like Stoneflies that I've found in our streams are genus Tallaperla. She also enclosed a photo of a Peltoperlid she had just found which -- like the nymph in the photo above -- had very clear "dark pigment spots located lateral to [the] ecdysial suture on [the] meso- and metanota" (Stewart and Stark, Nymphs of North American Stonefly Genera, p. 292.). Those spots are a defining characteristic of the genus Peltoperla -- they are not found on Tallaperla nymphs.
Well, what to say? Clearly this was one of those places where I had reached a hasty conclusion without doing my homework. In my defense, I was following the key in Peckarsky, Freshwater Macroinvertebrates, p. 62) which does not distinguish the two genera. But it was time to revisit this issue. So I turned to more detailed sources of information, consulting, specifically, 1) Stewart and Stark (the book mentioned above, pp. 292 --- 306) and 2) Merritt, Cummins, and Berg, ed., Aquatic Insects of North America, p. 318. In the latter we read: Peltoperla -- "Four paired, light-colored round spots on pronotum; paired dark pigment spots on meso- and metanota"; Tallaperla -- "Pronotum without paired, light round spots; prominent dark spots absent from meso- and metanota."
Stewart and Stark, of course, go into much greater detail, noting that there are three Peltoperlid genera that inhabit Appalachian ("our") streams: Peltoperla, Tallaperla, and Viehoperla. We can rule out Viehoperla for the nymphs that I've found. Number one, because it is not attested in the state of Virginia (though clearly, that's not conclusive). But number two, because the thoracic gills on Viehoperla nymphs are single, but they're double on the other two genera. All of the nymphs that I've found have "double" gills (see the picture above). (Viehoperla is discussed in detail, and fully illustrated, by Stewart and Stark on pp. 302-306.)
On the genus Peltoperla, Stewart and Stark (p.292) make the following points: "Diagnostic Characters: (1) roachlike habitus, (2) gills absent on prothorax, but PS2 and PS3 [those at the base of the meso- and metanota] double, and (3) dark pigment spots located lateral to ecdysial suture [the line running down the middle of the back] on meso- and metanota." They continue, "The other Appalachian genera Tallaperla and Viehoperla also have the general roachlike habitus, but Viehoperla nymphs have gills PS2 and PS3 single, and Tallaperla nymphs lack the dark meso- and metanotal pigment spots." In describing this genus elsewhere they add -- in agreement with Merritt, Cummins and Berg -- that these nymphs are "brown with circular pale markings on [the] pronotum."
So, for Peltoperla -- we should see "dark pigment spots" on the meso- and metanota and "four, paired, light-colored round spots on the pronotum." However, it is my understanding that the light-colored spots on the pronotum are not always there, and that the presence or absence of these should not be used for identification. (Private correspondence from an expert with the North Carolina Division of Water Quality.)
All of that to say, that I'm now convinced that the Roach-like stonefly pictured at the top of the page is genus Peltoperla.
So too is this one.
Both of these nymphs are from a small tributary to the Moormans in Sugar Hollow, the former found on 4/20, the latter (the photo directly above) on 6/13. And note that the nymph in the photo above is fully mature, with dark wing pads -- ready to hatch.
Contrast that with the nymph in the following photo.
This nymph was found in the Rapidan River on 5/4. It's also fully mature -- but there are no dark pigment spots on the meso- and metanota. Not Peltoperla -- it must be Tallaperla. So too is the following nymph found in the Rapidan River on 2/23.
So there you have it. Clearly we have both Peltoperla and Tallaperla Peltoperlids crawling around in our streams. Now, the streams where these different genera were found are really quite different in nature. Though the one -- the Moormans trib -- is small and drops sharply down the side of a mountain, the other -- the Rapidan -- is a wider stream with reduced gradient in the areas where I sample.
Still, they both make good habitat for Peltoperlids: clean, cold, rocky water in forested land where the stoneflies can find lots of leaf packs (that's where they like to live).
One final point. Though I cannot see any "paired, light spots" on the pronota of the Peltoperla nymphs in the pictures above, I can make them out, using my microscope, on a small nymph that I had preserved.
Maybe they're just harder to see when a nymph ages and darkens in color.
So we've learned something, thanks to my friend who enjoys the work of genus and species ID as much as I do, and who has a very good eye for detail.
Additional note: From what we have read, our Peltoperlas are probably Peltoperla arcuata and the Tallaperlas are Tallaperla maria. For more information on the types of streams in which these species ae found, see: http://www.hindawi.com/journals/psyche/1995/061675/abs/.