Thursday, March 29, 2012
Rhithrogena Flatheaded Mayflies and Diploperla Perlodid stoneflies: A Trip to the Lynch River
This is not a flatheaded mayfly that we see in a lot of our streams: last year I only saw them in Buck Mt. Creek. But today they were all over the bottoms of rocks in the Lynch River. They were prolific; this was the only flatheaded genus that I could find.
The "give away" feature for this genus of flathead is -- quoting from Beaty ("The Ephemeroptera of North Carolina," p. 22) -- "Abdominal gills 1 and 7 enlarged and meet ventrally forming a ventral disk."
This complete gill "oval" on the underside of the nymph is very clear in this microscope view.
Beaty also notes that this flatheaded genus -- in North Carolina, at least -- is "uncommon" and "intolerant" (the TV assigned in NC for the genus is 0.0), and that "A fair amount of color variation and overlap of characters can make Rhithrogena species determination difficult." From the brief descriptions that Beaty provides of 7 possible species, the nymphs I was finding today could be R. uhari -- but that's just judging by size (5-7 mm) and color of tergites.
Knopp and Cormier (Mayflies, p. 137) note that Rhithrogena flatheaded mayflies produce an important hatch for fly fishermen in the West, the "Western March Brown." But Rhithrogena is not considered important for fishermen in the East. "Scattered populations of Rhithrogena species also inhabit certain East and Midwest freestone rivers but are not known to produce very reliable hatches." I'd have to say that there will indeed be a "reliable hatch" of Rhithrogena in the Lynch River this spring, in fact it's probably already starting. Here are pictures of two other nymphs that I found this morning -- I saw hundreds -- and note that the first is fully mature with the black wing pads. (The second, by the way, since tergum "color" is important in species ID, might not be the same species as the other two that I've shown.)
While the rocks in the Lynch were covered with Rhithrogenas, the leaf packs were full of Perlodid stoneflies, in particular, Diploperla duplicata. We've been seeing these in various rivers all winter long, but they're starting to color up now as they start to mature.
This nymph was 12-13 mm long. I did find Isoperla namatas, but they were outnumbered by far by the Diploperlas, not something I would have predicted.
(More signs of spring in Virginia.)