This is the reason that I went to the Lynch River this morning. While I've found some Rhithrogena flatheaded mayflies at Buck Mt. Creek, they are prolific at the Lynch at this time of year -- at least in the riffles that I explore. The water there is cold and fast, and the Rhithrogenas are tailor made to "cling" to the rocks in these kinds of conditions. "Abdominal gills 1 and 7 enlarged and meet ventrally forming a ventral disk." (Steven Beaty, "The Ephemeroptera of North Carolina," p. 22) The gills on the underside of the nymph look like an oval "suction cup". You can almost see that in one of the live photos that I got this morning...
But it's only totally clear in a microscope view.
When you see these nymphs on the rocks, they look almost exactly like Epeorus pleuralis -- at least until they move. E. pleuralis nymphs never advance in a straight line -- they move diagonally on the rocks: Rhithrogena nymphs march straight ahead. The other way to tell them apart? Rhithrogenas have three tails (vs. the two on Epeorus pleuralis).
Rhithrogena is not considered an important insect for fly fishermen in the East: Knopp and Cormier (pp. 137-148) refer to it as the "Western March Brown." But were trout still common in the Lynch River -- and I'm told that the Lynch was once a very good trout stream -- I have no doubt that this is an insect the fishermen would imitate at this time of year.
Species? Still not sure, but I hope to solve that real soon. My guess is that these are either R. fasciata or R. uhari. Both are 5-7 mm: those in the photos that I took this morning were a full 7 mm being fully mature. Not that I didn't see a lot of nymphs that were not yet mature.
The other insect I saw in very large numbers this morning was the "net-winged midge" -- family Blephariceridae, genus Belpharicera: tolerance value, 0.0.
Note those 6 "suckers" in the middle of each of the segments. Again, it's that anatomical feature that makes this an insect that's well adapted to fast water conditions. While I saw a lot of the larvae, like those in the photos above, I saw even more Blepharicerids that had already turned into pupae. Odd-looking things, as you can see in this shot of a larva and pupa side-by-side. (Note the bulging eyes on the pupa.)
There's a detailed article online by Gregory Courtney on the "Biology of Blephariceridae" for those who would like to know about this hardy member of the order Diptera (http://www.ent.iastate.edu/dept/research/systematics/bleph/biology.html).
One more pleasing find in the river this morning -- another Rhyacophila ledra/fenestra free living caddisfly larva.
The "muscle scars" at the back of the head and on the pronotum could hardly be clearer.