The Leucrocuta flatheaded mayflies I've been finding this summer and fall appear to be L. hebe in terms of the species -- that's the "Pale Evening Dun" (PED) to fly fishermen, an important hatch throughout the summer. The nymph in the photo above was found in the Doyles River on 6/17; the one in the photo below was found in South River, Greene County, just last week, on 9/12.
This is a small mayfly, 6-8 mm when it's mature, but I begin to see them in the spring when they're very, very small. I believe I mentioned last year that when they're immature, it's difficult to pick them up with your tweezers without doing some damage: I no longer pick them up at that stage of the game.
Leucrocuta flatheads are typically "collected from small, clean streams, from riffles and pools" (Steven Beaty, "The Ephemeroptera of North Carolina," p. 19). I see a lot of them in the spring in very small streams, streams that are too small to fish (but there are trout in the Doyles and the South).
Beaty describes the genus in the following way: "Head wider than pronotum (pronotum widest near middle); dark freckles on anterior portion on head; claws with distinct denticles; gills of abdominal segments 1-7 similar in shape but 7th gill smaller; gill 7 lacks fibrilliform portion; three caudal filaments with no intersegmental setae." (Beaty, p.19) All of those features -- save for the denticles on the claws -- can be seen in the following photo:
What about the species ID? Beaty urges us to leave the ID at the level of genus, but he notes L. aphrodite and L. hebe as the species most commonly seen in his state, in addition, "Leucrocuta in the Piedmont are probably aphrodite."
Knopp and Cormier -- Mayflies: An Angler's Study of Trout Water Ephemeroptera, pp. 149-153 -- note that L. aphrodite and L. hebe both hatch out as PED's, though some fishermen call L. hebe the "Little Yellow Quill." L. aphrodite is common in the East, hatching mainly in the spring; L. hebe is found in the East and the Midwest and hatches in the summer and fall. They also help us to distinguish the nymphs (see their chart on p. 152). The key is the abdominal pattern. For L. aphrodite "tergites 9-10 [are] paler"; for L. hebe, the color is "grayish brown with paler U-shaped spotting on [the body]." On the nymphs that I'm finding, tergites 9-10 are not paler than the rest, but there are "U-shaped" spots on the body, especially on terga 4-5 and 7-8. Note this mature L. hebe that I found on 9/12.
I don't think there's much doubt that the Leucrocuta flatheads that I see in the summer and fall are L. hebe. For those that I see in the winter and spring -- I'll have to check them closer next year: it's certainly possible that both species show up in our streams.
(For other photos of L. hebe -- both the nymphs and the adults -- see Donald Chandler's posting on Discover Life: http://www.discoverlife.org/mp/20q?search=Leucrocuta+hebe.
Additional photos of L. hebe nymphs: