Back to the drawing board on this one, this beautiful, fully mature flatheaded mayfly from the Rapidan River. It was not Maccaffertium pudicum. The problem is, using Beaty's "Ephemeroptera of North Carolina" (version 4), I can't really determine the ID for sure. I'm hopeful that he'll take a look if I send the nymph to him in a vial.
Having looked carefully at all of the relevant features, I've arrived at two possibilities: M. mediopunctatum and M. wudigeum. Let me show you how I got there.
1. The nymph, and it's fully mature, is 10 mm long -- tops. Of the 15 species that Beaty describes, that feature alone eliminates all but the following:
2. Another key feature in Mac ID is the presence or absence of denticles on the protarsal claw (claw on the first leg). They are present on this nymph. Two photos.
3. And another key feature -- the presence or absence of posterolateral projections on abdominal segments anterior to segment 6. They are present on our nymph.
That leaves us with only two possible species ID's: M. mediopunctatum and M. wudigeum.
Beaty's species descriptions read as follows.
M. mediopunctatum -- nymphs 7-10 mm; 0-9 (usually 5) hairs and 4-8 (usually 5-6) spine-like setae on maxillary crown; protarsal claws usually with denticles; posterolateral projections present anterior to segment 6; sternal maculations variable though usually with dark crossbands on sterna 2-8 and 9 with dark, obliquely oriented lateral bands, sometimes connected anteromedially. A facultative summer species only found in the mountains, especially rivers. (Beaty, p. 34)
[Note: Beaty's earlier description of M. mediopunctatum (version 3.3) differs in one significant way. It ends: "sternal maculations variable though usually with dark crossbands on sterna 2-8, and 9 with dark, inverted "U". (p. 19) I've put the key words in Bold.]
On the hairs and spines on the crown -- I can see 8 hairs and 5 spines. (But the hairs are very difficult to see, and therefore to count. There might be smaller hairs that I can't detect with my microscope.)
On the sternal maculations -- I could be convinced that there are "obliquely oriented lateral bands" on segments 1-9. (Note the little red arrows.)
M. wudigeum -- nymphs 8-10 mm (?); no hairs and 5 spine-like setae on maxillary crown; protarsal claws with denticles; posterolateral projections present on segments 3-9; dark coloration on outside edge of pronotum, clear on extreme lateral edge; abdomen mostly brown dorsally, each segment with pair of submedian dark dots; ventrally pale. This recently described species will key to couplet 4 in Bednarik and McCafferty (1979). Collected from upper Wilson Creek and the lower Linville River. Listed by NC Natural Heritage Program as Significantly Rare (2012). (Beaty, p. 35)
Probably not as good a fit as M. mediopunctatum. Our nymph clearly has some hairs on the maxillary crown, and I'm not sure that the pronotum looks like it should (also not sure that it doesn't!).
However, it looks to me like the anterolateral projections do indeed extend from segments 3-9, and there do seem to be paired submedian dark dots on at least some of the abdominal terga.
Well, that's the best I can do. Time to consult with the master. I'm leaning towards M. mediopunctatum. But, I'm bothered by the last line in Beaty's description: "A facultative summer species only found in the mountains, especially rivers." It sure isn't summer, and this nymph is already fully mature.