We learn by making mistakes: or so it sometimes seems. After careful study of our flatheaded nymph from the Rapidan River -- and a little help from some friends (Steven Beaty, Jessica Fang) -- I find that my original entry was right. This is Maccaffertium pudicum, Tolerance value -- as it should be for this river -- 2.1.
Here's what I discovered.
1) I remeasured the nymph: it is indeed 11 mm. M. pudicum nymphs are 11-14 mm.
2) I was wrong about there being "denticles" on the protarsal claw. What I had pointed to in the photo in my previous entry was a "basal tarsal claw," not a tooth. There are no denticles on the protarsal claw of M. pudicum nymphs.
3) M. pudicum has posterolateral projections on segments 3-9. Here's a better photo of that (I removed the gills on the right side), but I also see a projection on 2. Also relevant to the ID, the projection on 9 is shorter than that on 8. (See p. 33 in Bednarik and McCafferty.)
4) There are 30-40 lateral setae on the maxilla. Tough to see all of them in my photo, but I count at least 31. [For this data, see p. 14 in Philip A. Lewis, "Taxonomy and Ecology of Stenonema Mayflies," published by the National Environmental Research Center in November, 1974, in the Environmental Monitoring Series. (Stenonema was changed to Maccaffertium for all but one species in 1979.]
5) While I can only see, using my microscope, 8 setae on the maxillary crown, pudicum should have 15-40. I've already noted that there might well be more setae on the crown than I can see.
6) And the real key, for me, in establishing that this is a pudicum nymph, is that the ventral pattern is an exact match for one of the two patterns that we should see on pudicum nymphs: notice the splotches on 6-8, the largest/darkest being on 7, and the oblique anteromedial lines.
This illustration is found in Unzicker and Carlson's "Ephemeroptera," p. 3.75, figure 3.214. Their essay is Chapter 3 in Aquatic Insects and Oligochaetes of North and South Carolina, edited by Brigham, Brigham, and Gnilka. This illustration was originally published in A.F. Bednarik and W.P. McCafferty, "Biosystematic Revision of the Genus Stenonema", Canadian Bulletin of Fisheries and Aquatic Sciences, 201 (1979), p. 69, figure 78. Their description (p. 33) reads as follows: "Sterna 4-8 mostly pale with brown markings varying from faint submedian dots and oblique dashes near anterior margins to median broad brown areas near anterior margins (fig. 78); 7 and 8 sometimes with brown maculation expanded to cover much of anterior parts of segments (fig. 79); 9 with brown lateral bands often connected or nearly so by brown cross band on anterior of sternum." (I've underlined a key phrase.)
Heading to Oregon on Thursday. Very excited. Get to see totally different stonefly, mayfly, and caddisfly species.