I was right about this one the first time (see the previous entry) -- it is, for me, a new taxon: small minnow mayfly, Heterocloeon petersi. I've continued to work on this one since the ID of H. curiosum just didn't seem right. The only thing this nymph has in common with H. curiosum is that both have procoxal or forecoxal gills, the fingerlike projections at the base of the front legs. These:
And that's it. Every H. curiosum male that I've seen has been the very same in color and pattern, which Beaty labels a "conspicuous dorsal pattern" (Beaty, "The Ephemeroptera of North Carolina," p.9). They always look like this:
There is only one small minnow nymph in Beaty's document that accords with this detail: Heterocloeon petersi. His description reads as follows (Beaty, "The Ephemeroptera of North Carolina," p. 9):
H. petersi -- nymphs 7-8 mm; labrum with 1 + 3-5 marginal setae; gills grey or grey-brown with light margin; no dorsal pattern. Found in larger clean mountain streams and rivers.
The lack of pattern is obvious in our photos. Here are some additional photos.
I can't confirm the number of marginal setae on the labrum: my microscope isn't strong enough for me to do an accurate count -- though I can see them. On the color of the gills, they have a greenish tinge in my photos because of the background and light. But I have no trouble convincing myself that, in the right lighting, they would be grey-brown, and I can indeed see a light margin on them.
You can also see the light margins in the photo at the top of the page.
In the end, because of the size of our nymph -- almost 8 mm -- our choices in Heterocloeon are H. petersi and H. berneri, and our nymph does not have the defining feature of H. berneri: it does not have "thoracic sterna and abdominal sternum 1 with protuberances bearing many bristles." (Beaty, p. 9.)
H. petersi is a small minnow species that has only been found in the southeast: in Virginia, North Carolina, Georgia, and Tennessee. (See: http://eol.org/pages/2762495/overview). For the location of other confirmed records in the state of Virginia, see: http://www.npwrc.usgs.gov/resource/distr/insects/mfly/va/91.htm.
Be sure to update your "EPT Species List for Central Virginia".