I've decided to go out on a limb with this one. When I find a new species -- i.e. one that I've not seen before -- I like to have Steven Beaty confirm my ID. But he's out of the office for a couple of weeks so I've decided to lay out the argument. Naturally, I'll correct this if I find that I'm wrong.
I'm quite certain that the common stonefly that I found yesterday at the Rivanna is Agnetina flavescens. The common name for this Perlid is the "Midwestern Stone" since it is found in a lot of midwestern states. But it is also attested in the mid-Atlantic and South -- in PA, MD, VA, NC, SC, and GA (Stewart and Stark, Nymphs of North American Stonefly Genera, 2002, p. 320. There are two things I lack for a positive identification: 1) species ID's are based on features found on nymphs that are fully mature, and my nymph is still very young; and 2) there are things I'd like to check with a microscope -- but I didn't preserve the nymph. That being said, let me present the evidence that I have.
1. The genus ID -- Agnetina -- follows from the fact that there is a regular setal row on the occipital ridge (back of the head), and anal gills are present.
That give us three species to choose from: A. annulipes, A. capitata, and A. flavescens. (There are only three species of Agnetina.) A. annulipes -- which I've found a number of times in the Rivanna -- is a dark brown nymph, on which "tergum 10 [is] mostly dark including the apex" (Beaty, "The Plecoptera of North Carolina, p. 15). It looks like this. (Beaty has confirmed this ID.)
While we could argue that the nymph I found yesterday was still very small and might get much darker as it matures, the very smallest A. annulipes nymphs I have found were already dark brown, including tergum 10. Look at this one from August 11, 2011.
On our nymph, tergum 10 is almost entirely light with a dark band on the anterior edge. (Look at the photos above). It is therefore not A. annulipes.
What about A. capitata? Have a look at Tom Murray's photo of A. capitata in Bugguide.net: http://bugguide.net/node/view/295042/bgimage. Note that the "lateral arms of [the] M-pattern on [the] head [are] directed laterally" (Beaty, p. 15). That is not the case with our nymph on which they are directed posterolaterally -- i.e. they point to the rear.
Our nymph is not A. capitata. I might add that Beaty comments that A. capitata was "Listed by NC Natural Heritage Program as Significantly Rare (2010)."
Thus, through a process of elimination, our Agnetina nymph must be Agnetina flavescens.
2. But let's go at this in a positive way by looking at Beaty's description of A. flavescens.
A. flavescens -- nymphs ?? mm; head pattern roughly M-shaped with arms directed posterolaterally and almost interrupted; a light triangular pale area between lateral ocelli; dorsum of abdomen banded, with dark bands on anterior half of segment, distal segments may have a narrow light band anterior to the dark band; apex of tergite 10 light with narrow dark pigment band interrupted mesally.
a) As we've already noted, the head pattern matches.
b) Since our nymph is still immature, I can't be sure that there is a "light triangular pale area" between the lateral ocelli -- but one does seem to forming.
c) The abdomen clearly is banded with dark bands on the anterior part of each segment. I can't tell for sure if there is a "narrow light band anterior to the dark band" -- but it could be the case. (Really need a microscope view.)
d) Tergite 10. Yes, the apex is light, but is the dark band at the head of that tergite "interrupted mesally"? This is another place where I wish I had a microscope view and/or a nymph that was fully mature. However, I think I can see a pale slot in the middle of that band in the following photo.
And that's all the "interruption" amounts to -- a pale slot. For verification, see the illustration of A. flavescens in Stewart and Stark, p. 318.
3. One last piece of evidence for us to use. There is a photo of A. flavescens on the internet at:
http://commons.wikimedia.org/wiki/File:Agnetina_flavescens_nymph.jpg. Please have a look. It looks a lot like our nymph; you can see the pale slot in tergum 10. But what is most striking to me is the pattern on the wing pads. Have a look at the wing pads on our nymph.
On each of the wing pads, there are two, pale, pointed projections pointing forward (note the arrows). The wing pads in the photo posted on the website noted above look exactly the same!
So I feel strongly that this nymph is Agnetina flavescens and have added it to our EPT list. If I need to correct this I'll let you know.