On Monday I argued that this "brown" Acroneuria Perlid (Common stonefly) that I found at Darden Towe Park appears to be Acroneuria arenosa. I'm still persuaded that that is the case, but I have two additional comments that need to be made.
1) I sent my photos to Steven Beaty, and he confirmed -- that yes indeed -- those are anal gills on our nymph, and that it could be A. arenosa.
He cautioned, however, that we have to consider another ID: Acroneuria evoluta. Here is how he describes both of those species ("The Plecoptera of North Carolina," p. 14).
A. arenosa...dorsum of head with M-shaped pattern, sometimes faint to absent; abdomen uniformly brown; anal gills present. Uncommon but widespread during the spring through fall.
A. evoluta...dorsum of head with interrupted M-shaped head pattern, appearing as a transverse row of 3 light spots in front of anterior ocellus; abdomen not banded; anal gills present. Uncommon. Occurs in Piedmont, Slate Belt, and southern Coastal Plain frivers. Nymphs more common during summer thru late fall.
Critical to the distinction is the "M-pattern" on the dorsum of the head. Before we decide which description seems most appropriate for our nymph, here is a photo of a similar nymph that I found in the Rivanna last year (11/14/13).
It has the same anal gills, and for that matter, the same type of thoracic gills on the sides: gray/black in color, and not very frilly. And the abdomen is uniformly brown; unbanded.
What about the "M" on the head? Well, it is "interrupted" (=A. evoluta), but does it appear "as a transverse row of 3 light spots"? I could go with that on the nymph that I found on Monday, but not with the nymph from last year. I could just as well say that on both nymphs there is an "M" pattern present (=A. arenosa),but not, in this case, "faint to absent."
Tough call. One I can't really make. But there is one other thing to consider: A. arenosa is attested in Virginia, specifically in our part of Virginia, A. evoluta is not. On this point, see: http://explorer.natureserve.org/servlet/NatureServe?searchSciOrCommonName=Acroneuria+arenosa&x=0&y=0, and http://explorer.natureserve.org/servlet/NatureServe?searchSciOrCommonName=Acroneuria+evoluta&x=10&y=7. Stewart and Stark make the same point (see Nymphs of North American Stonefly Genera, p. 316). Of course it is possible that we have found an Acroneuria species that, to date, has been overlooked in Virginia.
At the moment, I'm leaning towards A. arenosa, but the safe call is A. arenosa/evoluta.
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2) Point two is that it is not the case that every "brown" Acroneuria nymph that I've found in the Rivanna has anal gills. Look at this nymph, found on 10/4/13, also at Darden Towe Park.
No anal gills (also note that the thoracic lateral gills are different; they're frilly and white).
This would surely be the 3rd type of A. abnormis noted by Beaty: "dorsum of head without M-shaped head pattern and abdomen uniformly brown." (p. 14) Here is another example.
I would conclude that A. arenosa/evoluta and A. abnormis (brown type) are both present in the Rivanna. This is the only stream in which I've seen these brown Acroneuria Perlids.
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