One of the most common insects I saw at the Rivanna today is this very brown common stonefly (Perlidae), which in the past I've identified as Acroneuria abnormis. A. abnormis nymphs can show up in various forms. Let's look at Beaty on this:
A. abnormis -- male nymphs 15-20 mm, female nymphs 25-30 mm; dorsum of head with a well defined M-shaped head pattern, sometimes with interruptions; posterior margins of abdominal tergites light, dark tergal bands irregular; or dorsum of head without M-shaped head pattern and abdomen uniformly brown; anal gills always absent. ("The Plecoptera of North Carolina," p. 14)
We've looked at all of these patterns before, but to review...
1) well defined "M-shaped head pattern (terga banded)
2) "M" pattern "with interruptions" (terga banded)
and 3) "M-shaped head pattern" missing: abdominal terga brown
This last nymph was found in the Rivanna last year, and it's easy to see why I would have thought it was a "type 3" A. abnormis.
My entry today was to focus again on A. abnormis, pointing out that the A. abnormis type that I find in the Rivanna is one that I never see in our smaller, cleaner streams. In the other direction, I've never seen A. abnormis types 1 and 2 nymphs in the Rivanna. And then there's the question "will we ever know why?"
But then I ran into a problem. Remember, Beaty says of A. abnormis, "anal gills always absent."
Take another look at that nymph from this morning.
Do you see what I see? This nymph has anal gills!
And here's a microscope view.
I'll be damned. It can't be A. abnormis. So, is there an Acroneuria species that provides a good match for the Acroneuria nymphs that I find in the Rivanna? Yes there is: Acroneuria arenosa.
Beaty: A. arenosa -- male nymphs 14-17 mm, female nymphs 20-24 mm; 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. (p. 14)
Perfect fit. "M" pattern faint: abdomen totally brown; anal gills present
Two other surprises today at the Rivanna (Darden Towe Park in C'ville). 1) I saw many, many fingernet caddisfly larvae; I did not see a single common netspinner (common in here are the species H. venularis and H. rossi). The "commons" appear to be gone (pupating?); the fingernets are here in force.
2) I was amazed by the number of Heptagenia marginalis flatheads that I found on the rocks. And I did get some pretty good photos.
The tolerance value of Acroneuria arenosa is 2.4.
(For a second photo of A. arenosa, go to: http://www.discoverlife.org/mp/20p?see=I_DSC2&res=mx&flags=no_slide_show.)