The Rivanna River is my "river of choice" at the moment since there isn't much water in a lot of the streams that I visit. In the spring and summer, we had way too much rain; now we're getting nothing. But the Rivanna River has water.
I saw the same insects this morning that I've seen in other recent visits to the Rivanna -- common stoneflies (Acroneuria abnormis and Agnetina annulipes), Giant stoneflies (Pteronarcys dorsata), a sprinkling of small minnow mayflies (Baetis intercalaris, mostly), and the occasional fingernet caddisfly larva. This lack of variety is common for this time of year, but before long we'll start to see large winter stoneflies in here (Taeniopteryx burksi/maura) and some the mayflies of spring.
The A. abnormis common stones that I see in the Rivanna, you may recall, are different than those that I see in smaller streams that are in or close to the mountains. Most A. abnormis nymphs that we see have a well-defined yellow "M" on the head -- which is sometimes "interrupted" -- and the abdominal terga are banded; the posterior margins are light, the anterior margins are dark. As an example, we can look at the nymph that I found last week in the Rapidan River.
Every A. abnormis nymph that I've seen in the Rivanna is "type B," as it were; the terga are brown with no banding, and there is no "M" on the head -- at least not an "M" that we can see. (Note that there is an "M" crease or indentation on the head, it's just that it's not colored yellow.)
There is something I want to explore, and I'll let you know if I find anything. Could it be that the "type B" that we see in the Rivanna is a sub-species that is somewhat more tolerant of warmer water than those with the well-defined M? Could it be that habitat is relevant to the issue of which type of A. abnormis we find? I'd like to understand why I see this type nowhere else. I'll dig around in the literature and may check with Steven Beaty to see if any work has been done on this issue.
And the other stoneflies in the Rivanna this morning were...
1. The Giant stonefly, Pteronarcys dorsata. The rocks are covered with them. Very odd. The "striped" abdomen of which Beaty speaks ("The Plecoptera of North Carolina," p. 28) is very clear on this particular nymph.
2. And the common stonefly, Agnetina annulipes. Note that the posterior edge of the wing pads is starting to curve.
And another look at the A. abnormis that we find in the Rivanna. This was a spectacular nymph that measured, I would guess, at least 15 mm.