Thursday, August 31, 2017
Stenacron, Acroneuria arenosa -- and other things at the Rivanna
On the first rock I turned over this morning -- at the Rivanna at Crofton -- there were 3-4 Stenacron nymphs; took me by surprise. As you can tell by the wing pads on this one, it was fairly mature which made for some very nice photos.
The genus call on these nymphs -- Stenacron -- is fairly easy to make: they're long and slender, and the gills on terga 1-6 are sharply pointed with the gill on segment 7 being no more than a string. (See Steven Beaty, "The Ephemeroptera of North Carolina," p. 42) Here's a photo of one that I found some years ago.
For that matter, the species ID isn't too difficult either. This is Stenacron interpunctatum. Two things give it away. 1) The "mid-dorsal streaks" on the abdomen are very distinctive, and 2) the "pale spot anterior to [the] median ocellus [is] approximately as long as wide [and] may be triangular to irregularly shaped." (Beaty, p. 42) Bingo.
Beaty notes that S. interpunctatum is "the most common and tolerant Stenacron species in NC": probably the same here in VA. Still, I think it's one of the more attractive flatheaded mayflies.
I also picked a couple of common stoneflies (Perlidae) both of which turned out to be Acroneuria arenosa.
I've gone back and forth on the species ID for these nymphs: A. arenosa or A. evoluta? The difference is slight. While both have anal gills -- clearly visible in both of those photos -- the terga on A. evoluta are "uniformly brown" (Beaty, "The Plecoptera of North Carolina," p. 37), but on A. arenosa there's a "narrow, pale anterior band" on each of the segments. In the past, I've never seen that "pale, narrow band" so I've been tempted to go with A. evoluta. But I could easily see it on both of these nymphs.
Acroneuria arenosa, I'm a believer.
I made one other find, not something I've seen before in the Rivanna.
The dragonfly nymph -- Macromiidae. Might have to key this one out to the level of genus. This was a BIG, BIG nymph.