Thursday, March 24, 2011
Making the Case for a Third Large Winter Stonefly genus: Taenionema
In the entry I posted on 3/17 ("Another Secret Stream"), I suggested that the stonefly above was a large winter stonefly, genus Taenionema -- a genus I had not ID'd before in our streams. In this entry, I would like to make a detailed argument on behalf of that view.
When I first started lab work at StreamWatch with Rose Brown, on occasion we had what we were sure were large winter stoneflies, but we couldn't ID the genus. They clearly were not Taeniopteryx winters: they didn't look anything like them, i.e. like this:
And, they lacked "coxal gills," the defining characteristic of that particular genus.
But they also were not Strophopteryx nymphs: again they did not look the part, i.e. they did not look like this:
They lacked the "mottled" head, pronotum, and wingpads and the distinct "banding" on the abdominal gills: they also lacked the "triangular, ventroapical plate" that is characteristic of all large winter genera with the exception of Taeniopterx.
We set them aside. But eventually we decided that they must belong to the Nemouridae family of stoneflies since the tarsal segments on their legs were uneven in length. (As in the photo below.)
Now let me return to the present. Because I do think we have a third large winter genus, i.e. the nymph I found last week in two different places: 1) a "secret" tributary to the Moormans, and 2) Whippoorwill
Branch of the Mechum River.
Let me make my case by beginning with another look at that insect.
When I first saw this nymph in my tray last week, I was sure that it must be some kind of Nemourid, again because it looked nothing like the two large winters I knew: Taeniopteryx and Strophopteryx (should be clear from the photos above). But when I took a microscope look at this specimen, I realized that there was a problem with a Nemourid identification. 1) The tarsal segments on the legs of this nymph are equal in length = large winter stonefly:
and 2) this nymph does have a "triangular ventroapical plate."
So, off I went to my keys: 1) Barbara Peckarsky's Freshwater Macroinvertebrates of Northeastern North America, pp. 66-67, and Stewart and Stark's Nymphs of North American Stonefly Genera (Plecoptera), pp. 235-241. Based on the lack of "long dorsal hairs" on the "proximal cercal segments" (Peckarsky, p. 67) -- which we can see in the picture above -- this nymph had to be either Taenionema or Strophopteryx -- those are the only two choices. I went with Taenionema based on the color.
Let me read you what Stewart and Stark say about Taenionema nymphs: "Body 6.5-11mm, brown with darker brown mottlings on head and thorax." Now let me read you the choices Peckarsky provides:
Body dark brown, pattern indistinct; legs uniformly dark brown .... Taenionema
Body light brown or yellow, with distinct darker pattern on head and thorax; abdomen distincty banded.... Strophopteryx
Still, I wanted confirmation of my conclusion. So, I asked Billy van Wart with VA DEQ to look at the arguments I had made in my blog. (Let me quickly point out that Billy had only my photos to go on, and clearly he is in no way responsible for the validity of the determination I made.) He pointed out that the photo I had done of the "apical plate" and the "cercal segments" was inconclusive: Strophopteryx nymphs look exactly the same from the side. Sure enough -- this is a lateral view of a Strophopteryx plate.
He sent me back to Stewart and Stark to look at the "front on" view of the "ventroapical plate": these are on p. 237 for Strophopteryx (10.8 G and H), and p. 240 for Taenionema (10.10 G and H). The Strophopteryx plate narrows down and comes to a point: the Taenionema plate stays wide and rounded.
Here's a photo I just did of a Strophopteryx plate, front on:
And here's a look at the apical plate from the nymph I found last week:
I think the case is made for identifying these nymphs as large winter stoneflies, genus Taenionema.
So, why haven't we seen this genus before? Could it have something to do with habitat? Both of the streams in which these nymphs were found were small, narrow, clean water streams. Could that be a relevant factor? I suspect that it is.
I realize this entry will seem like "overkill" to a lot of my readers. But this is important to me, and I'm afraid the scholar inside sometimes takes over!