Tuesday, May 14, 2013
The problem of identifying our Rhithrogena flatheaded mayflies to the level of species
This is one of the flatheaded mayflies -- genus Rhithrogena -- that I found at the Lynch River on 5/2, and in the entry posted that day, I noted that I hoped I could ID these nymphs to the level of species. I've worked on the problem: I haven't resolved it. Let me show you why I'm still up in the air.
In his "Ephemeroptera of North Carolina" (p. 22), Steven Beaty describes seven species of Rhithrogena that occur in that state: R. amica, R. exilis, R. fasciata, R. fuscifrons, R. manifesta, R. rubicunda, and R. uhari. Given the size of our nymphs -- 5 mm -- and the general color and pattern of the abdomen and the head, I'd say we can eliminate all but two of these species: R. fasciata and R. uhari. These are described in the following ways (Beaty, p. 22):
R. fasciata -- nymphs 5-7 mm; tergites orange or orange brown. Spring and summer in Piedmont and Mountains.
R. uhari -- nymphs 5-7 mm; abdominal tergites light chestnut or cinnamon brown, with no pattern. Collected in winter and early spring.
So, what do you say? Is that abdomen "orange brown" or "cinnamon brown"? It does, I think, have a "pattern," so I was tempted to go with R. fasciata. But I decided to look into this further. I'm afraid that what I found just complicated the issue.
Let me begin with a reminder that I've found Rhithrogena in two of our streams: the Lynch River and Buck Mt. Creek. Here are photos of two of the nymphs that I found in Buck Mt. Creek in 2011.
With the sub-triangular white spot on its head, this second nymph is really quite striking, and I thought it might be a genetic anomaly. But then I saw Donald Chandler's photos of Rhithrogena on "Discover Life" (http://pick5.pick.uga.edu/mp/20q?search=Rhithrogena%20jejuna&btxt=microscope&burl=htpp://microscope.mbl.edu). He posts a photo of that very same nymph: it's Rhithrogena jejuna! And if you look at that page, you'll see another photo of R. jejuna which looks exactly like the nymph in the first photo, the one I found on 4/30/11.
I have to conclude that the Rhithrogena I find in Buck Mt. Creek is R. jejuna, a species that does not occur in North Carolina, but one which is found in the Northeast and the Midwest. And, it is an insect that is important to fly fishermen. Look, for example, at Ernie Schwiebert's Nymphs, Volume II: Stoneflies, Caddisflies, and other Important Insects, Including the Lesser Mayflies (p. 485). "The first of nine Rhithrogena species important in my experience is the eastern and midwestern Rhithrogena jejuna. It is the first of the genus to emerge on eastern and midwestern rivers, hatching from the close of the first week in May until Memorial Day." And R. jejuna, along with R. impersonata, is described in Knopp and Cormier (Mayflies, p. 139): "The occasionally encountered East-Midwest species R. jejuna and R. impersonata (synonymous with R. sanguinea) are pale olive brown and bright reddish brown, respectively, which, with their shape, makes them easily distinguishable from other mayfly nymphs that inhabit this region's streams and rivers."
"Pale olive brown"? I think I can buy that. That would mean that our Rhithrogenas (those in Buck Mt. Creek) are the same (R. jejuna) as those that occur in the Northeast and aren't connected to those that are found further South.
But what about those we find in the Lynch? I've found two types of Rhithrogenas up at the Lynch, two different colors. One looks like it, too, could be R. jejuna. For example, this nymph that I found there last year...
and this one as well, which I found on 5/2 of this year along with the one at the top of the page.
Still, most of the nymphs that I found this year were much darker in color...
and I also found dark brown nymphs in the Lynch River last year, this one on 3/28.
I have trouble going with "pale olive brown" for these darker nymphs, i.e., I doubt that they are Rhithrogena jejuna. But we have three other choices:
R. impersonata (bright reddish brown) -- Knopp and Cormier, Northeast and Midwest
R. fasciata (orange brown) -- Beaty, North Carolina
R. uhari (cinnamon brown) -- Beaty, North Carolina
This is where I give up. I really can't make such fine distinctions in color. Since R. jejuna seems to occur in our streams, perhaps these are R. impersonata, the other species that is common in the Northeast, and some of Chandler's photos of R. impersonata could be used for confirmation (http://www.discoverlife.org/mp/20q?search=Rhithrogena&guide=Groups_Insecta&mobile=iPhone). But "orange brown" and "cinnamon brown" seem to be good color choices as well.
That's where things stand at the moment. This one I haven't resolved.