The Giant stonefly in this picture -- one of the many I found in Sugar Hollow on Thursday -- is the same species that I always find in this small mountain stream. I've found the same species in the upper Doyles River, shortly after it flows out of the Blue Ridge. The one above is still immature, but it will get bigger and bigger as we progess through the winter, as we can see in the photos below taken in the early part of this year (same stream).
What is the species? In an entry posted on 2/2 of this year, I concluded -- through a process of elimination -- that this must be Pteronarcys scotti. But if you're a regular reader, you'll know that I've never been totally sure of that ID. So I've continued to work on this problem.
When I was reading Peter Claassen's Plecoptera Nymphs of America last week, looking at his key on Giant species, there was something that caught my eye: "Hooks appressed to body, small on segment seven, and inconspicuous on eight; cerci less than half as long as abdomen [Bold type added for emphasis]; front angles not noticeably produced... proteus." (p. 29) These points were meant to contrast with Pteronarcy biloba, on which the abdominal hooks are certainly not appressed on segments 1-8 -- they really stick out from the body -- on which the cerci are long, and on which the front angles of the pronotum are prominently "produced" (hook). To wit:
But it was the comment on the short cerci that drew my attention. On the cerci of P. proteus nymphs, Beaty comments: "...cerci more than half to three-quarters the length of the abdomen." ("The Plecoptera of North Carolina," p. 28) Hmm..... This is important since, in the entry I wrote on 2/2, my elimination of P. proteus as an ID for the nymphs I'd been finding was primarily made because of these comments on the length of the cerci (tails): clearly the tails on our nymphs are short -- very short.
So I decided to see if others had posted photos of P. proteus nymphs, and they had. There are excellent photos by Jason Neuswanger on Troutnut.com (http://www.troutnut.com/specimen/490), and on Bugguide.net, you can find P. proteus photos by Tom Murray and Donald Chandler (http://bugguide.net/node/view/196714/bgimage). The nymphs in their photos all have short tails -- they're an exact match for the nymphs that I have been finding.
Yesterday, I decided to contact Steven Beaty, seeking clarification. He generously took the time to give me a lengthy response. I won't quote him directly, but here's is the gist of what he had to say. 1) On the length of the cerci, yes, some P. proteus nymphs do have short cerci, and when he has time he intends to revise and re-word the entry on Pteronarcys in his Plecoptera document. 2) The critical difference between P. proteus and P. scotti -- very similar species -- is the ratio of the posterior surface of the "hook" on segment 5 to the width of segment 5. This is a point he had already made in his Plecoptera document: "...the length of [the hooks] on segment 5, as measured on the posterior surface, one-third to one-fourth the length of the tergite." (p. 28) Those on P. scotti nymphs are "one-fifth to one-sixth the length of the tergite."
Time to measure. In the photo below, the "posterior surface" of tergite 5 that has to be measured is set off in arrows.
The ratio on the nymph I found on Thursday was .28. Bingo! These are P. proteus nymphs.
But there is additional evidence for us to use. Look at the hooks on segments 6, 7, and 8. While the hook on segment 6 resembles those that precede it, that on segment 7 just barely sticks out from the body, and that on segment 8 is not much more than a bump. Beaty referred to the projections on terga 7 and 8 as "knobs". On P. scotti nymphs the "hook" on segment 6 is also knob-like while that on segment 8 is essentially non-existent.
When I went back to Claassen's lengthy description of P. proteus nymphs (pp. 30-31), I discovered that he said much the same thing about the "hooks" on segments 7 and 8: "...proteus has only a suggestion of hooks on abdominal segment eight, and even on seven the hooks are often inconspicuous, while the remainder of the hooks are always close to the body." (This is again said in contrast to the hooks on P. biloba.)
Point, set, and match. The Giants we find in our small mountain streams are P. proteus Giants. Now I can move on to other things! Oh, one other thing. The "common name" for P. proteus is the "Appalachian salmonfly," and it is primarly found in the northeast from Maine to Virginia (see:http://eol.org/pages/612870/overview): the common name for P. scotti is the "Carolina salmonfly," and it is primarily found in Tennessee, North Carolina, South Carolina, and Georgia (http://eol.org/pages/612869/overview).
(There is a detailed study of P. scotti, with illustrations of the fifth tergites of P. proteus and P. scotti, by Willam E. Ricker. See his Systematic Studies in Plecoptera (Bloomington, IN: Indiana University Publications, 1952, pp. 147-151.)