Thursday, December 15, 2011

And Then There Were Three: The Large Winter Genus Strophopteryx

This is the Large Winter genus that we normally start seeing at the start of December -- but I've been unable to get into a lot of my streams because of high, muddy water.    Today I finally made it to Buck Mt. Creek where I've found large numbers of these in the past.  This is the genus Strophopteryx, and we can ID it right down to species: Strophopteryx fasciata.

Strophopteryx nymphs are like Taenionema nymphs in having a prominent "ventroapical plate" (and both genera lack the "coxal gills" of Taeniopteryx nymphs).  That plate is very clear in this picture.

But, remember that when looked at straight on, the apical plates of Strophopteryx and Taenionema do not look the same.  Let me quote this from Beaty ("The Plecoptera of North Carolina," pp. 6 and 7).
For Strophopteryx: "apical half of ninth sternal segment concave laterally."  Look at this microscope photo of the "ninth sternal segment" of one of the Strophopteryx nymphs that I found this morning.

But on Taenionema nymphs (remember the photos from Tuesday), the "ninth sternal plate [is] broad apically."  Have a look.

Those photos make it easy for us to distinguish these two genera.  (And for good illustrations, look at Stewart and Stark, pp. 237 and 240).  But these genera are also distinct in terms of color and pattern.  From Peckarsky (Freshwater Macroinvertebrates, p. 67):

18a. Body dark brown, pattern indistinct...Taenionema
18b. Body light brown or yellow, with distinct darker pattern on head and thorax, abdomen distinctly banded....Strophopteryx

Looking at the photo at the top of the page, I'd have to say that this is clearly a Strophopteryx nymph.  But how do we know that it's S. fasciata?   Well, if you look at Beaty's discussion of Strophopteryx, you'll see that there are only three Strophopteryx species in North Carolina (and I'm guessing in our state as well): S. appalachia, S. fasciata, and S. limata.   The nymph in our photo -- just in visual terms -- cannot be S. appalachia, and it cannot be S. limata.  In Beaty's description of S. appalachia features, he notes "abdominal tergites lacking median transverse row of dots."  Look closely at the nymph in the photo at the top of the page, or, better yet, look at the microscope shot of the ventroapical plate: the tergites on our nymph clearly do have a median transverse row of dots.  S. limata is out of the question since for that species the "dorsum [is] uniformly brown."

Now, let's see what Beaty says for S. fasciata: "abdominal terga yellow with uneven, dark brown transverse bands on anterior half of each tergite; median row of transverse dark dots on each tergite; ventral tibial hair fringe present but sparse; no hair fringe on the basal segments of cerci."  For the lack of a fringe on the basal segments of the cerci (tails), again look at the microscope shot of the ventroapical plate.  For the sparse hair fringe on the ventral tibiae, look at the following photo.

Pretty clear to me that this is indeed S. fasciata.  So, all three Large Winter species I normally see in our streams are now present and accounted for.

Strophopteryx fasciata

Taeniopteryx burksi

and Taenionema atlanticum


It was a "stonefly" day at Buck Mt. Creek.  I did find a few flatheaded mayflies and few brushlegged mayflies in the 20 minutes or so I was looking.  But, in addition to the three Large Winter nymphs that I found, I found 8-10 Helopicus (Helopicus subvarians) Perlodid stoneflies and probably the same number of Clioperla (Clioperla clio) Perlodids.  And I got some very nice photos.



It was a very nice way to spend my birthday!

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