I've done my best to identify this Chloroperlid I found yesterday (I found at least 2 of them) to the level of genus: I'm not at all certain that I've succeeded. But I'm happy to share the evidence that I've assembled.
I plan to keep working on this and hope to find another, more mature, specimen in the near future.
I have consulted five sources in doing this work, from which I will cite on occasion. They are:
1. Barbara Peckarsky, et.al., Freshwater Macroinvertebrates of Northeastern North America, p. 73)
2. Steven Beaty, "The Plecoptera of North Carolina," pp. 9-10.
3. Bill P. Stark and Brian J. Armitage (Editors), Stoneflies (Plecoptera) of Eastern North America, Volume II: Chloroperlidae, Perlidae, and Perlodidae, pp. 2-3.
4. R.W. Merritt, K.W. Cummins, and M.B. Berg, An Introduction to the Aquatic Insects of North America, Fourth Edition, pp. 326-330.
5. Kenneth W. Stewart & Bill P. Stark, Nymphs of North American Stonefly Genera (Plecoptera), pp. 247-290.
Let me start with some additional photos of our nymph, and then look at parts of the anatomy that are important in making identification.
1) A close-up of the wing pads:
2) Two close-ups of the pronotum (note setae on anterior corners):
3) A close-up of the thoracic sterna (i.e. ventral view of "chest"):
4) A close-up of the lacinia:
5) And a close-up -- but not a very good photo -- of the cerci (tails):
Steven Beaty lists the following Chloroperlid genera from which we can choose: Alloperla, Haploperla, Rasvena, Suwallia, and Sweltsa. Peckarsky adds Utaperla, but Utaperla has not been attested in NC or VA (Stewart and Stark, p. 290).
Let me begin by eliminating some of those options. This nymph is not:
1. Haploperla -- because the hind wing pads diverge from the central axis of the body: the hind wing pads on Haploperla are parallel or subparallel to the body axis. They look like this:
2. Utaperla -- not only because that genus is not attested in the state of Virginia, but also because there are about 25 segments on Utaperla cerci (Stark and Armitage, p. 2): the number of segments on the tails of our nymph is 12.
3. Rasvena -- because mature Rasvena nymphs are less that 5.5 mm in length (Beaty) while our nymph is about 7 mm. Also, there are 4 longitudinal stripes on the tergites of Rasvena, not on the nymph in our photo.
4. Suwallia -- because Suwallia nymphs are 8 - 10 mm in length. (But this is an option I may need to reconsider.)
That would leave us with Alloperla or Sweltsa. Sweltsa is the Chloroperlid we've been seeing all winter: most Sweltsa nymphs have already hatched. This is a photo of Sweltsa:
This does not look like the nymph that I found yesterday. Look at the abdomen: short and wide, bulging a bit in the middle; now look at the abdomen of the nymph pictured at the top of the page. It's long and thin. Here's a microscope view of the two nymphs side-by-side: Sweltsa on the right, new nymph on the left:
The abdomens are strikingly different; so too are the heads in terms of their shapes. Here's another, closer view:
Something else -- Sweltsa nymphs are very hairy: the wing pads are hairy, the tergites are hairy, and, you may recall, there are "thick depressed black clothing hairs laterally on all thoracic sterna," (Beaty) as in the photo below.
Compare that "chest" with the one of our nymph in the earlier picture.
So the "negative" approach leads me to Alloperla. But, on the positive side, what do our keys say about Alloperla? I'll use Peckarsky for this.
Peckarsky (p. 73): "Pronotum with few or no setae on front and, especially, hind margins; setae present only on corners."
I think that's correct for our nymph: I see no setae on the front and hind margins.
Peckarsky continues: "apical 7-10 cercal segments with numerous vertical long setae between apical coronas forming a feathery fringe visible in lateral view."
I'm not sure about this. I am not sure if there are "long setae between [the] apical coronas forming a feathery fringe." Part of the problem is -- and I've tried and tried -- I cannot turn my nymph so that I can get a lateral view of the final cercal segments!
She adds: "setae absent on 8th tergite at mesal posterior margin; integument gold-brown."
This is the best view I can get of the 8th sternite, and I can't tell for sure if there are setae in the middle part of the sternite or not.
__________
So, when all is said and done, I'm not sure where I stand on the ID of this "new" Chloroperlid. I feel strongly that it is not, in terms of the genus, Haploperla, Utaperla, Rasvena, or Sweltsa. If pushed to decide an ID, I'd stick with Alloperla. But I think I still need to look more at Suwallia nymphs. What I need is more specimens on which key features will show up a bit better.
Later note: The illustrations of Chloroperlid pronotums in Stewart and Stark lend support for an Alloperla ID. Almost all other Chloroperlid genera -- including Sweltsa and Suwallia -- have visible setae on the anterior and posterior margins. See pp. 255, 279, and 282.
Also note Tom Murray's images of Alloperla at: http://bugguide.net/node/view/264459/bgimage.
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