Thursday, February 2, 2012

Pteronarcys scotti? Trying to ID Some of Our "Giants"

One of the small streams that I visit in a regular way in Sugar Hollow has a healthy population of Giant stoneflies (family: Pteronarcyidae).  I've been hoping to ID these to the level of species -- and I do think they're all the same species -- so I was pleased when I found this fairly large nymph in a leaf pack this morning.  Let's see how we can do -- but let me underscore, before I begin, my amateur status and also note that my conclusion is not unproblematic.

Steven Beaty, in "The Plecoptera of North Carolina" (p. 28), states that there are 5 species of Giants in North Carolina, all genus Pteronarcys.  They are: P. biloba, P. comstocki, P. dorsata, P. proteus, and P. scotti.   At the moment, I would say the Giant in the photo above is P. scotti, a conclusion I've reached, in part, through a process of elimination.  Let me run through some critical features of the other four species.

1. P. biloba: On P. biloba nymphs, the "anterolateral angles of [the] pronotum [are] conspicuously produced into hook-like processes." (Beaty, p. 28)  Here's a photo of P. biloba, and I've noted the hooks.

There are no such hooks on the front corners of the pronotum of the nymph in our picture.

2. P. comstocki: On P. comstocki nymphs, the "lateral angles of pro-, meso- and metanotum [are] produced into spine-like processes." (Beaty, p. 28).  There are no spines on the angles of our pronotum.
If you want to see what those spines look like, go to,

3. P. dorsata: On P. dorsata nymphs there are "no lateral projections on [the] abdominal segments." (Beaty, p. 28).  There clearly are lateral projections on the abdominal segments of our nymph, at least on some of the segments.  For a photo of P. dorsata showing the lack of projections, go to,

4. P. proteus: On P. proteus nymphs, the "anterolateral projections on [the] pronotum [are] reduced, barely discernible," and  the "cerci [are] more than half to three-quarters the length of the abdomen."  On our nymph, the anterolateral projections -- though they are not "hooked" and are not "spine-like" -- are clearly discernible, and the cerci clearly are not "half to three-quarters the length of the abdomen."

We're left with P. scotti.  Let's see what Beaty says about this one, but first, have another look at our nymph, on which I've noted some features.

Beaty's description reads as follows: "...lateral margin of frontoclypeus with a low rounded protuberance adjacent to each antennal pedicel."  I think that works out.  Look at the photo above, and then look at this microscope picture.

Back to Beaty.  "anterolateral projections on pronotum easily discernible" -- yes, they are.

Now I run into a problem.  "lateral hooks appressed and not conspicuous on abdominal segments 6-8, the length of those on segment 5, as measured on the posterior surface, one-fifth to one-sixth the length of the tergite; cerci somewhat reduced, about 0.4 the length of the abdomen; antennae and cerci may be pale with darker medial segments or with a pale medial band."

1) Look at the photo above in which the abdominal segments are numbered.  I can see "appressed" hooks/projections on segments 7-8, but the hooks on segment 6 seems pretty clear.  2) I'm not entirely sure how to measure the length of the hooks on segment 5, but the "outside" edge of the hook is .5 mm, while the entire tergite is about 2mm.  That would make the hook longer than it ought to be.  3) The cerci are 6-7 mm in length; the abdomen is 19-20 mm.  So, that might work out: if the abdomen is 19mm, then the cerci should be 7.6mm.  5) The antennae and cerci both have a "pale medial band."

So there you have it.  An exercise whose results are still inconclusive.  But, for the moment, I'd say the Giants we find in this stream are P. scotti.  Oh.  I should also note Beaty's words -- "Nymphs occur throughout the year in the Mountains,"  and they are "probably semivoltine."  Yes, this nymph was found in the mountains, and yes, the Giants in this stream are indeed semivoltine -- i.e. they take two years to mature.

Some other things that I found this morning.

1. Perlodid stonefly -- Diploperla duplicata (only one).

2. A Leuctrid -- "Rolled-winged stonefly."  Unfortunately, its tails had broken off.  I'll write something on Leuctrids when I start getting good pictures.

3. Perlodid stonefly -- Isoperla namata.  This is the most common Perlodid we see in the spring, and there are a lot of them showing up now in our streams.

4. Large winter stoneflies -- Taenionema atlanticum.  I've seen so many by now in these streams, that I'm ready to see something new.  But these were real beauties.

5. Another Wormaldia fingernet caddis.  This genus is fairly uncommon.

Remember that this is the genus on which the frontoclypeal apotome is symmetrical, either concave or convex.  This one is convex.

6. And finally, a new species of Rhyacophilid that I've not yet identified.  But it isn't R. fuscula, and it isn't R. nigrita -- the two species that I've already identified in this stream.  If I figure this out, I'll get back to you.

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