Tuesday, January 17, 2012

On the Genus ID of our Winter Nemourids

This Nemourid stonefly was found in Buck Mt. Creek on 3/1/11, and as you know, I just found a small Nemourid in the same stream on Sunday (1/15/12).  In past entries (2/20/11, 2/21/11, 3/2/11, and 3/10/11), I've argued that the our winter Nemourids are genus Nemoura.   I now have good reason to believe I was wrong.  Shoot!  The first clue -- Nemoura is not attested as a genus that's found in Virginia in Stewart and Stark's Nymphs of North American Stonefly Genera (Plecoptera), p. 198.  It is primarily a genus that's found in the West, though Nemouras have also been found in parts of the Midwest and in some states in New England.

So, let's go back to Peckarsky (Freshwater Macroinvertebrates of Northeastern North America, p. 67).
The first couplet in Nemourid genus ID questions whether there are gills on the "prosternum" (i.e. "cervical gills").  There are not.  We do see "highly branched" cervical gills on our spring Nemourids -- genus Amphinemura (look back to the entries for 3/28, 4/11, 4/15, 4/19, and 4/24).

We move to couplet 21a and b.

21a.  Pronotum with a distinct, single lateral fringe of spines of equal length, sometimes longer on posterolateral margins...22
21b. Pronotum without a distinct lateral fringe of spines, or fringe irregular with long and short spines interspersed...23

This is where I went wrong last year.  Look at the pronotum of one of the Nemourids I kept from last year.

There are tiny spines on the lateral edges, but they are not long enough to be considered a "fringe."
So, we skip couplet 22a and 22b -- which would have taken us to the genus Nemoura -- and move on to 23a and 23b.

23a. Foreleg with a few large spines on tibia and femur; outer margin of tibia with spines of similar length to those on surface and without fringe of long hairs...Paranemoura
23b. Foreleg with numerous large spines on tibia and femur; outer margin of tibia with row of large spines and/or fringe of long hairs...24

We need a microscope view of the foreleg -- especially the outer margin of the tibia.

There is very clearly a "fringe of long hairs," so we move on to 24.

24a. Foretibiae without outer fringe of long hairs...25
24b. Foretibiae with outer fringe of long hairs...26

We've already concluded that we've got 24b., so we go on to couplet 26, which brings us to a choice between Prostoia and Shipsa.

26a. Dorsal and ventral bristles of cercal whorls [the whorls on the tails] longer than lateral bristles; foretibia with hair fringe and row of stout spines along outer margin...Prostoia
26b. Only ventral bristles of cercal whorls longer than lateral bristles; foretibia with hair fringe, but without stout spines along outer margin...Shipsa

Well, you can forget checking on the length of the dorsal and lateral bristles of the cercal whorls -- the foretibiae would have to be slide-mounted to see that, and even then, Beaty says the two genera can be hard to distinguish ("The Plecoptera of North Carolina," p. 4).  But, I do think our specimen has "stout spines along the outer margin."  These are difficult to see, but I've tried to point them out in this picture.

That would mean that our winter Nemourids -- or at least those I have found up to this point -- are genus Prostoia.  And there is another good reason to choose Prostoia over Shipsa: the size of the nymphs.  Beaty says Prostoia nymphs are "small, 5-6mm," while Shipsa nymphs are 6-8.5 mm.  The largest nymph in my collection is 5mm.

I'll be checking on this genus ID as I find more of these Nemourids in the next month or so.  But in the meantime, I'd have to go with Prostoia.


Oh.  P.S.  In the pronotum photo above, you can make out tiny spines on the surface of the pronotum as well as the sides.  That matches the illustration of the Prostoia pronotum found in Peckarsky (#38 on p. 68).

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