Thursday, August 3, 2017

Is it possible now to identify Perlesta nymphs to the level of species?

I was pleased to see that in our new key -- Larvae of the Southeastern USA: Mayfly, Stonefly, and Caddisfly Species -- six species of Perlesta (Perlidae, common stoneflies) have been described: shubuta, cinctipes, decipiens, teaysia, frisoni, and adena.   At last, I can see which species we have in our streams in May and June.

Let's work on that nymph in the photo at the top of the page, one that I found in Buck Mt. Creek.  The key begins in this way (p. 224).

120  Abdominal terga with dark, freckle-like spots around bases of intercalary setae (fig. 3.245)......121
120'  Abdominal terga without freckled appearance.....123

Lots of dark freckles on this one.  We proceed to 121.

121 Dark transverse pigment band through ocellar area bearing scattered dark spots (Fig. 3.246) .... Perlesta shubuta
121' Dark transverse pigment band on head uniformly colored, without darker spots (Fig. 3.247) .....122

No question that there are scattered dark spots in that band on our nymph.

Pretty darn easy.  However, to date, this species has only been found in AR, IA, IL, KY, MI, MO, MS, and OK -- i.e. not in the state of Virginia.  Still, it's shubuta that our key yields, and I've found nymphs that key out the same in the Rivanna,

and in the Doyles.

Great!  But what about those Perlesta that I've found in the small streams in Sugar Hollow?  Can we ID them as well?   Here's a pic of one of those nymphs.

If you look closely you'll see that there are some freckles on the abdominal terga, but nothing like our nymphs from Buck Mt Creek, the Rivanna, and the Doyles -- i.e. it doesn't have a "freckled appearance."  So, I think we should go from couplet 120 to couplet 123.

123  Pronotum with a pair of dark parenthesis-shaped bars (Fig. 3.249); head without pale M-line .... Perlesta teaysia
123' Pronotum without dark parenthesis-shaped bars (Fig. 3.251); head with pale M-line, but sometimes incomplete ....124

The dark band on our nymph goes completely around the pronotum, it's not limited to the "parenthesis" marks on the lateral edges, and, the M-line on this nymph is clearly present and well-defined.  On to 124

124 Most of dark pigment on head restricted to area of frons anterior to lateral ocelli (Fig. 3.251); known from the Appalachians and areas east of the mountains ... NC, SC, TN, VA, WV .....Perlesta frisoni
124' Dark pigment on head coverng most of frons anterior to ecdysial suture; known from the Cumberland Plateau and areas to the north and west ... IN, KY, OH, TN .....Perlesta adena

The dark pigment is clearly restricted to the "area of [the] frons anterior to [the] lateral ocelli".  We've got Perlesta frisoni.

Same ID for another nymph from the same stream.

So our new key allows us to identify at least some of the Perlesta we find in our streams.  Very exciting news.  But when I asked Steve Beaty about this this morning, he said it's not safe to use this key.

That sent me back to his descriptions in "The Plecoptera of North Carolina,"  (pp. 48-49)  where I found the following statement.

Notes: North Carolina has very high diversity of Perlesta, most of which are currently undescribed in the immature stage.  While many nymphs appear highly freckled, some nymphs have few or inconspicuous freckles.  It is inadvisable to use a new Perlesta key (Stark, 2016 in press) [that's the key we've been citing] as it contains only 6 of the 17 regional species (two of which are not known from NC).  Preliminary use of the key has led to immediate failure for most NC specimens.  It is not uncommon to get 2-3 nymphal habitus forms at one site.

So that's that. Have to follow his advice and leave our Perlesta ID at the level of genus.  Probably stuck at that level for a long time to come.

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