Friday, May 11, 2012

Isoperla dicala -- "not" I. transmarina: Our Perlodid from the Rapidan River

Well I thought it was I. transmarina.  But not being sure, I sent some photos to Steven Beaty for comment.  He replied that it looked like I. dicala.  And sure enough that's what it is.  Let's have a look at Beaty's description of I. dicala ("The Plecoptera of North Carolina," p. 23), using photos as we go along.

I. dicala -- nymphs ~ 10 mm; apex of lacinia broad although still narrower than base, with row of setae below subapical tooth.

Yes.  The apex is narrower than the base, and yes, there are setae (hairs) below the subapical tooth.

He continues, "pale marks anterior to median ocellus sometimes indistinct, sometimes with darker border."

I would not call those marks indistinct on this particular nymph, nor does there appear to be a dark border.
But those characteristics are not always present, and this is a judgment call.

Back to Beaty: "body and abdomen often speckled, particularly posterior segments; dark longitudinal abdominal stripes with very narrow pale borders."

No question about the speckling on the posterior segments, and the pale borders are clear as well.

Isoperla dicala nymphs are "collected from the Mountains and Piedmont during the winter and spring" (Beaty, p. 23).  The tolerance value is "undetermined."  (For Donald Chandler's photo of I. dicala, go to:

I found two of these nymphs yesterday.  Here are photos of the second which, with one antenna broken off, is not as good as I would like.  They'll have to do.

Another Isoperla species to add to our list of Perlodid stoneflies found in central Virginia.

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