Thursday, September 19, 2013

Stonefly day at the Rivanna: Giants and Perlids, including Agnetina Annulipes

Another day when persistence was rewarded.  I've been trying all summer to reach some riffles in the Rivanna at Crofton where I've found good things in the past: today I finally got there.  And I found at least one of the insects I was hoping to see: this is the Common stonefly (Perlid) Agnetina annulipes, a nymph that, so far, I've not seen anywhere else.   I picked up quite a few in the tangled river weed on the tops of the rocks, but they're easy to miss since they're very small nymphs.  The largest I found was this one

which was a mere 9 mm.  Clearly, these nymphs are not yet mature -- the posterior edge of the wing pads is just starting to bend -- still, I don't think this species gets very big.

Let's review the species ID.  "A. annulipes -- nymphs ?? mm; head pattern roughly M-shaped with arms directed posterolaterally, some specimens may have an almost interrupted mask; dorsum of abdomen banded, with dark bands on anterior half of segment, sometimes segments 5 and 6 dark mostly to posterior margin; tergum 10 mostly dark including the apex.  Semivoltine.  Collected from the Mountains and Slate Belt.  Also recorded from SC and VA."  (Beaty, "The Plecoptera of North Carolina," p. 15)

For the genus ID, Beaty notes "Occiput with setal row or ridge complete...anal gills present."  Let's look at our nymph.

Perfect, but I'd say that the posterior extension of the dark band on tergite 6 is clearer than that on 5.

North Carolina does not assign a tolerance value to A. annulipes, suggesting that it is not often seen.


The other Perlid I commonly see in the Rivanna is our "common" species Acroneuria abnormis, and I found some today.  But the nymphs that I see in this river often differ from those I see everywhere else.

Let's look again at Beaty's description of A. abnormis.  "...dorsum of head with a well defined M-shaped head pattern, sometimes with interruptions; posterior margins of abdominal tergites light, dark tergal bands irregular; or dorsum of head without M-shaped head pattern and abdomen uniformly brown."  (Beaty, "The Plecoptera of North Carolina," p. 14)

Almost all A. abnormis stoneflies I see do have the "well defined M-shaped head pattern," and tergites on which the posterior margins are light.    They look like this

or like this.

(Look again at the entry posted just six days ago on 9/13.)  The  A. abnormis nymphs I see in the Rivanna are clearly the "or" version that Beaty notes:  "dorsum of head without M-shaped head pattern and abdomen uniformly brown."


One other odd stonefly today.

Very strange!  It's a Giant, of course, and it's clearly Pteronarcys dorsata, and I saw a lot of those nymphs today.  But this is what the others all looked like.

Note again the P. dorsata key features: no lateral projections on the abdominal segments with very pronounced, "produced" angles on the pronotum -- they're very pointed on this one.   So what is this strange looking critter?

It took me awhile to figure it out, but clearly it's a P. dorsata nymph that has just recently molted.  Nothing has darkened.  The antennae are white, the cerci and legs are still white, and the body's light brown.  Very cool!

Agnetina annulipes

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