Thursday, September 13, 2012

Agnetina annulipes: a New Perlid in Buck Mt. Creek

My title today was going to be "Slim Pickin's," but when I got home and looked at my photos, I realized I had found something new.

I started the morning at the Moormans River where I found very little: one A. nadineae small minnow mayfly, one Darner Dragonfly, and a few, small common stoneflies, Acroneuria abnormis.   So, on the way back to town, I decided to have a look at Buck Mt. Creek.  Again, initially, it looked pretty much like a bust.  There were the usual flatheaded mayflies, all small save for one Epeorus vitreus.  There were also a number of Brushlegged mayflies: I almost always find them in here.  And here too, I found a number of Acroneuria Perlids of various sizes.  Of interest, I found a couple of Macrostemum common netspinners: I'll post those photos some other time.

When I saw this common stonefly, it just seemed a little bit odd.  The color just wasn't right for an Acroneuria Perlid.  So, I took a few photos and preserved the nymph for further study.  I'm glad that I did: it's a new species for me -- Agnetina annulipes -- though I now think that the Agnetina nymphs that I find in the Rivanna are probably this species as well.

There are two features that define Agnetina: 1) there is a "complete" setal row on the occipital ridge -- this

-- and 2) it has anal gills; these

For the species ID, I turn to Steven Beaty's "The Plecoptera of North Carolina," p. 15.

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.  Aslo recorded from SC and VA.

The banding on the abdominal segments is quite clear in the microscope photo above; it's also clear from the live photo at the top of the page.  Also clear, is the fact that tergum 10 is "mostly dark including the apex."  I'd also say that on this particular nymph, the head pattern is an "almost interrupted mask."
That tergum 10, especially the apex, is dark is important; it is primarily this feature that distinguishes A. annulipes from A. flavescens, on which the "apex of tergum 10 [is] light with dark pigmentation faintly continuous mesally."  (Beaty, p. 15)

Agnetina annulipes is primarily found in the southeast.  In fact, its common name is the "Southern Stone."  Stewart and Stark (Nymphs of North American Stonefly Genera, p. 320) say that it is attested in AL, DC, FL, LA, MD, MS, PA, SC, and VA.  But Beaty finds it in NC as well.

Below are photos of the small Agnetinas I've found in the Rivanna at Crofton.  I'd hazard the guess that they were A. annulipes as well.



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