Thursday, October 20, 2016

While we're waiting -- the Perlodid stonefly, Clioperla clio

We're waiting for Bob's camera to be returned.  I was having "metering" problems: all of my macro photos in the last couple of months have been overexposed.  I used to shoot at a shutter speed of 1/60, or even 1/40, using a film speed of ISO 100.  But then I read that to avoid camera shake (blurred pictures) I should be shooting at 1/125 with a 60mm lens.   To adjust to that speed, I increased the ISO to 320, or even 400.  All of sudden in August, my photos were overexposed.  So I tried to adjust, increasing the shutter speed and lowering the ISO back to 100.  Still too light.  I went up to 1/250, even 1/320 -- still overexposed.  That's when I knew that I had a problem.  So the camera is being repaired.  Should be back sometime next week.

In the meantime, a couple of things.  The first, the species ID of this nymph.

I argued last month -- post of 9/24 -- that this was Acroneuria evoluta.  But last week I heard from an entomologist familiar with the stoneflies we find in Virginia, and he pointed out that he and Boris Kondratief -- the expert on all of these things -- have never found this species in our rivers.  He suggested that it was probably A. arenosa.  This is a problem I've noted before -- in my post of 6/12/15.  With the nymphs, the differences between arenosa and evoluta are slight.  On the top of the head of A. evoluta there is a faint "M" consisting of three lateral dots: on A. arenosa, those dots may or may not be present.  (You can see them on the nymph in the picture above.)  The other distinction is that the terga on A. evoluta are "uniformly brown, not banded," while on A. arenosa, the terga are "mostly uniformly brown but with an incomplete, narrow, pale anterior band."  (See Beaty, "The Plecoptera of North Carolina," p. 37 [vers. 4.0, 2015)  I've never seen that banding on the nymphs that I find, but, I've never found nymphs that are fully mature.  Maybe on those nymphs we'd see it.  In the end, the best way to establish species ID is by using adults (the terrestrials), and that's something I just don't do.  So, I can't claim to know for sure that my nymphs are A. evoluta -- could be A. arenosa -- and we'll have to leave it at that.

Now for the nymph at the top of the page.  When I finally get back to the streams, this is one that I'm likely to see: Perlodid stonefly, Clioperla clio.    But this is what they look like in the month of October.

It's always the first Perlodid stonefly I see, and it's very small in October, sometimes into November as well.

But they change in a hurry as we move on into winter.  Here's the progression.  (And note how the wingpads increasingly bend and diverge.)



February (also the one at the top of the page)

And by April, they're either almost mature or fully mature.

Pretty dramatic.

For the description of Clioperla clio we turn to Beaty, p. 52 (vers. 4).  "Robust nymph, 12-18 mm. Lacinia bidentate and mostly quadrate."

Yes.  "large, light medial area on dorsum of head (including most of ocellar triangle), a faint M-pattern sometimes present in ultimate instars; dark area along anterior margin sometimes with linearized anterolateral spots; dark bar along epicranial sutures almost connecting lateral ocelli to eyes, submental gills absent; pronotum surrounded by a dark border and with lateral edges pale; dorsum of abdomen with longitudinal stripes sometimes vague and diffuse; each tergite with up to 4-6 pale circular dots transversely across the segment, the 2 median dots often most prominent."



For a Perlodid stonefly, Clioperla clio is fairly tolerant -- TV of 5.2 -- consequently, I don't find it in the very best streams I explore (the small mountain streams in Sugar Hollow and the Rapidan River where it exits SNP).   On the other hand, I've never seen one in the Rivanna (a big river) where the insects are typically fairly tolerant.   It's the streams in between like Buck Mt. Creek and the Upper Doyles River.  There I see them in fairly big numbers.

Should be back at it by the end of next week.  Until then, I wait!

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