Thursday, September 22, 2016

It's Acroneuria lycorias -- not carolinensis -- at Buck Mt. Creek

Kind of messed up today with my photos.  Somehow, I had my camera set to "Aperture priority" mode (Av) -- instead of the "Shutter priority" mode (Tv) that I normally use.  Unfortunately, I had not chosen an aperture setting, so all of my photos were overexposed.  Alas!  As you can see in the photo above, by editing we can make a partial correction -- but the shading doesn't look right.

Anyway, I was curious to see what this common stonefly -- an Acroneuria nymph for sure -- was in terms of the species.   You may recall that there are two species of common stone that look almost exactly alike: A. carolinensis and A. lycorias.   (See the entry posted on 12/11/14.)  This is A. carolinensis from another stream.

While I see this one a lot in the mountain streams that I visit -- upper Doyles River, Rapidan River, north fork of the Moormans --  in 2013 I realized that in Buck Mt. Creek, I was finding A. lycorias instead.  This one.  (photo taken on 10/20/13)

 (A. carolinensis, by the way, has a TV of 1.2; A. lycorias, 2.1.)  How do we tell them apart?  There are two ways in which they're distinct: 1) A. lycorias has anal gills; A.carolinensis does not: and 2) A. carolinensis has faint, medial, longitudinal markings on some or all of the terga, on A. lycorias such markings are missing.

If you look again at the A. lycorias photo directly above, you can see both of those features.  For contrast, here's a picture of another A. carolinensis.

So which one did we find this morning?  Medial longitudinal markings seem to be absent.

I was hoping for another A. lycorias nymph; so, so far so good.  But are anal gills present?  I couldn't see them.  So I brought this nymph home and used my microscope to get really close.  And voila!

Sure enough, there they are.

Back to the Rivanna tomorrow.  I've already corrected my camera settings!

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