Thursday, September 15, 2016

Fall Stoneflies in the Rivanna: Two things I "hoped" to find; two things I "expected" to find -- and a surprise netspinner

The common stonefly (Perlidae), Agnetina flavescens (Midwestern Stone).  This is a species that I've only seen once before.  That was on 11/9/13: the Rivanna River at Darden Towe Park (Charlottesville), the same place I went to today.  And I was hoping to see it.  Agnetina nymphs have a setal row on the occiput, and anal gills are present.  A. flavescens is described by Beaty in the following way: "head pattern roughly M-shaped with arms directed posterolaterally and almost interrupted; a light triangular pale area between lateral ocelli; dorsum of abdomen banded, with dark bands on anterior half of segment...apex of tergite 10 light with narrow dark pigment band interrupted mesally." ("The Plecoptera of North Carolina," p. 15)

We can see all of those features on this tiny nymph save one: the anal gills.

The anal gills require a microscope view.

There is also a feature not noted by Beaty in his description which seems to be characteristic of A. flavescens: terga 8 and 9 are both pale with a single dark spot in the middle of 8 and three dark spots in 9.

While A. flavescens is not something I commonly see in the Rivanna, A. annulipes (Southern Stone) is par for the course at this time of year, and I found that species as well.

Much darker than A. flavescens, and the apex of terga 10 on this species is totally dark.  Again, we have to go to the microscope if we want to see anal gills.

On the species of Agnetina, Beaty notes: "Nymphs occur primarily in the Mountains though can be found in the Piedmont.  Uncommon."  Interesting.  A. annulipes, at least, is present in pretty good numbers in the Rivanna.  (A. capitata, by the way, is uncommon.  I've only seen that one at the Rapidan River.)

While I was pleasantly surprised to see both Agnetina that are found in this river, I was not at all surprised to see two other stones.  The first, the "brown form" of Acroneuria abnormis.

The abnormis nymphs that we normally see have a very clear pale yellow "M" on the head, and the terga are banded.  But with this type of abnormis, the head lacks that "M" pattern, and, as Beaty notes, the "abdomen [is] uniformly brown." (p.15)  It was a young one -- note the posterior edges of the wing pads -- but it was already a pretty big nymph: 17 mm.  By contrast, the A. flavescens nymph was only 8 mm while the A. annulipes was a mere 6!

Also around -- quite a few of them, actually -- the Giant stoneflies Pteronarcys dorsata.

This is the only "Giant" that I see in the Rivanna, and it's the most tolerant Giant species with a TV of 2.4.  It's easily distinguished by a number of features.  1) the lateral angles of the pronotum are "produced"; 2) there are longitudinal stripes on the abdomen (usually 3-5, only 2 visible here); and 3) there are no lateral projections/hooks on the abdominal segments.  (See Beaty, "The Plecoptera of North Carolina, p. 28)


It was a good day for stoneflies at the Rivanna, lots of them crawling around in the leafpacks.  I saw very few mayflies, and very few netspinner larvae in a stream that's filled with them in the summer.  But I did pick up one to ID.

I was completely shocked when I looked at this through the camera.  It's a Macrostemum!  How do we know?  The "carinate, flattened head".  (Beaty, "The Trichoptera of North Carolina," p. 77)  Let's have a close-up look.

You can see the carina -- the raised ridge circling the top of the head -- in this photo, and if you can't tell how flat that head is, here's another angle to use.

And with these gills, these larvae should have no problem getting oxygen out of the river!

Species?  Don't know.  There are two possibilites: M. carolina or M. zebratum.  On both, the head is "reddish brown".  (Beaty, p. 77)  The difference -- the size of the tubercle near the eye.  This.

On carolina it's "large"; on zebratum it's small.  I'd have to see both species to know which one I've found.  This is a common netspinner that I always see in the North Fork of the Rivanna in summer, but this is the first time I've picked one up in the main river.  While this one is brown, those in the North Fork have always been green.  (Photo from 7/16/11)


It was a pleasure to get back out to our streams.  The Rivanna will be my destination for the foreseeable future.  We're in semi-drought conditions: most of our streams are dreadfully low and warm.

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