Thursday, September 11, 2014

The autumn stoneflies in the Rivanna

I went to the Rivanna at Crofton this morning, where I found pretty much what I expected to see: the common stonefly, Agnetina annulipes; the giant stonefly, Pteronarcys dorsata; and the same common stonefly that I found at Darden Towe Park, Acroneuria arenosa (or possibly A. evoluta).

In the photo above, a fairly immature A. annulipes (measured about 9 mm), and here's one even smaller (only 5 mm).

Agnetina Perlids have a complete setal row at the back of the head (occiput), and anal gills are present.  While neither feature is crystal clear in my photos, if you look closely, they can be seen in this one.

Other key features: "head pattern roughly M-shaped with arms directed posterolaterally," "dorsum of abdomen banded, with dark bands on anterior half of segment," and "tergum 10 mostly dark including apex."  (Beaty, "The Plecoptera of North Carolina," p. 15)  The banding shows up the best on the small nymph in the photo above: we can see the arms on the M, and dark color of tergum 10, in just about all of the photos.  Actually, in this photo we can see all of the features.

I saw a lot of these Perlids today, and I'll continue to see them well into the fall.  They're fairly common in the Rivanna at this time of year.

Giant stonefly, Pteronarcys dorsata

And I saw a lot of these "giants" -- Pteronarcys dorsata.   This is a fairly tolerant species -- 2.4 -- and it's the only species of giant I've seen in the Rivanna.   Characteristics: "Lateral angles of pronotum produced, anterolateral ones almost hook-like;  no lateral projections on abdominal segments; each tergite with anterior and posterior abdominal spots sometimes confluent to give the abdomen a longitudinally striped appearance (3-5 stripes possible)."  (Beaty, "Plecoptera," p. 28)  Very striped it is.

Two interesting shots.  The first, a ventral view of the thoracic gills of the giants:

the second, a side view of one of my nymphs as it climbed on top of another.


Common stonefly, Acroneuria arenosa/evoluta

Since I've already covered the problem we have with the ID of this particular Perlid,  I'll just post some of my photos.  The anal gills (black/gray) are clearly visible on both of the nymphs that I photographed: on one, the M pattern is clearly there but faint -- which points to A. arenosa.   But, both nymphs are still immature.  What I need to see is what the head will look like on these nymphs when they're mature.


Common netspinner, Hydropsyche venularis

Though I found very few -- not sure I saw any -- common netspinners last week at Darden Towe Park, at Crofton, there were still some crawling in the vegetation on rocks.  I got some fairly good photos of this one, and it turned out to be Hydropsyche venularis, one of two species that I commonly see in this river.

The yellow marks on the head are key to identification.  On H. venularis, there are two pair of yellow dots on the top of the head, but the two often merge into one.

Still, to distinguish venularis from rossi, we must look at the muscle scars on the side of the head.  On H. venularis the rows of muscle scars "curve dorsad posteriorly." (Beaty, "The Trichoptera of North Carolina," p. 76)  You can see that with the top row in this microscope photo.


The weather is changing at last: cooler temps on the way.  So I should start to get out more often and start looking at smaller streams.

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