Thursday, November 29, 2012
Varieties of the common stonefly (Perlid) Acroneuria abnormis: back to the Rivanna
I had a doctor's appointment this morning that took me close to Darden Towe Park, so I decided to make a quick stop at the Rivanna. The dominant taxon this morning was, as it was last week, the large winter stonefly Taeniopteryx burksi/maura, and I'll post some photos of one of those nymphs in a moment.
But I want to focus attention today on the common stonefly in the photo above, a nymph that I also found in the Rivanna. This is one of the larger common stoneflies I've found recently with a body of 17 mm, cerci of 18 mm, and antennae of 13 mm. So, from "tip-to-toe," as it were, this nymph measured 48 mm, roughly 2 inches (1.89 in., to be exact)! It is probably an Acroneuria abnormis in terms of the species, the most common "common" we see, but let's come back to that in a moment.
In his description of Acroneuria abnormis nymphs, Steven Beaty notes that there are two different forms ("The Plecoptera of North Carolina," p. 14).
A. abnormis -- male nymphs 15-20 mm, female nymphs 25-30 mm; 1) dorsum of head with a well defined M-shaped head pattern, sometimes with interruptions; posterior margins of abdominal tergites light, dark tergal bands irregular; or 2) dorsum of head without M-shaped head pattern and abdomen uniformly brown; anal gills always absent. Widespread and nymphs occur year round. (I've added the 1/2 notation for clarification.)
Type 1 is the one that I normally see. It looks like this:
The "M" pattern on the head is very clear -- not "interrupted" in this case -- as are the light colored posterior edges of the abdominal tergites. There are no anal gills: rather, there is a fine silky setae on the inner sides of the cerci (tails) at the base.
As I browsed through my photos a short while ago, I also found, I think, a picture of the alternate form of A. abnormis: no "M" pattern on the head, and the abdomen is uniformly brown. This was found on August 8 last year (2011), that means it was still on the "young" side, and the head pattern and abdominal colors could change. Still, it does look quite a bit different than the nymph in the photo above: note the difference in the width and the length of the pronotum.
Now, let's look again at our nymph from this morning.
It's almost a hybrid of the two types that Beaty describes. The abdomen appears to be "uniformly brown," but there does appear to be an "M" pattern on the head -- in this case "interrupted."
I sent a copy of this photo to Steven Beaty, and he was kind enough to reply. His guess -- going by the photo -- would be that it is the "dark morph" of A. abnormis, but, he notes something important for us to remember. There are still species of Acroneuria common stoneflies for which the nymphs have not been described (in his documents, he notes this is true for A. arida and A. petersi), so, ultimately, we have to be careful to declare an ID for a nymph that does not quite fit into the mold.
Interesting. I should also note that I had found a similar common stonefly in the Rivanna at Crofton on 11/20 (last week). Same abdomen, same head pattern, and same odd shaped pronotum -- very wide and somewhat narrow.
At first I thought this might be A. arenosa, another species on which the abdomen is totally brown. But, A. arenosa has anal gills, the nymphs I am finding do not. Always something new to explore.
And now for some pix of one of the large winter stoneflies that I found this morning. This is a male, and it was 8 mm: that's quite a change from the first one I found back on 10/28 -- one month ago -- which was a mere 1.5 mm!
("Dirty nymphs" -- All of the nymphs that I found this morning were "dirty," covered with silt. The Rivanna River runs very muddy in times of high water, and I'm afraid in the fall and the winter, the leaf packs never clean up.)