Thursday, October 23, 2014

The common stonefly Acroneuria carolinensis -- or is it?!

I've had several e-mail discussions this year with Steven Beaty about genus Acroneuria common stones (Perlidae) -- a genus, he feels, is badly in need of re-search and revision.  This came up in April when I "might" have discovered an A. internata (see the entries of 4/20 and 4/22), and it came up last month when I found A. arenosa in the Rivanna, a nymph I had previously thought was a "brown" A. abnormis (see the entries of 9/1 and 9/4).   In those discussions, Beaty mentioned in passing that if I thought distinguishing those species from one another was tough, just wait until I tried to distinguish A. carolinensis from A. lycorias.

Huh.  I've seen -- I thought -- A. carolinensis a lot in the past -- and posted a whole lot of photos -- I've never looked for A. lycorias: I didn't know this ID was in doubt.

I commonly find what I thought were A. carolinensis nymphs in the Doyles River at what I call my upper site, which is where I went this morning.  (Actually, I started at Patricia Byrom Park.  But with the small streams there virtually dry, I backtracked to the Doyles.)  So I thought I would look into this problem.

No problem finding the nymphs: I found two in short order.  The one at the top of the page (another photo),

and this one.

So, which are they, Acroneuria carolinensis or Acroneuria lycorias?  Let's look at Beaty's descriptions.  ("The Plecoptera of North Carolina," p. 14)

A. carolinensis - male nymphs 17-19 mm, female nymphs 21-23 mm; dorsum of head with light M-shaped pattern; dorsum of abdomen banded, anterior half light and posterior margin of tergites dark; anal gills usually absent.  Relatively common in the northeastern Mountains and in the Sand hills year round but particularly in the early winter.

Perfect fit.  Light "M" pattern, tergites banded, dark in back, no anal gills.

A. lycorias -- male nymphs 15-18 mm, female nymphs 17-20 mm; dorsum of head with light well-developed M-shaped pattern; posterior margins of tergites dark; anal gills usually present, sometimes small.  ... Found in Mountains (mostly the Catawba River basin) and Sand Hills.  Though uncommon, nymphs can be collected year round.

As you can see, the species seem to differ only in one way:  the presence or absence of anal gills.  Of course the catch is that accoring to Beaty they are usually absent on carolinensis, and usually present on lycorias.  That sort of leaves the door open!

Since we can't clearly decide on the species using Beaty's descriptions, let's see if there's other information around.  As it turns out the confusion of the two species was noted as early as 1942 by Frison (T. H. Frison, Studies of North American Plecoptera: With Special Reference to the Fauna of Illinois:  Urbana, Illinois, Natural History Survey Division, pp. 281-284).    On A. carolinensis he says-- "particularly important features of the color pattern are the arrangement of the light and dark areas on the dorsum of the head and the banding of the abdominal tergites; in color pattern the nymph is very similar to lycorias, but it differs from the nymph of lycorias in the lack of anal abdominal gills." (p. 282)  It would seem that at that time, at least, the presence or absence of anal gills was taken as definitive.  Perhaps things have changed.

Frison does one other thing that might be useful in deciding this matter.  He provides full illustrations of both of the nymphs (A. carolinensis, on p. 282; A. lycorias on p. 283).   In those illustrations, the head patterns are not exactly the same.   The carolinesis head looks exactly like the heads of the nymphs that I found this morning.  Note two things: 1) the wide, light area at the back of the head forms an arc that reaches up to the sides of the eyes, and 2) that light area intrudes into the dark transverse band on the head on either side of the ocellar triangle.  I.e. this is what we see.


Neither of those things is true on the lycorias head.  That is to say, the light area at the back of the head essentially forms a closed rectangle at the back of the head --it isn't shaped like an arc, and it does not intrude into the transverse band.  Also, it's completely surrounded by a dark border.  So, fill in everything surrounding the rectangle with brown.

Hope your imagination is good: best I can do.

What that all comes down to is this: I feel pretty confident that the nymphs that I'm finding are Acroneuria carolinensis, and I'll continue to use that ID.  But I will be checking for any sign of anal gills.


Oh.  Lots of Strong-case makers in this river as well.  Psilotreta labida from what I can tell.

And there was a freshly molted Acroneuria nymph -- probably A. carolinensis -- and this little crittered crawled onto its head while I was taking my photos!

No comments:

Post a Comment