Thursday, December 11, 2014

Acroneuria carolinensis, Acroneuria lycorias: How to tell them apart

At the risk of trying your patience, I want to assert in this entry that it is possible to distinguish the nymphs of Acroneuria lycorias and Acroneuria carolinensis, even if the experts these days are hesitant to take a firm stand on the issue.   The nymph in the photo at the top of the page is Acroneuria lycorias: I found it at Buck Mt. Creek on 10/28.  This nymph,

I'd argue, is Acroneuria carolinensis.  It was found at Entry Run on 11/30.

You'll recall from previous entries (10/23, 10/28, and 11/30) that at issue in telling the two species apart is the presence or absence of anal gills.  T.H. Frison (Studies of North American Plecoptera, 1942, pp. 282-284) had argued that A. lycorias has them, A. carolinensis does not.  However, Beaty waffles on this, noting that on A. carolinensis nymphs "anal gills [are] usually present," while on A. lycorias nymphs "anal gills [are] usually absent, sometimes small."  (Beaty, "The Plecoptera of North Carolina," p. 14.)  The word "usually" (italicized by me for emphasis) sort of muddies the waters.

I want to argue that I've found both species -- that I can prove it -- and my arguments on this are three.

1. Every nymph that I've found that I regard as Acroneuria carolinensis has lacked anal gills.  I preserved the nymph from Entry Run.  Have a look for yourself.  ("Paraprocts" are the lobes adjacent to the anus.)

The only nymph that I've found that I regard as Acroneuria lycorias has them.  They are quite clear on the nymph found at Buck Mt. Creek.  (But you can't see them from the front.)

2. A. carolinensis nymphs, with a tolerance value (North Carolina) of 1.2, should only be found in very good streams: A. lycorias has a TV of 2.1; I would not expect to find the two together -- and I haven't.  As I've noted before, I find A. carolinensis in very clean mountain streams: the Upper Doyles River, not far from the SNP, and Entry Run which is in the Blue Ridge.    Buck Mt. Creek, home to my A. lycorias nymph, while still very good water, runs through fields and pastures.

3.  While I've made points 1. and 2. in previous entries, this one is new.  While it is true that the nymphs of both species look almost exactly alike, they differ in one significant way.  Look closely at the abdomens.

Acroneuria carolinensis

Acroneuria lycorias

Note 1) that the bands on the terga of the carolinensis nymphs are very irregular/wavy; those on the lycorias nymph are not.  They are almost uniform in thickness.  And 2) on carolinensis, there are medial extensions of the dark bands that in some nymphs actually turn into a longitudinal line.  There are no such extensions on the lycorias bands.  (Also of interest, I think, is that odd shaped medial extension on tergite 10 on the lycorias nymph.)

This is why I think that this is important.  Frison includes illustrations of A. carolinesis and A. lycorias nymphs in his study (pp. 282-283), and bingo, we can see the same distinct abdominal patterns on the two nymphs!  His nymphs look exactly like those in the pictures above.

Acroneuria carolinesis

and Acroneuria lycorias

I think we can tell them apart.

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