Saturday, April 22, 2017

Project for a rainy day: re-examining Amphinemura species ID

Rain at last, and it's supposed to continue through Tuesday.  Our streams really need it.

You'll recall that I found quite a few Amphinemura Nemourids last week at Buck Mt. Creek.  On Tuesday, I went to the Rivanna and they were in there as well.

Some time ago I worked on Amphinemura species ID and reached the conclusion that those that I've found in our streams are Amphinemura delosa.    This was based on the description of A. delosa found in a 1971 article by P. P. Harper and H. B. N. Hynes: "The nymphs of the Nemouridae of Eastern Canada (Insecta: Plecoptera)," Canadian Journal of Zoology, 49: 1129-1142.  Let me review their description (their words cited in BOLD) noting those features on the nymphs that I've found this week and last.  (See p.1131 of their article.)

1. Total length of mature nymph, 5-6 mm.  The nymphs that I found were all 5.

2.  Color medium to dark brown, head darker; antennae pale, first few segments darker; Yes.

3. legs brown; ... cerci pale, first few segments darker;  Yes.

4. Short bristles covering head capsule, those behind eyes longer and stout.  Very clear.

5. Pronotum rectangular, nearly as broad as head, covered with short bristles and hairs; pronotal fringe well defined, consisting of long pointed bristles.  They are indeed long and pointed.  Not real easy to see the bristles and hairs on the pronotum itself with all of the sand and silt on the nymph.

6. legs with long stout bristles, the longest femoral bristles longer than the greatest width of the femur.  They don't seem quite that long on my nymph, but I think they're close enough.  This nymph was still immature.

7. a few long hairs on the tibiae but no tibial fringe.  Yes.

8. prosternal gills in four tufts; distance between the median tufts about twice that between the median and lateral tufts; each tuft comprising about eight (in mature nymphs) filaments forming a whorl around a central axis.    That's a match for our nymph.

9. Finally, abdomen covered with long bristles, the longest marginal bristle longer than the mid-dorsal length of the corresponding tergum.


Looks like a slam dunk to me.  But just to be sure, I thought I'd see what Steve Beaty says in his latest work on "The Plecoptera of North Carolina" (Version 4.0, 2015).  Gives me pause.

To begin with, he notes that four species of Amphinemura have been found in North Carolina: appalachia, delosa, nigritta, and wui.  (Harper and Hynes, by the way, have described A. nigritta and A. wui but not A. appalachia. A. delosa and A. nigritta are very similar, but not exactly the same.)  But he cautions those of us using his guide to leave Amphinemura ID at the level of genus.  I'm not exactly sure why -- I need to check in with him on this matter.  I suspect he feels that we need to be cautious until the nymphs of all four species have been fully described.  Then again, he may question the reliability of a study done nearly 50 years ago.  There is good news: B.P. Stark (of Stewart and Stark) is apparently preparing a "nymphal key to the eastern species of Amphinemura," but it will not include a description of A. appalachia.  Bummer.

All things considered, I think that at the moment the safe thing to call the nymphs I've been finding is Amphinemura sp. (delosa?).  I'll let you know if that changes.

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