Sunday, April 12, 2015

It appears to be, for us, another Nemourid -- Ostrocerca truncata....and some other great photos today

So many insects, so little time.  That's what it felt like today in one of our small streams in Sugar Hollow.   And I found a real mix of things.  But the big news -- what appears to be a new, for us, species of Nemourid (Forest Fly), Ostrocerca truncata.

Let's see if I can convince you of the ID.  First on the genus.  From Steven Beaty: "Nymphs 5-7 mm; pronotum with an irregular lateral fringe of moderate spines; anterior thoracic gills absent; fore tibia with two rows of large, heavy bristles along outer margins and a sparse fringe of silky hairs; .... tenth tergite of male extended and apically cleft. " (The Plecoptera of North Carolina," p. 3)

1. This nymph was exactly 6 mm in length.

2. My photos of the "irregular lateral fringe of spines" on the pronotum aren't great.  Still they prove the point.

3.  The "anterior thoracic gills" that are absent are those that we see on genus Amphinemura Nemourids.  These.

4. Spines on the fore tibia.  Definitely two rows on the outer margin.

5. Apical cleft in the 10th tergite.   Yes.


To identify this to the level of species I have to use data from the following article on Nemourids:  P.P. Harper and H.B.N. Hynes, "The nymphs of the Nemouridae of Eastern Canada (Insecta: Plecoptera)," Canadian Journal of Zoology, Vol. 49 (1971), pp. 1129-1142.  The relevant page for us is 1136 on which they describe the nymphs Ostrocerca albidipennis and Ostrocerca truncata.  Also relevant, figures 26 (on 1136) and 38 (on 1138).

Let me highlight some of the features of O. albidipennis, since the authors note, on O. truncata, "the morphology is almost identical with that of the previous species" -- which was O. albidipennis.

1. "Pronotum nearly quadrate, narrower than head."  Yes.

2. "legs covered with scattered bristles, the longest bristles on the femur about one-third as long as the greatest width of the femur."  I'd agree.

3. "whorls of cercal bristles regular at the base of the cercus, the longest bristles about one-third the length of the segment, but becoming irregular towards the tip of the cercus."  Verified in the following photos.

4.  "ocelli marked as purplish dots in riper nymphs."  Yes.

5. "developing genital organs in the last instar male nymph forming an apical protuberance about as long as broad, as seen in top view (Fig. 39)."    NOT TRUE

Rather, this protuberance is that of Ostrocerca truncata.   "developing genital organs in the last instar male nymph form an apical protuberance which is much longer than wide in top view (Fig. 38). "  Bingo!  The photo of the protuberance on the 10th tergite of our nymph is a perfect match for Fig. 38 in Harper and Hynes (Ostrocerca truncata)

I have to leave this ID as a "tentative" one --  Beaty says we should shy away from working on Ostrocerca species ID.  Still, I think we can make a pretty good case.  (Oh.  Beaty notes -- on Ostrocerca -- "A spring emergent taxon.  Mountains only.  Nymphs are rarely collected."  Guess I got lucky.


Other photos today.  Got some beauties.

1. Mature Malirekus hastatus Perlodid.  19 mm.

2. A fairly mature Chloroperlid (green stonefly), genus Sweltsa.  Note how it has flattened its back legs over the abdomen.

3. Two Pycnopsyche gentilis larvae (Northern case-makers) in various stages of converting their 3-sided cases of leaves into cases of pebbles.

(Note the Isoperla similis nymph in this photo.  I found 8-10 of them today.)

4. A beautiful photo of a Leuctrid (needlefly) nymph.

5. The common stonefly, Eccoptura xanthenses eating a freshly molted Epeorus pleuralis flatheaded mayfly.

6. And a critter I found by scooping up sand from the bottom -- the Spiketail dragonfly (Cordulegastridae) genus Cordulegaster.


I had expert advice today -- and supervision -- as I was doing my microscope work!

No comments:

Post a Comment