Monday, February 24, 2014

The Rapidan in winter: pronggilled mayflies, I. montana Perlodids, and lots of Lepidostomatids

But before we talk about the dominant taxa...  When I picked up this gorgeous Perlodid, I thought I'd found something new -- and it is new, for me, for the Rapidan River.  Before today, I'd only found this one in the small streams in Sugar Hollow.  Isoperla similis: tolerance value, 0.8 and "relatively uncommon" (Beaty, "The Plecoptera of North Carolina," p. 24).  I think I've ID'd it before, but it's worth another look.  Here's Beaty's description:

I. similis gr. -- nymphs 12-15 mm; apex of lacinia slightly narrower than base and slightly constricted medially, with row of 4-5 stout setae below subapical tooth; head brown with a pair of pale spots near labral suture, a pale M-shaped mark anterior to median ocellus and pale marks anterolateral to the lateral ocelli; abdomen brown with a light median longitudinal stripe and with a pair of faint submedian pale dots on each segment.  Head and body covered with dark clothing hairs. ... Collected from headwater Mountain streams and medium rivers during the winter and early spring.

1. This nymph was 13-14 mm.

2. The lacinia matches the description, though I count 5-6 stout setae below the subapical tooth.

3. The head is as described.  Have a look.  (You can also see the "hairy" body in this photo.)

4. And for the abdomen, the medial stripe and the submedian dots are clear.

Isoperla similis, and I guess we could call the Rapidan a "medium" river at this location.

And another beauty this morning -- Giant stonefly, Pteronarcys biloba.  This measured about 1 3/4" -- about 45 mm.


But the dominant taxa -- Perlodid stoneflies, Isoperla montana/sp., pronggilled mayflies, the little case-maker Lepidostomatid, and on the rocks -- the flatheads Epeorus pleuralis.   I easily saw over 100 I. montanas in sorting through 10-12 leaf packs in one small part of the stream.  What a hatch of "Yellow Sallies" there must be up here in the spring!

1. Isoperla montana/sp.  (Caught one eating a pronggilled mayfly.)

2. Pronggilled mayfly, Paraleptophlebia sp. (mollis?).  One that survived being in a tray full of I. montanas!

3. Flatheaded mayfly, Epeorus pleuralis.

3.  And some of the many, many Lepidostomatids.


The Rapidan was high and fast and I was restricted to looking at leaf packs and rocks that were in close to shore.

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