Tuesday, September 23, 2014

Buck Mt. Creek in the autumn, and some notes on Giant stoneflies

This morning when I went out to Buck Mt. Creek, I wasn't expecting to see a whole lot -- and my suspicions were right.  This is not the best time of year to be looking for insects.  Things are small, and a lot of mayflies and stoneflies are still in the substrate just starting to grow.  I know a lot of groups that monitor streams like to sample in September and October -- I don't always understand why.  Do we really get a good sense of the insects that live in our streams by looking this early?  You can find netspinners -- but they tend to downgrade the stream.  You won't see a lot of stoneflies -- which upgrade the streams -- until November and December.  By then most groups have stopped going out.  I'd venture to say that a lot of volunteers have never seen a small winter stonefly (Capniidae) or a large winter stonefly (Taeniopterygidae) if they don't sample from November -- February.  That's a shame.

Anyway, I didn't see anything that I wanted to photograph today -- save for the stream itself (note the water is low).  Saw some flatheaded mayflies, some small brush-legged mayflies, one small minnow mayfly, a few netspinners and fingernet larvae -- all of them small, and quite a few A. abnormis Perlids (common stoneflies), none even close to being mature.  I guess the rivers to do at the moment remain the Rivanna, the Rapidan, and Entry Run/South River.

I. Giant stoneflies.

You'll remember that I picked up this Giant -- Pteronarcys biloba -- at Entry Run on Sunday.  And since we'll be seeing a lot of Giants from now through the spring, I thought I might note the "quick" way to distinguish the three species I've encountered so far: P. biloba, P. proteus, and P. dorsata.   For full details, see Beaty, "The Plecoptera of North Carolina," p. 28.    But to tell them apart, you only need to examine the pronotum (especially the corners) and the sides of the abdominal segments.

1. Pteronarcys biloba

On P. biloba nymphs, the front corners of the pronotum (anterolateral angles) are "conspicuously produced into hook-like processes." (Beaty)

But what we can see right away are the "lateral projections" or "hooks" on abdominal segments 1-8.  As I noted, they were very pronounced on the nymph that I found on Sunday.

Additional photos of P. biloba nymphs.

2. Pteronarcys proteus

This is the species I find in very small mountain streams, but it's in the Rapidan River as well.  The front and rear angles on the pronotum are not at all produced, no hooks whatsoever.  "Anterolateral projections on pronotum reduced, barely discernible."  (Beaty)

As for the abdominal lateral hooks, they're visible, but they're "appressed" (Beaty).  I.e. they don't stick out nearly so far as those we see on P. biloba, and they are only distinct on segments 1-6.  The projection on segment 7 is small; that on 8, barely discernible.  Also worth noting, the cerci (tails) on the P. proteus nymphs I have found are much shorter than those on P. biloba nymphs.

Some other P. proteus photos.

3. Pteronarcys dorsata

And we have Pteronarcys dorsata, the most tolerant of the Giants.  It's the only species I've seen in the Rivanna, but I've also found one in Buck Mt. Creek.  On the pronotum, "the lateral angles of [the] pronotum [are] produced, anterolateral ones almost hook-like." (Beaty)  Actually, the rear angle is virtually "pointed."

For the sides of the abdominal segments, there are "no lateral projections on [the] abdominal segments." (Beaty)  Sure can't argue with that.

Some additional photos of P. dorsata.


That's all there is to it.  Tolerance values: P. biloba, 0.0; P. proteus, 0.4; P. dorsata, 2.4.

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