Saturday, February 18, 2012

Pteronarcys biloba: The "Giant" of the Rapidan River

Time to take a close look at the spectacular Giant stonefly that haunts the leaf packs of the Rapidan River.
This is the only species of Giant that I've seen in this stream -- and it's quite a beast!  This was close to 2" long, and look at those abdominal "spikes".  I always wonder if the Brook trout that inhabit this river lick their chops when they see one of these coming -- or do they dive for cover?!

Let's take a look at Beaty's description of Pteronarcys biloba ("The Plecoptera of North Carolina, p. 28).

P. biloba -- lateral margin of frontoclypeus with small triangular tubercles adjacent to, and partially obscuring, antennal pedicels; anterolateral angles of pronotum conspicuously produced into hook-like processes, although North Carolina specimens less so; abdominal hooks divergent, often with low knob on the posterior margin of one or more of the pairs, and conspicuous on abdominal segments 7-8; antennae and cerci with a pale yellow medial band; cerci greater than half the length of the abdomen.  Nymphs occur year-round in the Mountains from June through February.  Semivoltine.

I did not preserve this nymph, so I can't verify the size and shape of the tubercles at the front of the head next to the antennae.  But the other features we can see clearly without special magnification.

1. Are the front corners of the pronotum "hook-like"?  Yes.

2. Are there low knobs on the rear margins of the "spikes" that stick out from the abdominal segments?
Yes.  In fact I can see them on every pair of projections.

3. And, is there a medial, yellow band on the cerci, and are the cerci more than half the length of the abdomen?  I did not measure the cerci -- but they are indeed very long, and they probably matched Beaty's description.  (Compare these to the short cerci of P. scotti, the Giant featured the entry posted 2/2).

Finally, a close look at the antennae.

Quite a magnificent creature, and with a TV of 0.0, it's absolutely intolerant of impairment.

The insects I found at the Rapidan River this morning were pretty much what I expected to see, but, as always, there were a couple of surprises.  The first was the sheer number of Isoperla nr. namata Perlodid stoneflies I found in the leaf packs.  There were hundreds.  This was clearly the dominant taxon in the exploring that I did today.  And, a lot of these nymphs are showing signs of maturing: the rear wing pads are starting to flare out from the body.

The other surprise?  I found three small minnow mayflies.  And though I was expecting them to be Heterocloeon amplum -- they were not:  they were Baetis pluto.  Notice the three tails, with the middle one much shorter than the two on the outside, and, note that tergite 5 is pale while 6 and 7 are dark (= B. pluto).

So, there are at least two species of small minnow mayflies in our streams in the winter.   (You may recall that I previously found a Baetis pluto in the South River on 1/12.)

Here are a few other nice photos from today's journey up to Madison County.

1. A pretty spectacular free-living caddisfly larva: Rhyacophila fuscula.

2. A lovely fingernet caddis: genus Dolophilodes.  (I've never seen the common Chimarra in here.)

3. Another Malirekus hastatus Perlodid stonefly.

4. The Common stonefly (Perlid), Paragnetina immarginata.

5. And, the spiny crawler that I only see when I come to this stream -- Ephemerella subvaria.  I saw a lot of them on this trip, and they're getting ready to hatch as the "Hendrickson" in April and May.

No comments:

Post a Comment