Wednesday, February 23, 2011

Stream Report: The Rapidan River off Graves Mill Rd.

The site I explored today was about a mile below the entrance to the National Park.  And I made some interesting finds.  (What I did not find was any large or small winter stoneflies!)

1. The insect pictured above is a "predaceous diving beetle" (family: Dytiscidae).   This is an insect we hardly ever see, and I actually found two of them here.  Both were in leaf packs.  They are fittingly named, since they'll go after just about anything they can get into their jaws: today, their game would have been common stoneflies, pronggilled mayflies, and black fly larvae.

Dytiscids are aeropneustic insects.  That means they get their oxygen out of the air -- not out of the water (hydropneustic).   To do this they use a "breathing tube" which is located between their tails.  Since it's hard to see in the picture above, let me show you a specimen that's been preserved.

To breathe, they must surface every so often or live close enough to dry land where they can quickly access the air.  Both of the Dytiscids that I found today were in leaf packs, but in leaf packs that were right next to shore and were only partly submerged.

2. Also in the leaf packs were lots of pronggilled mayflies.  And they appear to be fairly mature.  The reader may recall that the only genus I've seen in this region is Paraleptophlebia: to fly fishermen, that's the "Blue Quill," and it hatches in early spring.  A nice photo follows.

3. Also in leaf packs -- lots of Peltoperlids (Roach-like stoneflies).  I had mentioned in an earlier entry that we often find them in spring, and they're often in leaf packs, and that when we find them, we often find lots of them in the same place at the same time.  Exactly what happened today.  I didn't keep any for my reference collection, but I did get a few photos.

And another hovering by a Uenoid caddis and a Glossosomatid (Saddle case-maker) caddis that's slipped out of its case.

4. I also learned a good lesson today which is one that we all should remember:  common netspinning caddisflies -- even those of the same genus -- can often be quite different in pattern and color.  I was curious about the genera of the two larvae below, so I preserved them to look at at home.  The first has a very black head with brownish abdomen; the second was light green in color with a yellow pattern on the head and the thoracic sclerites.  Surely, I thought, these are different genera.

Wrong.  Both were genus Hydropsyche, one of the common genera we see.  What identifies this particular genus?  There are two things to look for.  1) The fore trochantin (shoulder blade?) is forked.
Take a look at the picture below.

The "forked fore trochantin" means a larva is either genus Cheumatopsyche or genus Hydropsyche.  So, we've narrowed it down.  2) For the other defining feature, we have to look under the neck.

Under the neck is a horizontal dark bar known as the "prosternal plate".  If there are two dark marks (sclerites) below it (as in the photo above), our larva is Hydropsyche; if the sclerites are missing, it's Cheumatopsyche.  (Neither of these genera -- you may recall -- has a very good tolerance value; it's between 5 and 6).  Both of our common netspinner larvae turned out to be genus Hydropsyche.

5. I once again found that beautiful spiny crawler, wearing its "coat of many colors" (see previous Rapidan entry).  And as I was pushing it to pose for some pictures, it moved into a stance that we commonly find with spiny crawlers.

It raised its abdomen up and pointed the tips of its tails towards the front of its body.  This is a defensive posture.  It's a way of warding off enemies when they get close.  And, it's something we often see when we collect these nymphs in the spring.  If you see a mayfly curled up like this on your net in the spring, you can be pretty sure that you've got a spiny!

6.  One final photo -- a brushlegged mayfly.  I also found a fair number of these, and they too were snugged up in the leaf packs.   (The bottoms of rocks were the domain of Epeorus flatheaded mayflies, Uenoid case-making caddisflies, and a few saddle case-makers).

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