Thursday, August 4, 2011

The "Golden Stone" Hatch in the Rapidan River

The Rapidan River in Madison County is rich in stoneflies -- lots of "Common Stoneflies," Perlids, which hatch out as "Golden Stones" in fly fishing terms.  As I walked through the river this morning looking for insects, I saw Perlid "shucks" all over the rocks.  A couple of photos, beginning with a double.

And then this one, in which we can see how the "shuck" was cracked open to allow the flying adult emerge.

There are two Perlid genera that inhabit this stream: Acroneuria and Paragnetina.  The one in the photo above is a Paragnetina imarginata that I found this morning, and here is a beautiful Acroneuria (carolinensis) that will surely crack its shell and fly off before very long (or will it be part of next year's class?)

But I also found a member of the "new generation".  Take a look at this photo, another of this Acroneuria nymph, but note what's crawling around in the bottom left of the picture.

Now, that is what you call a small stonefly, especially when we know that the mature nymph was about 1 inch in length!   Let's let him take the stage on his (or her) own.  It's in a petri dish which is 3 1/2" in diameter.

This turned out to be a small Paragnetina common stonefly, but it took microscope work to make that identification.  The crucial factors?  There is a row of spinules along the back side of the head (the "occipital ridge"), and "subanal gills" are not present.  Here's a look at that ridge.

The Rapidan -- like all of the streams I'm visiting at the moment -- was full of netspinning caddisfly larvae, flatheaded mayflies (Maccaffertium and Epeorus vitreus), and small minnow mayflies (genus Acentrella).  There were also some small Giant stoneflies and lots of small Peltoperlids (Roach-like stoneflies).   But the other thing I found here was lots of BIG brushlegged mayflies, some that were clearly ready to hatch.  For example...

And I'm quite sure that many brushlegged mayflies have already emerged -- as "Slate Drakes" or "Leadwings," to use the fly fishing terms.  In fact, I found evidence of this as I was snapping photos of stonefly shucks.  Have a look.

Now, it's not a very good photo -- it was in an awkward place on the rock -- but it's clearly a brushlegged "shuck".  The brushlegged mayfly (Isonychiidae), in contrast to most other mayflies, crawls out of the stream onto rocks to emerge (see Knopp and Cormier, Mayflies, p. 84).   The species of brushlegged mayfly that we have in the East is Isonychia bicolor, and Knopp and Cormier (p. 85) note that it emerges here from late spring into fall.

My photography today was limited by heavy cloud cover.  Still, I got photos of a couple of cased caddisflies that I can use to close out this entry.  The first is of a Glossosomatid (Saddlecase Maker), crawling out of his stony "tortoise shell" case.  The second is of one of many Brachycentrids (Humpless case makers) that are starting to show up in our streams.  This one appears to be staring down that tiny common stonefly.  (Final photo -- another pic of the Acroneuria Perlid.)

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