Sunday, July 17, 2011

I Went, I Found, I Came Home

I went to the North Fork of the Rivanna this morning in search of one thing: I also found other good things, but this is the larva that I was after.  Common Netspinner (Hydropsychidae), genus Macrostemum.  Very few volunteer samplers ever see this one since it's not all that common -- in our watershed, that is -- and it's only here from mid-summer through early fall.  Another look.

Check out the head.  It slopes down from the back to the front -- in fact, it's a little caved in.  But that's not easy to see without magnification, and I've worked with monitors in the past who had no clue what this was -- despite the obvious frilly abdominal gills.  The other problems with identifying this as a netspinner are that it's an odd shade of green, and, it's big and fat, considerably longer and fatter than the netspinners that we commonly see.

For keying this out, let's use Peckarsky (Freshwater Macroinvertebrates, p. 101).  There we read: "Head with a broad, depressed, flat, dorsal area surrounded by an extensive arcuate carina; anterior margin of tibia and tarsus of foreleg with a dense brush of pale setae."  I was able to get a microscope shot that shows us both of those features.

Very, very odd-looking critter as netspinners go.  And it's less tolerant than the two genera we most often see (Cheumatopsyche and Hydropsyche).   In the latest list of tolerance values posted by the North Carolina Division of Water Quality, Cheumatopsyche netspinners have a value of 6.6; Macrostemum netspinners come in at 3.4.  One more photo of a Macrostemum larva since they aren't that easy to find and not that easy to photograph either!

The vegetation growing on rocks in the North Fork was absolutely crawling with netspinner larvae -- only 4 of which were genus Macrostemum.   If I lifted a rock out of the water and let it sit a few seconds with the vegetation exposed to the air, I'd see netspinner larvae wiggling all over the place.  I certainly saw hundreds of larvae today, possibly thousands.   I'm convinced that the main hatch of netspinners in this part of Virginia occurs in the fall.

A few other nice photos to look at, though I don't want to detract from the focus of today's entry.
There were also a lot of brushlegged mayflies hiding away inside the vegetation on top of the rocks.
A lot.  And they were pretty mature.  I'd expect them to hatch by the end of the summer.  Here's a pretty good shot (note the dark wing pads).

There were also flatheaded mayfly nymphs present -- all Maccaffertium -- and I found three small minnow mayflies, 1 was a small three-tailed Baetis, the other two were Heterocloeon curiosum, the same species I found in the main stem of the Rivanna last week.  The deciding factors: a circle of gray pigment in the center of each of the gills, and "procoxal gills" --  fingerlike gills sticking out from the base of the front legs.  Got a good microscope photo of that.

And, I found yet another Hagenius Clubtail dragonfly (I understand from the folks at that this is Hagenius brevistylus).  I may have to revise my thinking that these are rare in our streams.  By the way, they are always tangled up in vegetation and sampling with the protocol that's normally used would not shake them loose.  It's unlikely that volunteer groups are going to see them.

And finally, I found an unexpected treasure: a "Whirligig beetle" larva -- family Gyrinidae.  I have never seen very many of these -- a few in Buck Mt. Creek, a few in Mechunk Creek, and now one here in the North Fork.  And this was a big one -- I'd say about 1 1/4" in length -- and in its fully mature very bright colors.

All in all, a nice summer day at the stream.

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