Tuesday, July 19, 2011

The Trumpetnet Caddisfly (Polycentropodidae): Genus Polycentropus

I went to the Doyles River in Doylesville last week -- but I didn't find anything new or exciting.  There are still some common stoneflies here, genus Perlesta, and I also saw a young Acroneuria.  Lots of netspinners, fingernets, and flatheaded mayflies (genus Maccaffertium, and a few Epeorus vitreus).  I also found a few small minnow mayflies, genus Baetis, and I'll show you a photo of one of those in a minute.  But the water was low, and with the heat we're having this week, it's bound to drop even more.  So, for the moment, I think I'll stick with the big river -- the Rivanna itself: that seems to be where the action is at the moment.

But I did find the interesting caddis larva in the photo above.  This one fooled me.  It was long, maybe 3/4", and big and orange.  And with abdominal segments that are pretty constricted, my guess at the stream was that it was a freeliving caddis.   However, when I downloaded my photos at home and noticed the gray "freckles" (muscle scars) on top of the head, my thoughts went right away to Polycentropodidae, a "trumpet net" caddisfly larva.  There's only one way to know that for sure -- you have to check the shape of the fore trochantin using a microscope.  The fore trochantin (the joint where the front leg meets the body) should come to a sharp point -- sort of looks like a pick-axe.

So, no doubt about it: it was indeed a Polycentropodid.  What about the genus ID?  That's also easy to determine using a good microscope.  This one is Polycentropus.  How do we know?  The "suture" on the anal proleg of Polycentropus consists of a black "X".

Polycentropodids, by the way, have a tolerance value of "6.0" in the VA DEQ list of tolerance values: in the NC DWQ list, the genus Polycentropus comes in at "3.1".    Since I was thrown off by the color, let me remind you of what they normally look like: this is a photo of one from the Doyles, that I posted back at the beginning of May.

Two more photos.  The first, a lovely little flatheaded mayfly, Epeorus vitreus.  We find these all summer long in our "good," cold water streams -- though never in very large numbers.

The other, a small minnow mayfly, genus Baetis.  I don't know the species.  But I've been finding this species in a lot of streams now (at least I think it's the same species I'm seeing).  It's very small -- every one I've seen has been small -- and not very colorful.  But  they may be small and lacking in color because they're immature.  Note that the wingpads are not very long.

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