Monday, January 17, 2011

Stream Report: Powells Creek (near Crozet)

Powells Creek never disappoints.  Every time I visit this small stream that flows just west of Crozet, I find a lot of nymphs and larvae, and the diversity is always good.  I'll focus on a few very nice finds, but let me begin with an inventory of sorts.  Today I found: 1) large winter stoneflies -- both Taeniopteryx and Strophopteryx (the former large, the latter still small); 2) a few small winter stoneflies, all of them close to hatching; 3) flatheaded mayflies -- both Stenonema (Maccaffertium) and Epeorus, the former very large, the latter really tiny!; 4) some very large midges (Chironomidae); 5) lots of Uenoid case making caddisflies; 6) lots of netspinners (either Cheumatopsyche or Hydropsyche); 7) a fair number of fingernet caddis (I'll say more about these in a later entry); 8) a few common stoneflies (Perlidae) -- genus Eccoptura; 9) and a couple of Perlodid stoneflies -- genus Diploperla.  Oh, and I saw some black flies, genus Prosimulium.

Now for some of the highlights.  I saw a couple of Brushlegged mayflies (family: Isonychiidae), the mayfly pictured above (the photo above is a live photo taken this summer at Buck Mt. Creek).  I think this is one of the prettiest mayflies we see, especially when it's fully spread out in this way.  This is a mayfly that hatches from late spring to late fall (fly fishermen know it as the "Slate Drake"); thus we see the nymphs in large numbers in samples we take throughout that period. (Photo below of an unsorted tray from the Lynch River in May, 2010 with lots of Burshlegged mayflies.)   Still, I saw quite a few of them in November and December, and now in January.  So obviously, a few of them are with us all year.

This is a mayfly we find throughout our watershed, but it's a mayfly that likes cold, well oxygenated, fast flowing water (it's a strong swimmer), so look for them in those types of streams.  Among the streams that I visit, they are prominent in Buck Mt. Creek, the Lynch River, and the Doyles River.

With its "cigar-like" shape and well-defined gills that stick out to the sides, the Brushlegged mayfly is one that novice samplers come to recognize with little practice.  Also easy to see on mature nymphs (around 3/4" long) are the many hairs on the insides of the front legs, and the intermeshed hairs in the tails.  These serve different purposes.  With the front legs shaped like the signal for a "field goal" (!), those hairs interlock, forming a net that traps micro-organic matter for food.  The intermeshed hairs in the tails, on the other hand, help the tails to work like a paddle to propel the nymph forward.

Another discovery that I made today -- the Powells is filled with Uenoid caddis cases, and they're much bigger than those I've been seeing in other streams.  Not only that, some of them seem to be lining up close to one another as they prepare for pupation (I'll try to get a photo of this later this winter).  Below is a photo of a mature Ueonid larva in a very nice case.  Note the larger pebbles on the sides of the case, a characteristic of Uenoid construction.

I also found two Perlodid stoneflies -- genus Diploperla.  This is the only site, to date, where I have found this particular genus.  Here is a microscope view:

I don't think this is a genus that we see commonly in local streams (feel free to correct me on that if you know that I'm wrong).  In any event, this specimen was large enough that I was able to get a picture of one of the defining characteristics of this particular genus:  the terminal lacinial spine is 1/2 as long as the lacinia itself.

(The lacinia is the projection with two brown spines in the upper part of the picture.)  Also quite clear in this photo is the "notch" in the labium which helps us to ID Perlodid nymphs.

Finally, I picked up a stonefly today that was totally black, and I was excited to go home and see what it was.  It turned out to be a small winter stonefly in the process of transformation into its terrestrial form.
As you can see, its adult wings have already emerged.  The nymphs are never this color; they're yellow, or brown, sometimes highly patterned.  But the adult is totally black.

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