Saturday, August 31, 2013

Montana Part II: Perlodid Stonefly "Skwala"

Last Monday we floated -- and fished -- the Upper Clark Fork River, east of Missoula, and I found an important stonefly in this part of the country, the Perlodid stonefly, genus Skwala (probably Skwala americana).   The "Skwala hatch" is one of the first major fly fishing events for guides working out of Missoula: it normally happens in April.   Most guides focus on the Bitterroot River, but obviously there are Skwalas in other local rivers as well.

This is not a stonefly that's found in the East, so to make this identification I turned to Stewart and Stark's Nymphs of North American Stonefly Genera, pp. 448-451.  The critical features for us to see are 1) the pattern on the head,  2) the morphology of the lacinia,  and 3) the shape of the mesosternal "Y-arms".

Head: "Head dark with lighter M mark forward of anterior ocellus and incomplete light mesal bar; 2 oval, light occipital spots inside eyes, broken with reticulate dark lines and bordered behind by narrow band of stout spinules." (Stewart and Stark, p. 448)  In this photo, we can easily see everything but the band of stout spinules behind each of the spots (though they're easy to see with the microscope).

Lacinia: "Lacinia bidentate with tuft of setae on a low knob below subapical tooth and continuous inner row of marginal setae; complete band of submarginal setae...; terminal tooth about 0.4 times total outer lacinial length and subapical tooth about 0.6 times length of terminal tooth."  (p. 448)
Here's what that looks like in a microscope view.

Shape of Y-arms:  "Y-arms of mesosternum reach anterior corners of furcal pits." (p. 451)  They look like this, as illustrated by Stewart and Stark on p. 450.

I'd note one other feature for confirmation.  On the abdomen, the medial longitudinal line consists of a series of dots and slashes --

that feature, too, is illustrated by Stewart and Stark, p. 449.  The Perlodid stonefly, Skwala.

Now if I can only get out to Montana in April when these nymphs mature and hatch: it's supposed to be fantastic fishing!

Naturally, I found other insects on Monday as well, some of which require more work for species identification.  More on that later, but here are some photos.

1. A "Trico" (family, Leptohyphidae; genus, Tricorythodes).  We had prolific hatches of Tricos every morning, no matter where we were fishing.

2. The flatheaded mayfly Nixe, species unknown.

3. A pronggilled mayfly, Paraleptophlebia bicornuta.  The "tusks" at the front of the head give this one away.

4. Some small minnow mayflies that I need to ID.  (Very small, 4-5 mm)

5. And a very large, very black,  common netspinner larva.  The genus is Arctopsyche.  I think it's A. californica, but I need to look into that further.

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