Wednesday, October 16, 2013

The Skwala Perlodid stonefly: Back -- briefly -- to the Northwest

Just back from Oregon where we've been visiting our daughter in Portland.  On Sunday, we went to the North Fork of the Lewis River in southern WA where I just couldn't resist looking through leafpacks and turning over some rocks.  I found a number of stoneflies, all the same -- the Perlodid stonefly, genus Skwala: S. americana or S. curvata, I'm not really sure which.  You may recall that I found one in Montana in August as well, and as I noted in the entry I posted, the "Skwala hatch" is a major hatch in the Northwest in March and April.

For identification I've used Stewart and Stark, Nymphs of North American Stonefly Genera, pp. 448-451 (good illustrations on p. 450).   There they describe S. americana -- but remember they're describing a nymph that's fully mature: the nymphs I found were still very small.

1. 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."  Works very well.

2. lacinia

"Lacinia bidentate with tuft of setae on a low knob below subapical tooth and continuous inner row of marginal setae."  You bet.

3. mesosternal ridge

"Y-arms of mesosternum reach anterior corners of furcal pits."  For sure.

I hope I get to see some mature nymphs some spring.  But even these immature nymphs were colorfully patterned.


I only had a short time to look for some insects, but I also managed to find a few mayflies -- the small minnow mayfly, Baetis tricaudatus, and what appeared to be a small Leucrocuta flathead.

1. B. tricaudatus, male

(and check out those eyes!)

Baetis tricaudatus, female

2. And the small Leucrocuta


(Lucia Falls on the North Fork of the Lewis.  Spectacular!)

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