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.
"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.
"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
Baetis tricaudatus, female
2. And the small Leucrocuta
(Lucia Falls on the North Fork of the Lewis. Spectacular!)