And this is why I was fishing in Montana last week. The "Skwala hatch" was on. It's a well-known hatch in the Missoula area, and the Bitterroot River is the place where most people go to fish it, and I had wonderful fishing there on the last day of March. The scenery wasn't bad either!
The nymph in the photo at the top of the page is a female, the males are much smaller, and I regret not taking a photo of a male as well. We saw some, but I had already put my camera equipment away. The nymph in this photo measured 24 mm.
The most thorough description of the Skwala nymph is once again to be found in Stewart and Stark (Nymphs of North American Stonefly Genera, pp. 448-451). The key features are these:
1. "Head dark with lighter M mark forward of anterior ocellus and incomplete mesal bar; 2 oval, light occipital spots inside eyes, broken with reticulate dark lines and bordered behind by narrow band of stout spinules." These features are easy to see on our nymph, as are some "erect white, silky hairs behind [the] ecdysial suture of [the] head."
2. "Lacinia bidentate with tuft of setae on a low knob below subapical tooth and continuous inner row of marginal setae...terminal tooth about 0.4 times total outer lacinial length and subapical tooth about 0.6 times length of terminal tooth." The "knob" below the subapical tooth is very distinctive, almost forming a shelf. (Note that in my photo the tip of the apical tooth seems to be broken.)
3. "Long submental gills, projecting portion about 2.5-3.0 times as long as their basal diameter." Yes.
4. "Pronotum dark with light lateral margins, light mesal band and light reticulate interior markings; complete row of short marginal spinules."
5. "Y-arms of mesosternum reach anterior corners of furcal pits."
6. "Abdominal terga brown with mesal and lateral pairs of small light spots; densely covered with intercalary setae and margined posteriorly with a closely set row of short, stout setae." We can see all of those features in this photo.
Don't think we need anymore evidence to verify the genus ID. It's a Skwala.
Regular readers might recall that I've found small Skwalas before, both in Montana and Oregon. The photo below was taken in Oregon in October, 2013.
There are two species of Skwala found in Montana -- S. americana and S. curvata. Can we tell which one this is? The answer is "No". To distinguish the species, to begin with, we need a male nymph -- which is not what we have. And, the males are distinguished by the relative size of the lobes on the terminalia. We have no way to proceed, so we are left with our genus ID. Good enough for me!
But I'm now back in Virginia, and I hope to head up to the Rapidan River tomorrow. Lots going on here at the moment.