I knew as soon as I saw it that I had not seen this stonefly before. And it was BIG -- 25-26 mm. But I was clueless about the ID, even the genus. I had to key it out. And for that I turned to our major source of information -- Kenneth W. Stewart & Bill P. Stark, Nymphs of North American Stonefly Genera (Plecoptera), pp. 403-406.
(Note: the "Key to Mature Nymphs of Perlodidae" in Stewart and Stark -- pp. 361-364 -- won't get you to Isogenoides. The ID follows from couplets 19 to 20 to 22 to 23. But none of the preceding dichotomous choices takes you to item 19. I decided where to begin by matching my nymph with the anatomical features of Isogenoides illustrated on p. 405 (Fig. 14.28).)
1. "Body light brown or yellow with brown markings: intersegmental membranes cream to light yellow; antennae and legs yellow, cerci brown." Stewart and Stark are using an I. zionensis nymph as their model. Since that species doesn't occur in Montana, we can't be locked in by the colors. Still, the intersegmental membranes on our nymph are, indeed, "cream to light yellow." (See the photo below.)
2. "Head with yellow M mark forward of anterior ocellus; ocellar triangle enclosing a brown spot; occiput with large yellow spots, enclosed by brown, well-developed row of occipital spinules, interrupted mesally." All true.
3. "Lacinia triangular, bidentate with a distinct knob below subapical tooth." Yes, and in their illustration of this, they show three, stout setae on that knob, just as we see in my photo below.
4. "Prominent submental gills, projecting portion about 3 times their basal diameter." They're really big.
6. "Y-arms of mesosternum meet posterior corners of furcal pits; a distinctive median, longitudinal ridge connects fork of Y with a transverse ridge." Here that is. Note that the median ridge is very light.
7. "Meso-metanota with pairs of large round anteromesal light spots and faint pattern." Let's look at our diagnostic photo again. I've pointed to those light spots with the four, short yellow lines, showing the four on the mesonotum.
Stewart and Stark also describe the setae on the legs and the cerci, but I don't think we need anymore. It is an Isogenoides Perlodid nymph for sure.
But can we go further? Can we determine the species ID. I think that we can, and I'll show you on what that is based.
To begin with, according to Stewart and Stark, there are only two species that occur in Montana: elongatus -- the Elongate Springfly -- and colubrinus -- the Blackfoot Springfly. John Sandberg suggested to me that for the stoneflies we find in Montana, I might take a look at the follow monograph: Richard W. Baumann, Arden R. Gaufin, and Rebecca F. Surdick, "The Stoneflies (Plecoptera) of the Rocky Mountains," Memoirs of the American Entomological Society, 31 (1977), pp. 1-208. On p. 126, the authors provide us with a "Key to the Species of Isogenoides," and they provide the following couplets when it comes to the nymphs.
1. Conspicuous denticles present along margins on ventral cusps of both mandibles....2
Denticles present along margin on ventral cusp of right mandible only....colubrinus
2. Body color dark brown ... elongatus
Body color very light, almost yellow...zionensis
Since I. zionensis is out of the running, all that we have to do is look for denticles on ventral cusps of the mandibles. As it turns out, denticles are present on both.
Note: John Sandberg tells me that the key in Baumann et.al., isn't the greatest. Best to call this one Isogenoides sp. (elongatus/colubrinus).