The Isoperlas I found last week in Montana in the Blackfoot and Clark Fork Rivers were Isoperla roguensis. Species ID is possible using an excellent article by John Sandberg -- "The Isoperla of California (Plecoptera: Perlodidae); larval descriptions and a key to 17 western Nearctic species," Illiesia, 7(22): 202-258. I am grateful to Steven Beaty for sending this to me. Sandberg's very detailed description of I. roguensis can be found on pp. 244-246. It includes color photos of the nymph and the features of the lacinia.
"Male larva. Body length of mature larva 9-12 mm." The nymph in my photo at the top of the page was a full 12 mm.
For description of the top of the head, let's use the following photos.
"Dorsum of head with contrasting pigment pattern and fine dark clothing setae, anterior frontoclypeus margin unpigmented...posterior ocelli with partially enclosed large light areas along outer lateral margins; interocellar area variable , from completely dark to partially light." (This one is completely dark.) "Occiput with irregular spinulae band extending from below eye to near median epicranial suture." You can't see the entire band in my photo, but I've noted the direction of that band from the base of the eye to the epicranial suture. I'll try to get a better photo tomorrow.
"Abdominal terga variable, usually with two distinct longitudinal dark stripes; wide light median longitudinal band sometimes bisected by faint, light brown longitudinal median band; lateral pair of dark longitudinal stripes usually not extending to lateral margins." Good view of those stripes on this small nymph (10 mm) that I found in the Blackfoot.
Isoperla roguensis -- the "Rogue Stripetail" -- but fly fisherman go by the name of the hatch, the "Yellow Sally." It's a big hatch in the rivers in the Missoula area, occurring in June and July. Hope to see it some day!