These stoneflies--abundant during midday--seemed at first to be flying around with plain yellow bodies, but on closer inspection turn out to have striking, fiery red abdomens.
My guess would be genus Sweltsa, of which I believe there to be ~8 species in MT. It looks pretty similar to a (mostly) dorsal view of Sweltsa townsesi, which is pictured in American Stoneflies: A Photographic Guide to the Plecoptera by Bill B. Stark, et. al. However, I believe that species is only present in the Northern Sierra Nevada Mountains of CA and NV.
Troutnut on Jul 17, 2019July 17th, 2019, 8:03 pm EDT
Thanks Roger. I'll put it there for now, along with another specimen of similar but not identical coloration and a few mm greater length.
FYI, today and tomorrow I'm adding quite a few more specimens from this year's Montana trip and would definitely appreciate any ID thoughts if you have time. I'll message Dave about the caddis soon, too.
Jason Neuswanger, Ph.D.
Troutnut and salmonid ecologist