Salmonflies
Pteronarcys californica
The giant Salmonflies of the Western mountains are legendary for their proclivity to elicit consistent dry-fly action and ferocious strikes.
I'm calling this one Pycnopsyche, but it's a bit perplexing. It seems to key definitively to at least Couplet 8 of the Key to Genera of Limnephilidae Larvae. That narrows it down to three genera, and the case seems wrong for the other two. The case looks right for Pycnopsyche, and it fits one of the key characteristics: "Abdominal sternum II without chloride epithelium and abdominal segment IX with only single seta on each side of dorsal sclerite." However, the characteristic "metanotal sa1 sclerites not fused, although often contiguous" does not seem to fit well. Those sclerites sure look fused to me, although I can make out a thin groove in the touching halves in the anterior half under the microscope. Perhaps this is a regional variation.
The only species of Pycnopsyche documented in Washington state is Pycnopsyche guttifera, and the colors and markings around the head of this specimen seem to match very well a specimen of that species from Massachusetts on Bugguide. So I am placing it in that species for now.
Whatever species this is, I photographed another specimen of seemingly the same species from the same spot a couple months later.
Common Names of Trout Stream Insects
Common Names of Trout Stream Insects
Common names are more confusing than Latin names, because there are usually many names for each bug and many bugs for each name. People like them anyway. Most of these names are better for talking about body colors (like Sulphur) or fly patterns (like Adams) than for discussing actual insect species. A common name might tell you roughly what an insect looks like. A few of them can tell you what species you're dealing with, especially if you're familiar with your area's hatches and what people call them. However, many common names aren't even common—they were just made up by authors trying to name a species for readers they imagine are deathly allergic to Latin, as if "pale white-winged watery dun" is really any more common in streamside conversations than Anafroptilum.