The famous nocturnal Hex hatch of the Midwest (and a few other lucky locations) stirs to the surface mythically large brown trout that only touch streamers for the rest of the year.
This one seems to lead to Couplet 35 of the Key to Genera of Perlodidae Nymphs and the genus Isoperla, but I'm skeptical that's correct based on the general look. I need to get it under the microscope to review several choices in the key, and it'll probably end up a different Perlodidae.
About "Mystery Creeks": If you recognize one of these, you already understand why I'm keeping it a secret. I'm not as strict as some anglers about hiding where I fish, mostly because I don't expect to substantially affect fishing pressure on already well-known or simply unpopular waters. But there are some gems where I don't want to add a single unfamiliar bootprint to the mix, due to the fishing, their wild character, or keeping a friend's secret. They're all "Mystery Creek" here—even the lakes.
Landscape & scenery photos from Mystery Creek # 186
This one was preserved in ethanol and partially dissected, so it doesn't have its natural wild coloration, and it's missing some appendages. I'm posting it for identification purposes.