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.
Described by Ross, H. H. Studies of Nearctic aquatic insects. I. Nearctic alder flies of the genus Sialis (Megaloptera, Sialidae). Bulletin of the Illinois [State] Natural History Survey 21:57-78 [Errata: printed on a free slip of paper inserted in the work]. (1937).