Tiny Baetis mayflies are perhaps the most commonly encountered and imitated by anglers on all American trout streams due to their great abundance, widespread distribution, and trout-friendly emergence habits.
This wild-looking little thing completely puzzled me. At first I was thinking beetle or month larva, until I got a look at the pictures on the computer screen. I made a couple of incorrect guesses before entomologist Greg Courtney pointed me in the right direction with Psychodidae. He suggested a possible genus of Thornburghiella, but could not rule out some other members of the tribe Pericomini.
These interesting insects are not mentioned in trout fishing literature, but they turn up once in a while in my Eastern samples and I understand they are also common in the Midwest.
Larva & pupa biology
The larvae very much resemble pennies from one side. Their whole bodies are an excellent suction cups, their legs are strong for clinging to rocks, and they are somewhat translucent and extremely well-camouflaged. The result is an extremely well-adapted fast-water insect which probably rarely if ever ends up in the drift for the trout.