I have probably caught more trout and panfish on a simple fly made from thread and one other material. I typically tie it on 2xl to 3xl shank hooks, but most commonly on the Tiemco 200R or equivalent from size 8 down to 18.

The pattern was introduced to me as the "Monroe Leech".

In addition to thread, the one material is marabou. Tie in at the rear of the hook some marabou with the tips extending past the bend to create a short stubby tail. Don't trim the marabou. Twist the marabou (not the tail portion) into a rope and wrap up the shank. Tie off, trim, and finish the head.

In small sizes, only use a few strands of marabou to create a small nymph. More strands and larger hooks can be used to imitate leeches, damselfly larvae, or even minnows. Use whatever color suits you. I use black, olive, cinnamon, purple, burgandy, and grey most often.