The Mop Fly is exactly what it sounds like: one strand cut from a car-wash mitt or microfiber mop, lashed to a hook behind a bead. It imitates nothing in particular. The fat, segmented body reads as a cranefly larva or a grub, and the material breathes and wiggles in the current the way a real soft-bodied bug does. Anglers argue about whether it counts as "real" fly tying; trout do not.
Tie it on hooks 8-12 in yellow, olive, tan, or pink. Yellow and tan lean toward the cranefly-larva look; pink is pure attractor. A tungsten bead gets it down.
This is a searching pattern for tougher water - run it when flow is normal to high and the water is stained, off-color, or muddy, in temps from the upper 30s to the mid-60s. Fish it dead-drift under an indicator or on a Euro rig, ticking bottom. It works best as the heavy point fly with a smaller nymph on a dropper above: the mop gets you down and grabs attention, the dropper gets the finesse eats.