Few flies divide anglers like the mop fly. Tied from a single fuzzy strand of a car-wash or cleaning mop, it is about as far from traditional fly tying as you can get, and it catches trout so well that it has become a staple of the modern nymph box, purists’ grumbling notwithstanding.
What it imitates
The soft, segmented, slightly buoyant mop body does a convincing job of imitating a fat grub, crane-fly larva or big caddis larva, the kind of juicy, high-calorie morsel a trout will move for. The material breathes and pulses in the current in a way trout clearly find edible.
How to fish it
Fish it dead-drift near the bottom like any nymph, on a tight line or under an indicator. Often tied on a jig hook with a bead to get down fast, it makes a great point fly (the heavy, deep fly) with a smaller nymph like a Pheasant Tail on a dropper.
Tip Use the mop as your anchor fly. Its weight (especially on a beadhead jig hook) gets your rig down fast, so run it as the point fly and trail a smaller, more imitative nymph off it, the mop gets it deep and often gets eaten itself.