This is one of my favorite patterns for Roaring River. Some say the mop fly represents the crane larvae. Honestly I think it's an attractor fly. Works great when it's cloudy or when the sun is peeking in and out. I have landed some of my bigger rainbows on this fly. I like to use 5X tapered leader with 5X tippet. Works great as a double dropper underneath a strike indicator and I like to fish it about a foot above the bottom or in the feeding lane.