In all honesty I doubt this fly caught anything this month especially as I found it in a tree! However one of popular topic at the minute, even making the mainstream press. The mop fly. Designed to no doubt replicate a worm or similar ground dwelling bug they sink super fast once wet and if fished properly do move well through the water. On a cold January day casting something like this on top of a fish in the hope it will turn round and aggressively grab it may sound like a last resort, but you never know!