Mop Fly Pattern Chenille is thick and fluffy (equivalent to an XL size chenille). To some, the MOP fly imitates nothing. Others agree that, in certain colors, it represents crane flies, grubs and other fat larvae that trout can't resist. Cut to length, secure one end on the hook (add a collar or not), and burn the tag end with a lighter or torch so it wont fray. 2½ yards of material per pack.