Fish Head Hends give the front of the streamer its shape and provide a surface for attaching 3D eyes. The plastic head slides through the eye of the hook once the fly is tied. It also adds a little weight to the front.
Options include sizes of 5, 6, or 7 mm. Check that it fits through the eye and that there is enough room for the head before finishing the ties. The clear or colored finishes allow it to blend into the fish’s design; the eyes are then attached to this base.