These hollow lead tubes can be added to most standard fly tying tubing to add weight and help get tube flies to the appropriate depth in the water column. The weights can be permanently applied when tying the fly, or added later in the field. They were originally designed for the Pro Flexitube, but work well with any standard tube. It's a terrific method of achieving a weighted fly in single-handed rod applications which has been one of the only shortcomings of the tube fly approach to fly fishing.
