An important fly tying material that serves to weight nymph, wet and streamer flies are lead wires. If you wind them to the hook shank, you will be able to apply a great load to your fly and you will get a better drop in the water column or a variety of effects that attract fish to the strike - eg the fly load on the headl causes a jig effect when the fly fall on the head and the other parts of the body acts as a living fish prey.
In addition to the lead wires, there are also ribbing and color wires and tinsels that serve to strengthen the fly as well as to make your fly more attractive to the fish!