Consequences of forced silking.

The forced silking of a spider to obtain major ampullate (MA) silk for experiments is a standard practice; however, this method may have profound effects on the resulting silk's properties. Experiments were performed to determine the magnitude of the difference in the forces required to draw silk from the MA gland between unrestrained spiders descending on their draglines and restrained spiders from which MA silk was drawn with a motor. The results show that freely falling spiders can spool silk with as little as 0.1 body weights of force, which generates a stress that is about 2% of the silk's tensile strength. In contrast, forcibly silked spiders apply as much as 4 body weights of force with an internal braking mechanism, and this force creates silk stresses in excess of 50% of the silk's tensile strength. The large forces observed in forced silking should strongly affect the draw alignment of the polymer network in the newly spun fibers, and this may account for the differences in material properties observed between naturally spun and forcibly spun MA silks. In addition, the heat produced by the internal friction brake during forced silking may set the upper limit of forced silking speed.