There is widespread interest in producing high-performance fibers that min-dc the behavior of natural silks, especially spider drag line. Given the multiple roles of drag line in nature, it is pertinent to explore whether spiders can tailor the tensile properties of this material to match its intended use. Here we distinguish between the ability of spiders to control the quality (intrinsic stress-strain response) versus the amount (load-bearing cross-section) of drag line. The mechanical characteristics of drag line spun during a vertical climb differ from those of drag line spun when the spider crawls on a horizontal surface. Also, the intrinsic stress-strain response of drag line spun during a vertical climb is significantly more reproducible (i.e. dependable) than when this fiber is produced under other conditions. Implications for biomimetic polymer science are discussed.