Orb-weaving spiders rely on sticky capture threads to retain prey long enough to be located and attacked. The evolution of viscid silk is associated with the high diversity of araneoid orb-weaving spiders, in part because it is cheaper to produce than the primitive dry cribellate fibrous adhesive used by deinopoid orb-weaving spiders. Unlike cribellate threads, viscid glue contributes little to the tensile strength of the capture spiral it decorates. However, viscid silk utilizes a unique suspension bridge mechanism, absent in cribellate silk, which increases total stickiness by recruiting the adhesion of multiple glue droplets. Here, we analyze the relationship between stickiness of viscid silk and various biomechanical and architectural features of webs that may influence its evolution, across a sample of 17 ecribellate orb-weaving species. The force required to break capture spiral fibers is the single most important factor explaining about 90% of the variation in stickiness among orb spiders. Stickiness increases linearly with capture spiral strength, but with a significant safety factor because stickiness is always less than the force required to break the silk. Our results thus indicate that evolutionary changes in the stickiness of the capture spiral are largely constrained by the strength of the fiber the glue is placed on rather than by the chemistry of the glue itself. This implies that orb webs function optimally when threads are able to detach and adhere repeatedly to struggling prey.