The scopulae (from the Latin words scopa, “twigs, broom, brush”, and scopula, “a small brush”) of a spider consist of small, dense tufts of fine hairs directly under the claws, and in some cases (e.g., the mygalomorph tarantulas) extending along the entire ventral portion of each tarsus and metatarsus. These structures provide a multiplicity of contact points with the substrate upon which the spider passes; so much so, in fact, as to create a natural adhesion between the spider and the substrate. The result is that spiders possessing dense pads of scopulae are, in general, able to scale smooth vertical surfaces–including glass–with ease.
The faculty afforded by these structures is remarkable. One spider biologist (Foelix, 1996, pp. 18-19) points out that, under experimental conditions, spiders in the genus Cupiennius (mygalomorph hunting spiders in the Ctenidae family, native to the tropical rain forests of Central and South America) were able to hold ten times their body weight without slipping, while sitting on a vertical plate of glass. However, Foelix adds that each scopula hair splits into thousands of even finer cuticular extensions, so that–for example–a crab spider with but 30 scopula hairs on each foot is able to achieve 160,000 microscopic contact points between its feet and the substrate it passes over. The resulting adhesion, says Foelix, is not due to suction or electrostatic forces, but results from sheer physical adhesion, enhanced by the capillary forces provided from a micro-thin film of water. Absent the latter water film, the scopulae fail to adhere to smooth surfaces, and the spider loses the climbing faculties the scopulae otherwise supply.
Foelix, Rainer F. 1996. Biology of Spiders, 2nd Edition. Oxford University Press.