Friction and adhesion in the tarsal and metatarsal scopulae of spiders

Friction and adhesion forces of the ventral surface of tarsi and metatarsi were measured in the bird spider Aphonopelma seemanni (Theraphosidae) and the hunting spider Cupiennius salei (Ctenidae). Adhesion measurements revealed no detectable attractive forces when the ventral surfaces of the leg segments were loaded and unloaded against the flat smooth glass surface. Strong friction anisotropy was observed: friction was considerably higher during sliding in the distal direction. Such anisotropy is explained by an anisotropic arrangement of microtrichia on setae: only the setal surface facing in the distal direction of the leg is covered by the microtrichia with spatula-like tips. When the leg is pushed, the spatula-shaped tips of microtrichia contact the substrate, whereas, when the leg is pulled over a surface, setae bend in the opposite direction and contact the substrate with their spatulae-lacking sides. In an additional series of experiments, it was shown that desiccation has an effect on the friction force. Presumably, drying of the legs results in reduction of the flexibility of the setae, microtrichia, spatulae, and underlying cuticle; this diminishes the ability to establish proper contact with the substrate and thus reduces the contact forces.