Comparative morphology of pretarsal scopulae in eleven spider families

Many wandering spiders bear attachment pads (scopulae) on their tarsi, consisting of hierarchically-branching adhesive setae. Amongst spider families and even species, these show remarkable differences in morphology. Using scanning electron microscopy, the scopula microstructure of sixteen spider species was described, with the focus on pretarsal scopulae (claw tufts). Area and shape of the claw tuft, seta and setule density, as well as seta and spatula dimensions were analysed and compared. Claw tufts of the majority of species studied show a similar gradient in size and shape from anterior to posterior legs: the dimension of pads increases, while setal density decreases. Commonly, there is also a gradient of both the seta and spatula size within the claw tuft: Setae become larger from the proximal to the distal part of the pad, and spatulae size increases in the same direction at the level of individual seta. Often, different hierarchical levels of claw tuft organisation are differently expressed in different species: Species with lower setal density usually have broader setae. Smaller spatula size often implicates higher setule density. Evolutionary and ecological aspects of the scopula origin are discussed.