The evolution and function of spider feet (Araneae: Arachnida): multiple acquisitions of distal articulations

The tip of the legs concentrates the interactions that a spider has with the substrate where it lives. We review the morphology and evolution of spider feet, discussing the functional anatomy of their articulations and proposing a coherent terminology. All spiders consistently have two tendons to operate their feet and show a stereotyped sequence of levation of the pretarsus and its claws prior to detachment from the substrate. A pair of slit sensilla, the foot slits, provide a reliable landmark across most spiders. The evolutionary reconstruction of morphological variants using a composite tree of spiders indicates that similar morphologies arose independently, with multiple acquisitions of one to four distal articulations. A distal articulation appeared repeatedly at the foot slits, the podotarsite, and at least three independent origins of highly articulated feet correspond with cuticular structures to retain the flexor tendons in the proper ventral position. Our results indicate that while in some spiders the adhesive setae were added to articulate feet, in other taxa the sequence was opposite. We conclude that a limited repertoire of feet articulations appeared and reversed many times in spider evolution, and combine in many ways to produce a highly diverse functional unit.