The whole is more than the sum of all its parts: collective effect of spider attachment organs.

Dynamic attachment is the key to moving safely and fast in a three-dimensional environment. Among lizards, hexapods and arachnids, several lineages have evolved hairy foot pads that can generate strong friction and adhesion on both smooth and rough surfaces. A strongly expressed directionality of attachment structures results in an anisotropy of frictional properties, which might be crucial for attachment control. In a natural situation, more than one leg is usually in contact with the substrate. In order to understand the collective effect of hairy foot pads in the hunting spider Cupiennius salei (Arachnida, Ctenidae), we performed vertical pulling experiments combined with stepwise disabling of the pads. We found the attachment force of the spider to be not simply the sum of single leg forces because with leg pair deactivation a much greater decrease in attachment forces was found than was predicted by just the loss of available adhesive pad area. This indicates that overall adhesion ability of the spider is strongly dependent on the antagonistic work of opposing legs, and the apparent contact area plays only a minor role. It is concluded that the coordinated action of the legs is crucial for adhesion control and for fast and easy detachment. The cumulative effect of anisotropic fibrillar adhesive structures could be potentially interesting for biomimetic applications, such as novel gripping devices.