Comparative Contact Mechanics

Abstract

In this chapter, we briefly discuss three hotly debated issues of the current research on adhesive foot pads, (1) the validity of the contact splitting theory and its implication for scaling laws, (2) the role of fluid secretions in hairy adhesive pads, and (3) the controversy about the subdivision of biological adhesive systems into ‘dry’ and ‘wet’ adhesives. We present the first data on comparative adhesion measurements in arachnids on (a) smooth pads, (b) patterned soft pads, and (c) hairy adhesive pads covered with tenent microtrichia. The data generally indicate stronger adhesion in systems with multiple contacts. Taking our new data on hairy adhesive pads in arachnids into account, we attempt to explain, why the evolution of spatular sizes is governed by phylogenetic constraints and the previously postulated relationship between body mass and degree of contact splitting does not hold. Furthermore, we provide several arguments against the view of previous authors, who classified the hairy adhesive pads of spiders and mites as wet adhesives, and stated geckoes as the sole organisms relying on dry adhesion.