Why are so many adhesive pads hairy?

Many arthropods and vertebrates possess tarsal adhesive pads densely covered with setae. The striking morphological convergence of 'hairy' pads in lizards, spiders and several insect orders demonstrates the advantage of this design for substrate adhesion. Early functional explanations of hairy adhesive organs focused on the performance on rough substrates, where flexible setae can make more intimate contact. Recent theoretical and experimental work shows that the hairy design can also help to achieve self-cleaning properties, controllable detachment and increased adhesion. Several arguments have been proposed to explain why adhesive forces are maximised. First, the 'Force scaling' hypothesis states that when adhesive forces scale linearly with the dimensions of the contact, adhesion is increased by dividing the contact zone into many microscopic subunits. Second, the 'Fracture mechanics' argument implies that adhesion is maximised when the size of adhesive contacts is smaller than the critical crack length. Third, the 'Work of adhesion' model suggests that adhesion increases due to the bending and stretching of setae and associated energy losses during detachment. Several morphological traits of hairy adhesive pads can be explained by the need to maximise the work of adhesion, while avoiding the sticking of setae to each other (self-matting). Firstly, if setae are oblique and convex toward the foot tip as typical of most hairy pads, arrays should achieve greater adhesion. Secondly, a branched seta morphology not only confers the advantage that setae can adapt to roughness at different length scales but also prevents self-matting and increases the work of adhesion. It is predicted from the 'Work of adhesion' model that adhesion of pads with unbranched setae cannot be increased by subdividing the contact zone into ever finer subcontacts, because this would increasingly cause self-matting. However, contact splitting can increase adhesion if setae are branched. The greater density of setae in large animals has been interpreted by 'Force scaling'. However, the existing data can be explained by the effect of seta branching and by a fundamental difference between 'wet' and 'dry' adhesive systems. As insects employ adhesive fluids, they can cope with small-scale surface roughness even with relatively blunt seta tips, whereas the dry systems of lizards and spiders require extremely fine endings.