hairy setae - 2611 The Journal of Experimental Biology 209,...

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2611 Introduction The foot of a fly is a most admirable and curious contrivance, for by this the flies are enabled to walk against the sides of glass, perpendicularly upwards, and to contain themselves in that posture long as they please; nay, to walk and suspend themselves against the undersurface of many bodies, as the ceiling of a room, or the like. .. ” Hooke, R. (1665). Many animals that live on trees or regularly visit plants have evolved adhesive organs on their feet. Despite the wide diversity of animals using adhesion and the variety of structures employed, tarsal adhesive organs come in only two basic designs: (1) pads with a relatively smooth surface profile and (2) pads densely covered with specialised, ± m- or nm- sized setae. ‘Hairy’ adhesive organs have evolved independently at least three times in lizards (Irschick et al., 1996; Williams and Peterson, 1982), at least three times in insects (Beutel and Gorb, 2001), and occur in three phylogenetically distant groups of spiders (Coddington and Levi, 1991; Rovner, 1978). This suggests that hairy pads represent an optimised design for surface attachment. What is the advantage of this convergently developed morphology? No later than in the 17th century, the pioneers of light microscopy were intrigued by the highly regular structures of fly feet and speculated about their function (Hooke, 1665; Power, 1664). Adhesive hairs were thought to work by interlocking with microscopic protrusions of the substrate. 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
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hairy setae - 2611 The Journal of Experimental Biology 209,...

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