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First published online June 29, 2006
Journal of Experimental Biology 209, 2611-2621 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02323
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Why are so many adhesive pads hairy?

Walter Federle

Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK


Figure 1
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  		Fig. 1. (A) Ventral view of first tarsal segment of the ladybird Coccinella septempunctata (setae scratched away in the center to show the underlying cuticle. Note that setae stand in rows; the distance in the proximal-distal direction is greater than perpendicular to it. Scale bar, 20 µm. (B) Relationship between pad adhesion (work of adhesion of a setal array) and seta angle, as predicted from the Work of adhesion model and the assumption that sloped setae must be spaced further apart in the direction of the slope to avoid self-matting [function g({theta}) of Eqn 10 (see Appendix), plotted here for a seta aspect ratio (l/2r) of 10].

 

Figure 2
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  		Fig. 2. Typical curvature of setae and control of attachment and detachment. (A) Lateral view of setae in the longhorn beetle Clytus arietis; note the vertical, non-adhesive orientation of the seta tips and the corrugations on the dorsal sides, which probably prevent self-matting. Scale bar, 20 µm. (B) Schematic diagrams of two possible seta orientations, convex proximal (Bi; not found in natural systems) and convex distal (Bii; typical orientation). Distally convex setae can easily switch between attachment and detachment by proximal and distal leg movements, respectively.

 

Figure 3
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  		Fig. 3. Relationship between contact density (NA) of hairy pads and body mass for animals from a variety of taxa. Data from the studies by Arzt et al. (Arzt et al., 2003Go) and Kesel et al. (Kesel et al., 2003Go) (only Evarcha arcuata).

 

Figure 4
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  		Fig. 4. Schematic diagram illustrating two possible strategies of compensating small-scale surface roughness (roughness smaller than the dimensions of the pad). Good adhesive contact can either be achieved `dry' with very fine seta tips or `wet' by secretion of a fluid that can fill out substrate cavities. A fluid may not only be necessary in smooth adhesive pads (e.g. many insects and tree frogs) as shown here but also in hairy pads, when setae are relatively large and blunt as in many insects.

 

Figure 5
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  		Fig. 5. Model used to estimate the effect of seta branching. For an explanation of symbols, see List of symbols.

 




© The Company of Biologists Ltd 2006