In vivo dynamics of the internal fibrous structure in smooth adhesive pads of insects

Citation

J.-H. Dirks, MingHe Li, A.J. Kabla and W. Federle

Acta Biomaterialia 8:2730-2736 (2012)

Abstract

Many insects with smooth adhesive pads can rapidly enlarge their contact area by centripetal pulls on the legs, allowing them to cope with sudden mechanical perturbations such as gusts of wind or raindrops. The short time scale of this reaction excludes any neuromuscular control; it is thus more likely to be caused by mechanical properties of the pad’s specialized cuticle. This soft cuticle contains numerous branched fibrils oriented almost perpendicularly to the surface. Assuming a fixed volume of the water-filled cuticle, we hypothesized that pulls could decrease the fibril angle, thereby helping the contact area to expand laterally and longitudinally. Three-dimensional fluorescence microscopy on the cuticle of smooth stick insect pads confirmed that pulls significantly reduced the fibril angle. However, the fibril angle variation appeared insufficient to explain the observed increase in contact area. Direct strain measurements in the contact zone demonstrated that pulls not only expand the cuticle laterally, but also add new contact area at the pad’s outer edge.

Figure sample

Hypothetical model for the passive contact area increase of smooth pads. (A) Schematic drawing indicating the orientation of the cuticle fibrils in a fixed adhesive pad and the orientation of the axes used in this study. The lateral (transverse) y-axis is orientated perpendicular to the image plane. (B) A pulling movement of the pad on the surface may reduce the angle a of the cuticular fibres. Assuming a constant length of the fibrils, reducing the fibril angle will reduce the structure’s height (h). If the structure’s height decreases, the average spacing between the fibres ds (measured within the x–z-plane) will be reduced too. This ‘‘compression’’ might increase the bending stiffness of the adhesive pad. (C) If the volume of the fibrous cuticle is constant (a x b x h), decreasing its height to h 0 should enlarge the contact area by a factor of h/h 0 .