Comparison of smooth and hairy attachment pads in insects: friction, adhesion and mechanisms for direction-dependence

Adhesive pads on the legs of animals can be classified as either 'smooth' or 'hairy' (fibrillar). It has been proposed that the hairy design conveys superior and controllable adhesion. However, no study has yet compared the basic performance of both systems. As such, we measured single-pad friction and adhesion forces in sample hairy (Gastrophysa viridula) and smooth (Carausius morosus) pads and simultaneously recorded contact area. Adhesion and friction forces per unit pad area were very similar in smooth and hairy systems. Insect pads of both types adhere via a thin film of liquid secretion. As found previously for the smooth system, forces in the fibrillar system strongly decreased with larger amounts of fluid secretion present, suggesting that the fluid mainly serves to maximize contact on rough substrates. One essential prerequisite for the control of surface attachment during locomotion is the direction-dependence of adhesive pads. We compared the mechanisms of direction-dependence in smooth and hairy systems by performing proximal and distal slides. Both types of pad exhibited a large drop in friction when moved away from the body, although this effect was more extreme for the hairy system. Direction-dependence is explained in both smooth and fibrillar systems by the instability of the tarsal chain, causing the whole pad to peel off. In the fibrillar pads, anisotropy additionally arises from the direction-dependence of individual setae.