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

Abstract

SUMMARYAdhesive pads on the legs of animals can be classified as either ʻsmoothʼ or ʻhairyʼ (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.

Observations of shear adhesive force and friction of Blatta orientalis on different surfaces

“…In general, claw-mediated adhesive insects can attach to a horizontal or vertical surface only by interlocking and so the adhesive abilities increase with the surface roughness [5,19,20], in agreement with our observations. In particular, the claw-mediated adhesion occurs when the surface asperity size is comparable or larger than the claw tip diameter [3,4,51], here estimated to be 12.3 µm.…”

“…Many authors have studied a multitude of insects, especially thanks to the availability of microscopic analysis instruments (Field Emission Scanning Electron Microscope (FESEM) and Atomic Force Microscope (AFM)), in order to understand and measure their adhesive abilities; in the course of the last decades, beetles [1][2][3][4][5][6], aphids [8][9][10], flies [7,11,12], bugs [13], ants [14][15][16][17], cockroaches [18][19][20][21][22][23][24], spiders [25][26][27] and geckos [28][29][30][31][32][33][34][35][36][37][38] have been extensively studied.…”

Mechanisms of fluid production in smooth adhesive pads of insects

“…This discrepancy can be explained by the specific detachment movements of adhesive pads during locomotion [3,34,[39][40][41][42] and by the ability of insects to control adhesive contact area. Many insects make very little contact with their adhesive organs when walking upright and use their soft pads primarily while climbing when adhesive forces are required [16,34,41,43].…”

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

“…Many insects possess fluid-mediated adhesive pads to safely adhere to almost all known surfaces [1,2]. Whilst the adhesive pads of several insect groups such as flies and beetles are densely covered with flexible setae, the smooth pads found in other insects such as ants, stick insects and cockroaches are "pillow-like" soft structures.…”