Walking with Sticky Feet: Biomechanics of Surface Attachment in Insects
Walter Federle
Department of Zoology, University of Cambridge
Many insects are capable of running upside down on smooth and often slippery plant surfaces, carrying loads and withstanding pull-off and shear forces of more than 100 times their own body weight. The mechanisms of how this impressive performance is achieved and of how insects master the conflicting tasks of running and making stable surface contacts are still poorly understood.
Insect adhesive organs are either 'smooth' pads characterised by a specialised, soft cuticle, or 'hairy', i.e. densely covered with flexible adhesive setae. In both groups, the control of attachment and detachment during locomotion is largely passive and involves sophisticated mechanical systems. Insect adhesion is mediated by small volumes of fluid secreted into the pad contact zone. This secretion is an emulsion consisting of volatile, hydrophilic droplets dispersed in a persistent, hydrophobic phase. Single pad force measurements revealed that the secretion does not generally increase attachment but does so only on rough substrates, where it helps to maximise contact area.
Shear stress of adhesive pads increases with velocity, consistent with the effect of a continuous fluid film in the contact zone. However, adhesive pads exhibit considerable static friction even when secretion has accumulated. Insects appear to prevent sliding by virtue of non-Newtonian properties of their two-phasic adhesive secretion rather than by direct contact between the cuticle and the substrate.
Research into adhesive structures of animals and in their mechanisms to control attachment forces may reveal general principles useful for the development of 'biomimetic' adhesives.
