Liquid secretion and setal compliance: the beetle's winning combination for a robust and reversible adhesion

This paper is a brief review and discussion of the recent literature on the hairy adhesive pads of beetles, with the focus on two features of these pads, firstly, compliant setal tips and secondly, a liquid secretion, that together guarantee robust cycles of attachment/detachment on smooth and rough substrates. The compliance is required to ensure sufficient contact between the setal tips and the substrate with a minimum of elastically stored energy at the contact interface. The secretion fills potential gaps between both surfaces, generates capillary adhesive forces, and enhances self-cleaning of these microstructures. Furthermore, the secretion might prevent setal dehydration and subsequently maintain setal tip compliancy. The paper also pinpoints a series of open questions on the physical mechanisms at play to passively regulate the contact forces developed by these hairy pads during locomotion.