Division of labour and sex differences between fibrillar, tarsal adhesive pads in beetles: effective elastic modulus and attachment performance

Many beetles employ arrays of adhesive setae to control attachment during locomotion. Here we investigate whether and how variation in seta structure, both between sexes and between tarsal pads on the same leg, determines the mechanical properties and adhesive performance of fibrillar arrays. We vertically compressed individual adhesive pads to determine their effective elastic modulus. Distal adhesive arrays were significantly softer than middle and proximal ones. Variation in stiffness was mainly due to different seta diameters, but calculated elastic moduli of seta cuticle were relatively constant at 5-16 GPa. Consistent with their greater compliance, distal pads generated higher adhesion and friction on rough substrates. However, the greater stiffness of proximal pads conveys a superior ability to push. Proximal pads of males were less direction dependent than distal pads and generated larger pushing forces in the distal and lateral directions. In females, proximal pads also produced higher friction forces than distal pads, but only in the lateral direction. Video recordings of vertically climbing beetles confirmed that each pad was used differently. When legs above the body centre of gravity were pulling, beetles mainly engaged the distal pads, whereas legs below the centre of gravity mainly pushed with the proximal pads. Attachment performance was additionally compared between sexes on different substrates. Our findings demonstrate the presence of sex-specific specialisations of the fibrillar system as well as a division of labour between different adhesive pads on the same tarsus.