Tunable adhesion for life and sex: functioning mechanisms of adhesive hairs in diving beetles

Male diving beetles use specialized adhesive setae, in spatula or circular form, to mount on female elytra during underwater courtship; co-evolution of the contact surfaces has attracted much attention since Darwin.  We for the first time directly measured and compared the performance of a single seta of each form.  While the circular setae behave like typical suckers, the spatula ones with a modified shallow sucker and channels, found only in male Cybister beetles, use the combined mechanisms of suction and viscous resistance for adhesion.  To decipher the physical mechanisms of the unusual velocity-dependent adhesion, we construct a conceptual “water-leaking model” combining spatula seta’s surface geometry and properties, as well as its force and deformation throughout adhesion.  Comparison between simulation and empirical results reveals three functioning mechanisms: (i) water flowing through imperfectly sealed microfluidic channels leads to velocity-dependent adhesion; (ii) stalk-pulling action increases pressure difference to compress the channel wall, triggering seal-off of the micro-channels; (iii) stalk elasticity provides buffer for energy storage, further increasing the adhesion capacity.  Such tunable adhesion mechanisms found in spatula setae not only allow the male Cybister beetles to succeed the premating courtship mount and easy release for respiration, but also provide us insights for future design of bio-inspired underwater adhesion devices.