title: Underwater attachment using hairs: the functioning of spatula and sucker setae from male diving beetles.
abstract: Males of Dytiscinae beetles use specialized adhesive setae to adhere to female elytra during underwater courtship. This coevolution of male setae and female elytra has attracted much attention since Darwin. However, there has been little examination of their biomechanical functioning despite increasing knowledge on biofibrillar adhesion. Here, we report and compare, for the first time, the mechanisms of underwater attachment using two hair types, the primitive spatula and derived 'passive' sucker, found in male diving beetles. Results from interspecific scaling of protarsal palettes and adhesion by single seta suggest better performance in the later-evolved circular (sucker) setae. Spatula setae with a modified shallow sucker and channels use the combined mechanisms of suction and viscous resistance for adhesion. Velocity-dependent adhesion provides sufficient control for resisting the female's erratic movements while also detaching easily through slow peeling. Direction-dependent shear resistance helps reorient setae surfaces into a preferred direction for effective adhesion. Seta deformation using different mechanisms for circular and spatula setae reduces the force that is transmitted to the contact interface. A softer spring in spatula setae explains their adhesion at lower preloads and assists in complete substrate contact. Attachment mechanisms revealed in adhesive setae with modified spatula and passive suckers provide insights for bioinspired designs of underwater attachment devices.
authors.affiliation: Department of Physics and Institute of Biophysics, National Chung Hsing University, Taichung 40227, Taiwan.
authors.forename: Ying
authors.initials: Y
authors.lastname: Chen
authors.affiliation: Department of Physics and Institute of Biophysics, National Chung Hsing University, Taichung 40227, Taiwan.
authors.forename: Ming-Chih
authors.initials: MC
authors.lastname: Shih
authors.affiliation: Department of Physics and Institute of Biophysics, National Chung Hsing University, Taichung 40227, Taiwan.
authors.forename: Ming-Huang
authors.initials: MH
authors.lastname: Wu
authors.affiliation: Department of Entomology, National Taiwan University, Taipei 10617, Taiwan.
authors.forename: En-Cheng
authors.initials: EC
authors.lastname: Yang
authors.affiliation: Department of Physics and Institute of Biophysics, National Chung Hsing University, Taichung 40227, Taiwan Department of Life Sciences, National Chung Hsing University, Taichung 40227, Taiwan kjchi@phys.nchu.edu.tw.
authors.forename: Kai-Jung
authors.initials: KJ
authors.lastname: Chi
mesh_terms: D000268:Adhesiveness; D000818:Animals; D001517:Coleoptera / Q000033:anatomy & histology / Q000502:physiology*; D003198:Computer Simulation; D005121:Extremities / Q000033:anatomy & histology / Q000502:physiology*; D017276:Friction; D006197:Hair / Q000502:physiology*; D007101:Immersion / Q000503:physiopathology*; D008297:Male; D008954:Models, Biological; D058618:Sensilla / Q000033:anatomy & histology / Q000502:physiology*; D033081:Shear Strength / Q000502:physiology; D013314:Stress, Mechanical; D014783:Viscosity
keywords: attachment–detachment process; biofibrillar adhesives; diving beetles; suction; underwater attachment; viscous adhesion