Beetle adhesive hairs differ in stiffness and stickiness: in vivo adhesion measurements on individual setae

Leaf beetles are able to climb on smooth and rough surfaces using arrays of micron-sized adhesive hairs (setae) of varying morphology. We report the first in vivo adhesive force measurements of individual setae in the beetle Gastrophysa viridula, using a smooth polystyrene substrate attached to a glass capillary micro-cantilever. The beetles possess three distinct adhesive pads on each leg which differ in function and setal morphology. Visualisation of pull-offs allowed forces to be measured for each tarsal hair type. Male discoidal hairs adhered with the highest forces (919 +/- 104 nN, mean +/- SE), followed by spatulate (582 +/- 59 nN) and pointed (127 +/- 19 nN) hairs. Discoidal hairs were stiffer in the normal direction (0.693 +/- 0.111 N m(-1)) than spatulate (0.364 +/- 0.039 N m(-1)) or pointed (0.192 +/- 0.044 N m(-1)) hairs. The greater adhesion on smooth surfaces and the higher stability of discoidal hairs help male beetles to achieve strong adhesion on the elytra of females during copulation. A comparison of pull-off forces measured for single setae and whole pads (arrays) revealed comparable levels of adhesive stress. This suggests that beetles are able to achieve equal load sharing across their adhesive pads so that detachment through peeling is prevented.