Slippery pores: anti-adhesive effect of nanoporous substrates on the beetle attachment system

Traction experiments with adult seven-spotted ladybird beetles Coccinella septempunctata (L.) were carried out to study the influence of surface structure on insect attachment. Force measurements were performed with tethered walking insects, both males and females, on five different substrates: (i) smooth glass plate, (ii) smooth solid Al2O3 (sapphire) disc, and (iii-v) porous Al2O3 discs (anodisc membranes) with the same pore diameter but different porosity. The traction force of beetles ranged from 0.16 to 16.59 mN in males and from 0.32 to 8.99 mN in females. In both sexes, the highest force values were obtained on smooth solid surfaces, where males showed higher forces than females. On all three porous substrates, forces were significantly reduced in both males and females, and the only difference within these surfaces was obtained between membranes with the highest and lowest porosity. Males produced essentially lower forces than females on porous samples. The reduction in insect attachment on anodisc membranes may be explained by (i) possible absorption of the secretion fluid from insect adhesive pads by porous media and/or (ii) the effect of surface roughness. Differences in attachment between males and females were probably caused by the sexual dimorphism in the terminal structure of adhesive setae.