Nano-Porous Substrates Reduce Beetle Attachment Force

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Traction experiments with the seven-spotted ladybird beetles Coccinella septempunctata (L.) (Coleoptera, Coccinellidae) were carried out to study the influence of surface structure on insect attachment. Force measurements were performed with tethered walking insects using a load cell force transducer. For each beetle, forces were measured on five different substrates: (1) smooth glass plate; (2) smooth solid Al2O3 (sapphire) disc; (3 – 5) porous Al2O3 discs (anodiscs, back side) with the same pore diameter (220 – 235 nm), but different porosity (28, 42 and 51%). Males (N = 10) and females (N = 10) were used in experiments (10 single runs on each surface). Additionally, inversion tests were performed after each traction force measurement. The force ranged from 0.368 to 10.370 mN in males and from 0.514 to 6.262 mN in females. In both sexes, the highest force values were obtained on the smooth glass and sapphire surfaces, where males generated considerably higher forces compared to females. On all three porous substrates, forces were significantly reduced in both males and females, and the only difference for surfaces was obtained between two extremes: anodiscs with the highest (51%) and lowest (28%) porosity. Males produced essentially lower forces than females on anodiscs samples. Experimental insects performed well and showed normal locomotion on both smooth surfaces. On all anodiscs samples, beetles usually were not able to get a grip and slid over the surface, refused to walk and came to a standstill or even turned over on their backs. When substrates were inverted to 90° and 180°, insects were still able to remain attached to both the glass and sapphire samples, but failed on anodiscs. The reduction of insect attachment on anodiscs surfaces is explained by (1) possible absorption of the secretory fluid from insect pads by porous media and (2) effect of surface roughness.