Frictional characteristics of the beetle head-joint material

Frictional properties of the head part in the beetle head–thorax articulation were studied in two beetle species, Pachnoda marginata and Geotrupes stercorarius (Coleoptera, Scarabaeoidea). Using the microtribometer, the head part of the joint was brought into contact with the flat glass and slid along a distance of 120 μm. The friction experiment was carried out with two conditions of the head material: fresh and air-dried. The tribosystem, consisting of the sapphire sphere in contact with flat glass, was used for comparison. Frictional coefficient of the joint material on glass was significantly lower than that of the sapphire sphere on glass. The material of the joint cuticle is rather stiff and the surface is hydrophobic (the contact angle of distilled water was 88.3°). It is suggested here that the high stiffness of the joint material and hydrophobicity of the joint surface are parts of the mechanism minimizing friction in insect joints.