Friction properties of the head articulation in the beetle Pachnoda marginata (Coleoptera, Scarabaeidae)

Abstract

The head articulation of the beetle Pachnoda marginata is a micro-tribological system which consists of the ventral convex structure called gula and the corresponding concave surface of the prothorax. The surfaces of both parts are in contact and are expected to be optimised for friction reduction. The relationship between structure, mechanical properties of the cuticle material and friction properties of this micro-tribological system are investigated. The surface and material structure of the gula and its prothoracic counterpart were studied in fractured pieces of the cuticle using scanning electron microscopy (SEM). Friction force measurements and contact area estimations between the gula and a glass plate were carried out for different normal loads (0.1–10.0 mN) using two different microtribometers. The tribological behavior of the gula cuticle was studied on the (1) fresh, (2) dry, and (3) dry chemically (chloroform-methanol) treated samples. The dry samples exhibited a considerably rougher surface compared to the fresh ones. Furthermore, the chemical treatment led to some decrease in surface roughness. The fresh gula cuticle had the largest contact area and the highest friction coefficient. The drying out of the cuticle led to a decrease in both the contact area and friction coefficient. The friction coefficient was the lowest in the chemically treated gula, although the contact area was larger than in the dry condition. The tribological results were partly explained by direct measurements of the contact area fitted by the Hertz and JKR contact models.