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The Journal of Experimental Biology 205, 2479-2488 (2002)
© 2002 The Company of Biologists Limited

Roughness-dependent friction force of the tarsal claw system in the beetle Pachnoda marginata (Coleoptera, Scarabaeidae)

Zhendong Dai^1,2, Stanislav N. Gorb^1,* and Uli Schwarz¹

¹ Biological Microtribology Group, Division II, Max-Planck-Institute of Developmental Biology, Spemannstrasse 35, D-72076, Tuebingen, Germany
² College of Mechanical and Electric Engineering, Nanjing University of Aeronautics and Astronautics, 29 Yudao Street210016, Nanjing, China

^* Author for correspondence at present address: Evolutionary Biomaterials Group, Max-Planck-Institut fuer Metallforschung, Heisenbergstrasse 3, D-70569 Stuttgart, Germany (e-mail: s.gorb{at}mf.mpg.de )

Accepted 15 May 2002

This paper studies slide-resisting forces generated by claws in the free-walking beetle Pachnoda marginata (Coleoptera, Scarabaeoidea) with emphasis on the relationship between the dimension of the claw tip and the substrate texture. To evaluate the force range by which the claw can interact with a substrate, forces generated by the freely moving legs were measured using a load cell force transducer. To obtain information about material properties of the claw, its mechanical strength was tested in a fracture experiment, and the internal structure of the fractured claw material was studied by scanning electron microscopy. The bending stress of the claw was evaluated as 143.4-684.2 MPa, depending on the cross-section model selected. Data from these different approaches led us to propose a model explaining the saturation of friction force with increased texture roughness. The forces are determined by the relative size of the surface roughness R_a (or an average particle diameter) and the diameter of the claw tip. When surface roughness is much bigger than the claw tip diameter, the beetle can grasp surface irregularities and generate a high degree of attachment due to mechanical interlocking with substrate texture. When R_a is lower than or comparable to the claw tip diameter, the frictional properties of the contact between claw and substrate particles play a key role in the generation of the friction force.

Key words: friction, locomotion, leg, cuticle, insect, biomaterials, biomechanics, material properties, Pachnoda marginata, Scarabaeidae, Coleoptera

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