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First published online November 24, 2003
Journal of Experimental Biology 207, 67-74 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00716

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Biomechanics of ant adhesive pads: frictional forces are rate- and temperature-dependent

Walter Federle^1,*, Werner Baumgartner² and Bert Hölldobler¹

¹ Zoologie II, Biozentrum, Am Hubland, D-97074 Würzburg, Germany
² Institute of Anatomy and Cell Biology, University of Würzburg, Koellikerstrasse 6, D-97070 Würzburg, Germany

^* Author for correspondence (e-mail: wfederle{at}biozentrum.uni-wuerzburg.de)

Accepted 23 September 2003

Tarsal adhesive pads enable insects to hold on to smooth plant surfaces. Using a centrifuge technique, we tested whether a 'wet adhesion' model of a thin film of liquid secreted between the pad and the surface can explain adhesive and frictional forces in Asian Weaver ants (Oecophylla smaragdina).

When forces are acting parallel to the surface, pads in contact with the surface can slide smoothly. Force per unit pad contact area was strongly dependent on sliding velocity and temperature. Seemingly consistent with the effect of a thin liquid film in the contact zone, (1) frictional force linearly increased with sliding velocity, (2) the increment was greater at lower temperatures and (3) no temperature dependence was detected for low-rate perpendicular detachment forces. However, we observed a strong, temperature-independent static friction that was inconsistent with a fully lubricated contact. Static friction was too large to be explained by the contribution of other (sclerotized) body parts. Moreover, the rate-specific increase of shear stress strongly exceeded predictions derived from estimates of the adhesive liquid film's thickness and viscosity.

Both lines of evidence indicate that the adhesive secretion alone is insufficient to explain the observed forces and that direct interaction of the soft pad cuticle with the surface ('rubber friction') is involved.

Key words: friction, adhesion, attachment, tarsus, arolium, rubber friction, Asian Weaver ant, Oecophylla smaragdina

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