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Beetles’ secretion and adhesion

MSC : B. Abou

For a living being, the ability to adhere to a substrate requires some specific properties to circumvent the roughness of most encountered surfaces.

Beetles’ secretion

Pads of beetles are covered with long, deformable setae, each ending in a micrometric terminal plate coated with secretory fluid. It was recently shown that the layer of the pad secretion covering the terminal plates is responsible for the generation of strong attractive forces.

We have quantitatively evaluated the rheological properties of the attachment pad secretion in the beetle Leptinotarsa decemlineata (Coleoptera, Chrysomelidae) for the first time, using passive microrheology (Collaboration : S. Gorb, Kiel University). The beetle secretion was found to behave as a purely viscous liquid, of high viscosity (about 100 times the water viscosity), over the range of frequencies investigated. This indicates that beetle locomotion might not need a specifically viscoelastic behaviour.

We describe a scenario for the contact formation between the spatula at the setal tip and a smooth substrate, during the insect walk (Collaboration : C. Gay, MSC). We then estimate the duration of the attachment process of the spatula onto the substrate, and hence of the insect pads, on the basis of the above rheological measurements. It turns out that the estimate T ≃ 10 ms obtained for the contact duration is compatible with the duration observed in video recordings of walking beetles (S. Gorb 2010, personal observation).

One might wonder, however, why the secretion viscosity is so high (100 times the viscosity of water). A lower viscosity would ease the insect’s locomotion and speed up its pace. It is not unreasonable to suggest that high viscosity, and correspondingly high molecular weight, ensures slow evaporation—a crucial issue at such small length scales. Additionally, high viscosity ensures that the adhesion is robust under unexpected and rapidly varying conditions.

Extensive collection of femtolitre pad secretion droplets in the beetle Leptinotarsa Decemlineata allows nanolitre microrheology, J. R. Soc. Interface, vol. 7, page 1745 (2010)

We are now interested in the secretion rheological properties of insects with various adhesion mechanisms. Collaboration : W. Federle, Cambridge University.

Nom des membres actuels ou anciens

GAY Cyprien, ABOU Bérengère

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