Impact of chemical manipulation of tarsal liquids on attachment in the Colorado potato beetle, Leptinotarsa decemlineata.

Insect tarsal attachment forces are thought to be influenced by the viscosity and surface tension of a thin film of adhesive liquid (wet adhesion). In beetles, this fluid has been shown to be composed mainly of lipophilic substances that are similar to the cuticular lipids. In this study we investigate whether and how the chemical composition of footprint lipids affects attachment forces in the Colorado potato beetle, Leptinotarsa decemlineata. After application of standardised mixtures of synthetic n-alkanes or alkenes, or a concentrated hydrocarbon extract to the surface of the elytra, we tested the beetles' attachment performance using a beam force transducer. The results show that only the unsaturated components, but not the straight-chained alkanes reduced friction forces, confirming that attachment performance is influenced by the chemical composition of the adhesive secretion. We estimated the volume of footprint droplets and calculated a mean thickness of the liquid layer of 0.04 microm. The measured friction exceeded the viscous and capillary force expected for a film of this thickness. Therefore, alternative mechanisms (i.e. shear-thinning and solid-like behaviour) for the generation of attachment forces and their dependence on the chemical composition of the liquid are discussed.