The Role of Surface Viscosity in the Escape Mechanism of the Stenus Beetle

Beetles of the species Stenus comma live and hunt close to ponds and rivers, where they occasionally fall on the water surface. To escape this jeopardized state, the beetle developed a strategy relying on the excretion of a secretion containing the substances stenusine and norstenusine. They reduce surface tension and propel the bug to the saving river bank. These substances were synthesized and analyzed with respect to their equilibrium and dynamic adsorption properties at the air-water interface (pH 7, 23 ± 1 °C). The surface dilatational rheological characteristics in a frequency range from 2 to 500 Hz at molar bulk concentrations of 20.6 mmol L(-1) were studied using the oscillating bubble technique. Both alkaloids formed surface viscoelastic adsorption layers. The frequency dependence of the surface dilatational modulus E could successfully be described by the extended Lucassen-van den Tempel model accounting for a nonzero intrinsic surface viscosity κ. The findings confirmed a dual purpose of the spreading alkaloids in the escape mechanism of the Stenus beetle. Next to generating a surface pressure, a transition to surface viscoelastic behavior of the adsorbed layers was observed.