Eur. Phys. J. E (2014) 37: 109
https://doi.org/10.1140/epje/i2014-14109-y

Regular Article

Dew condensation on desert beetle skin

¹ Dept. Physics and Appl. Math., University of Navarra, Pamplona, Spain
² Laboratoire de Physique et Mécanique des Milieux Hétérogènes, Unité Mixte de Recherches 7636 Centre National de la Recherche Scientifique, École Supérieure de Physique et Chimie Industrielles - Université Pierre et Marie Curie - Université Paris Diderot, 10 rue Vauquelin, 75231, Paris, France
³ LPMC-CNRS-UMR 7336, Université de Nice, 06108, Nice Cedex 2, France
⁴ Dept. of Enviromental Biology, University of Navarra, Pamplona, Spain
⁵ Department of Life Sciences, Natural History Museum, Cromwell Road, SW7 5BD, London, UK
⁶ OPUR, 60 rue Emeriau, 75015, Paris, France
⁷ Service des Basses Températures, Commissariat à l’Energie Atomique-Grenoble & Université Joseph Fourier, 17, rue des Martyrs, 38504, Grenoble, France

^* e-mail: wens@unav.es
^** e-mail: daniel.beysens@espci.fr

Received: 13 August 2014
Revised: 20 October 2014
Accepted: 22 October 2014
Published online: 20 November 2014

Abstract

Some tenebrionind beetles inhabiting the Namib desert are known for using their body to collect water droplets from wind-blown fogs. We aim to determine whether dew water collection is also possible for desert insects. For this purpose, we investigated the infra-red emissivity, and the wetting and structural properties, of the surface of the elytra of a preserved specimen of Physasterna cribripes (Tenebrionidæ) beetle, where the macro-structure appears as a series of “bumps”, with “valleys” between them. Dew formation experiments were carried out in a condensation chamber. The surface properties (infra-red emissivity, wetting properties) were dominated by the wax at the elytra surface and, to a lower extent, its micro-structure. We performed scanning electron microscope on histological sections and determined the infra-red emissivity using a scanning pyrometer. The emissivity measured (0.95±0.07 between 8–14μm) was close to the black body value. Dew formation occurred on the insect’s elytra, which can be explained by these surface properties. From the surface coverage of the condensed drops it was found that dew forms primarily in the valleys between the bumps. The difference in droplet nucleation rate between bumps and valleys can be attributed to the hexagonal microstructure on the surface of the valleys, whereas the surface of the bumps is smooth. The drops can slide when they reach a critical size, and be collected at the insect's mouth.