Adaptive camouflage: What can be learned from the wetting behaviour of the tropical flatbugs Dysodius lunatus and D. magnus

The neotropical flatbug species and show a fascinating camouflage principle. Its appearance renders the animal hardly visible on the bark of trees. However, when getting wet due to rain, bark changes its colour and gets darker. In order to keep the camouflage effect, it seems as if some species benefit from their ability to hold a water film on their cuticle and therefore change their optical properties when wetted by water too. This camouflage behaviour requires the insect to have a hydrophilic surface and passive surface structures, which facilitate the liquid spreading. Here we show morphological and chemical characterisations of the surface, especially the cuticular waxes of Scanning electron microscopy revealed that the animal is covered with pillar-like microstructures which in combination with a surprising chemical hydrophilicity of the cuticle waxes, render the bug almost superhydrophilic: Water spreads immediately across the surface. We could theoretically model this behaviour assuming the effect of hemi-wicking (a state in which a droplet sits on a rough surface, partwise imbibing the structure around). Additionally the principle was abstracted and a laser patterned polymer surface, mimicking the structure and contact angle of -wax, shows exactly the behaviour of the natural role model - immediate spreading of water and the formation of a thin continuous water film changing optical properties of the surface.