Material properties of photomechanical infrared receptors in pyrophilous Melanophila beetles and Aradus bugs

Jewel beetles of the genus Melanophila and some pyrophilous species of the flat bugs genus Aradus show a pyrophilous behaviour and have developed so-called photomechanical infrared (IR) receptors. In a spherical photomechanical IR sensillum incoming IR radiation is converted into micromechanical action, finally stimulating the dendritic tip of a mechanosensitive sensory cell. The tip is located inside a tiny cuticular sphere with a diameter of about 12 μm. The material properties of the different cuticular components of this sphere are of great importance for stimulus generation. We measured the modulus and hardness of the outer exocuticular shell of the sphere and the mesocuticle inside the core. Measurements were made by nanoindentation at sensilla which were partly cut open under dry as well as under rewetted (i.e. quasi-natural) conditions. We found that in the rewetted sensilla the outer exocuticular shell of the sphere in the Melanophila sensillum is about 50% harder and 20% stiffer than reference exocuticle, and that in both species especially the rewetted mesocuticle of the inner core of the IR sensilla is significantly softer (about 80% in Melanophila) and more compliant (about 90% also in Melanophila) than the reference mesocuticle. The findings can be interpreted as special adaptations of the cuticular microdomains of photomechanical infrared sensilla to enhance thermomechanical performance and, thereby, sensitivity.