Thermomechanical properties of the stimulus transducing cuticle in the infrared organ of Merimna atrata (Coleoptera, Buprestidae).

The pyrophilous Australian "fire-beetle" Merimna atrata strongly depends on the occurrence and localization of forest fires for its reproduction. As a special adaptation to its unusual biology, elaborate infrared (IR) organs have evolved in this species. The IR-organs consist of a specialized cuticular portion, the absorbing area, innervated by a sensory complex. The sensory complex contains a thermosensitive multipolar neuron with a specialized dendritic region, the terminal dendritic mass, and a mechanosensitive unit represented by a chordotonal organ (CO). Evidence for the IR-receptive function so far has only been provided for the multipolar neuron. Based on morphological data, it has been hypothesized that the CO could also be involved in IR-reception by measuring minute thermal deformations of the absorbing area. To test this hypothesis, we investigated structural features like cuticle thickness, reduced Young's modulus and hardness of the absorbing area. The results were used in finite element simulations to analyze the thermomechanical behavior and performance of the IR-organ. Our findings indicate that considerable thermal deformation of the absorbing area occurs, supporting the hypothesis that the CO could function as photomechanical IR-receptor. Interestingly, at the innervation site of the CO the lowest relative displacements of the absorbing area were found. This may indicate that the CO as putative photomechanic IR-receptor has not been adapted according to the requirements of highest sensitivity. Probable benefits of the bimodal innervation by a thermosensory and a mechanosensory unit and their possible interaction for an improved performance of the IR-organ are discussed.