Characterization of beetle Melanophila acuminata (coleoptera: buprestidae) infrared pit organs by high-performance liquid chromatography/mass spectrometry, scanning electron microscope, and fourier transform-infrared spectroscopy

Abstract

The beetle Melanophila acuminata (De Geer) is able to detect infrared radiation emitted from forest fires with two infrared receptors (pit organs) located on the metathorax next to the coxal cavity of the second set of thoracic legs. Each pit organ houses ≈70 single IR sensilla, which probably transduce incoming IR radiation into a mechanical event. These pit organs may exhibit chemical differences from the cuticle covering other parts of the beetle, which could account for their enhanced infrared detection. Infrared pit organs and the elytra, the cuticle covering the wings, were subjected to LC/MS analysis by extracting the cuticle with solvents that varied in polarity. The resulting chromatograms and mass spectra were used to identify differences in solute mobility and composition. Energy dispersive spectroscopy and point dwell maps indicated only the presence of carbon, oxygen and nitrogen for the pit organs, wings and elytra. FT-IR transmission spectra were obtained for the pit organs, eye scales, coxal cavities and wings. FT-IR analysis detected chemical bonding for the sensilla of the pit organ, which agreed with the energy dispersive spectroscopy results. Sensilla absorb IR radiation at the maximum emission wavelength of forest fires, and there is no indication of a complex chemical reaction occurring upon IR absorption. Therefore, the degree of expansion of the cuticular apparatus of the IR sensilla seems to be the sole contributing factor for the photomechanical portion of infrared detection.