Fine structure and physiology of the infrared receptor of beetles of the genus Melanophila (Coleoptera: Buprestidae)

Buprestid beetles of the genus Melanophila (Coleoptera: Buprestidae) possess paired thoracic pit organs. Each pit houses about 70 tightly packed infrared sensilla that enable the beetles to detect forest fires at long range. The cuticular apparatus of a single infrared sensillum consists of an endocuticular sphere with an average diameter of 15 μm. Each sphere is innervated from below by the dendritic outer segment of a single sensory neuron that shows the same ultrastructure as a typical hair mechanoreceptor (e.g., sensillum trichodeum). Several transitional stages between cuticular mechanoreceptors and infrared sensilla can be found in the neighbourhood of the infrared sensillum field (intermediate sensilla and so-called suppressed systems). First electrophysiological recordings show that the receptors at the bottom of the pit are true infrared receptors. The sensilla respond in a fast phasic manner to infrared radiation and even an exposition time of 2 ms is sufficient to release a single spike. All morphological and physiological findings suggest that the infrared sensilla have evolved from ordinary cuticular mechanoreceptors and that mechanical events are still part of the transduction process. A model about the possible function of the infrared receptors of Melanophila is presented.