Sensory equipment and adaptations to the fire habitat of the antennae of the Australian ´firebeetle´ Merimna atrata (Coleoptera; Buprestidae)

The ‘Australian firebeetle’ Merimna atrata approaches fires in Eucalyptus forests for reproduction. Beetles stay on a postfire area as long as burning wood or hot ashes emit heat and smoke. Abdominal infrared receptors protect the beetles from landing on hot spots; however, until now fire-specific adaptations of the antennae have not been investigated in more detail. This affects the localization of olfactory sensilla used for the perception of smoke and in addition mechanisms to protect delicate sensilla against desiccation and pollution. Moreover, nothing was known about antennal thermo-/hygroreceptors in Merimna atrata. We found strong evidence for a functional grouping of the sensilla into receptors used on the ground or in flight, respectively. A first group comprises the outer visible sensilla, i.e. mechanosensory bristles, short gustatory sensilla and a small field of very short olfactory sensilla. They are used when the beetle is running around on the fireground on burnt bark or ashes. A second group of sensilla is hidden in closeable cavities on antennomeres 4–11. If the cavities are closed, the sensilla inside are fully protected. If the cavities are opened in flight, the beetles can make use of many multiporous basiconic sensilla and multiporous basiconic grooved peg sensilla for smoke detection. Minute modified sensilla coelocapitula occurring in small numbers in the cavities too, most probably serve as thermoreceptors. As a result the placing of sensilla deserving protection in closeable cavities and the reduction in number and length of the external sensilla can be interpreted as adaptations to the fire habitat.