Abstract
Infrared (IR) receptors are so far known only in boid and crotalid snakes and in three genera of pyrophilous beetles that seek out forest fires. Pyrophilous insects can also be found in other orders, however, so it can be hypothesised that IR receptors also occur in some of these species. We investigated the pyrophilous Australian flat bug Aradus albicornis and found a small number of dome-shaped sensilla (diameter 13 μm) on the prothorax, which have previously not been described. Ultrastructural investigations revealed that the sensilla are characterised by a fluid-filled inner compartment enclosed in a round cuticular shell. The cuticular apparatus is innervated by the dendrite of a ciliary mechanoreceptor, which is fluidically coupled to the inner compartment. Electrophysiological recordings demonstrated that the sensilla respond to brief warming by red laser light or to broadband IR radiation. Depending on the radiation intensity (4.4–549 mW/cm2 tested, threshold measured as 11.3 mW/cm2), first spike latencies varied between 3.4 and 7.5 ms. Thus, our findings demonstrate that A. albicornis most probably possesses photomechanic IR sensilla resembling the metathoracic IR sensilla of buprestid beetles of the genus Melanophila. In the Melanophila sensillum, IR radiation causes thermal expansion of a fluid, which rapidly deforms the dendritic membrane of a mechanosensory cell. The existence of photomechanic IR receptors in both beetles and bugs demonstrates a remarkable convergent evolution towards this particular biophysical transduction mechanism and suggests that it provides selective advantages over other possible solutions.
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Acknowledgements
We thank Mike Cantelo and Brian Inglis from the Department of Environment and Heritage (DEC, Wanneroo, Western Australia) for their generous support and the permission to visit bushfires. We are grateful to the Nees Institute at the University of Bonn for the use of the SEM. Horst Bleckmann provided lab space, and Ernst Heiss (Innsbruck) assisted in identifying the flat bugs and provided additional Aradus species. Supported by a grant of the German Federal Ministry of Education and Research to H.S (Grant No. 01RIO644A). We thank Tom Matheson (Leicester, U.K.) for thoroughly improving the language.
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Schmitz, A., Gebhardt, M. & Schmitz, H. Microfluidic photomechanic infrared receptors in a pyrophilous flat bug. Naturwissenschaften 95, 455–460 (2008). https://doi.org/10.1007/s00114-008-0344-5
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DOI: https://doi.org/10.1007/s00114-008-0344-5