Electrophysiological characterisation of the infrared organ of the Australian "Little Ash Beetle" Acanthocnemus nigricans (Coleoptera, Acanthocnemidae).

This study characterises the response properties of the sensilla located on the prothoracic disc organ of the beetle Acanthocnemus nigricans, such as intensity response functions and temporal coding properties. Warming the sensilla by a red laser accelerated their ongoing spiking activity, cessation of the stimulus suppressed their firing as revealed by extracellular recordings. Convective heat sources also increased sensillum activity, but stimuli of other modalities failed to elicit responses. The response threshold was between 11 and 25 mW/cm2 and latencies ranged between 20 and 40 ms. Repeating stimuli with frequencies between 5 and 20 Hz were reliably resolved by the sensilla. This temporal resolution enables the disc sensilla to represent behaviourally relevant changes in heat stimuli in a thermally patchy environment. These findings complement our knowledge on the sensory physiology of pyrophilous insects by hinting at two different, elementary orientation strategies evolved in the three pyrophilous beetle species described. A. nigricans seems to be best adapted to short-range orientation on freshly burnt areas.