Abstract
Jewel beetles of the genus Melanophila possess a pair of metathoracic infrared (IR) organs. These organs are used for forest fire detection because Melanophila larvae can only develop in fire killed trees. Several reports in the literature and a modeling of a historic oil tank fire suggest that beetles may be able to detect large fires by means of their IR organs from distances of more than 100 km. In contrast, the highest sensitivity of the IR organs, so far determined by behavioral and physiological experiments, allows a detection of large fires from distances up to 12 km only. Sensitivity thresholds, however, have always been determined in non-flying beetles. Therefore, the complete micromechanical environment of the IR organs in flying beetles has not been taken into consideration. Because the so-called photomechanic sensilla housed in the IR organs respond bimodally to mechanical as well as to IR stimuli, it is proposed that flying beetles make use of muscular energy coupled out of the flight motor to considerably increase the sensitivity of their IR sensilla during intermittent search flight sequences. In a search flight the beetle performs signal scanning with wing beat frequency while the inputs of the IR organs on both body sides are compared. By this procedure the detection of weak IR signals could be possible even if the signals are hidden in the thermal noise. If this proposed mechanism really exists in Melanophila beetles, their IR organs could even compete with cooled IR quantum detectors. The theoretical concept of an active amplification mechanism in a photon receptor innervated by highly sensitive mechanoreceptors is presented in this article.
Highlights
With 13 recent species jewel beetles of the genus Melanophila mainly can be found in the boreal and temperate forests of the holarctic zone (Bellamy, 2008)
It could not be demonstrated that Melanophila beetles could be lured by the smell of smoke (Evans, 1964) or that beetles resting at temperatures of 25◦C could be aroused by smoke
These stalks are missing in the photomechanic IR sensilla of pyrophilous Aradus bugs which are quite similar to the Melanophila IR sensilla but are not enclosed in a pit organ (Schmitz et al, 2010)
Summary
With 13 recent species jewel beetles of the genus Melanophila mainly can be found in the boreal and temperate forests of the holarctic zone (Bellamy, 2008). As an adaptation to the pyrophilous way of life the 1 cm long black beetles (Figure 1A) are equipped with special antennal smoke receptors (Schütz et al, 1999) and one pair of metathoracic IR organs (Evans, 1964; Vondran et al, 1995; Schmitz et al, 1997). An IR organ consists of a little array of dome-shaped sensilla which is situated at the bottom of a little pit (Figures 1B, 2A,B). The inner spherule of each sensillum is innervated by a ciliary mechanosensitive cell (Figure 2C; Vondran et al, 1995; Schmitz et al, 2007).
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