Matched filter properties of infrared receptors used for fire and heat detection in insects

Abstract

About 25 insect species are attracted by forest fires and thus can be found on freshly burnt areas after fires. In three genera of pyrophilous beetles and one genus of pyrophilous bugs, infrared (IR) receptors have been discovered. From a technical point of view, insect IR receptors can be classified into two classes: Bolometer-like sensors innervated by thermoreceptors and so-called photomechanic sensors which are innervated by mechanoreceptors. Despite of their different functional principles, insect IR receptors all show the same built-in filter properties. Remarkably, these filters were already preset by the absorption spectra of the gases in the atmosphere and the chemical composition of the insect cuticle. The atmospheric windows can be regarded as valuable filters (Filter 1) because emission maxima of relevant IR sources like fires or warm-blooded creatures are located within the MW(mid-wavelength)IR and LW(long wavelength)IR windows. Filter 2 is given by cuticular absorption. Insect cuticle can be regarded as a composite material consisting of biopolymers that show strong IR absorption bands in the MWIR. Because both filters perfectly match, an IR-sensitive pyrophilous insect is able to efficiently sense MWIR radiation emitted by a forest fire. Thus, filters could be used without further modifications enabling the underlying sensory cells to perceive a maximum of temperature increase and/or thermal expansion.