Beetles of the genus Melanophila and certain flat bugs of the genus Aradus approach forest fires. For the detection of fires and of hot surfaces, the pyrophilous species of both genera have developed infrared (IR) receptors, which have developed from common hair mechanoreceptors. Thus, this type of insect IR receptor has been termed photomechanic. Compared to a hair mechanoreceptor, a photomechanic IR sensillum shows the following special features, (i) The formation of a complex cuticular sphere instead of the bristle; the sphere consists of an outer exocuticular shell as well as of a cavernous microfluidic core, (ii) The enclosure of the dendritic tip of the mechanosensitive neuron inside the core in a liquid-filled chamber. Hence, we propose that a photomechanic IR sensillum most probably acts as a microfluidic converter of IR radiation into an increase in internal pressure in- side the sphere, which is measured by a mechanosensitive neuron. A simple model for this biological IR sensor is the Golay sensor, which is filled with a liquid instead of gas. Here, the absorbed IR radiation results in a pressure increase of the liquid and the deflection of a thin membrane. For the evaluation of this model, analytical formulas are presented, which permit the calculation of the pressure increase in the cavity, the deformation of the membrane and the time constant of an artificial leak to compensate ambient temperature changes. Some organic liquids with high thermal expansion coefficients may improve the deflection of the membrane compared to water.
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