A description of the fine structure of the infrared radiation (IR) sensor of Melanophila acuminata (De Geer) and of the role of the wings during flight in mechanically modulating incoming IR into on and off pulses, supports a thermopneumatic model of IR perception by this insect. The model postulates that pulsed IR, corresponding to wavelengths emitted by fires, enters an enclosed, airtight, thick-walled cavity in each sensor through an apical epicuticular waveguide and is absorbed by a thin, chitinous endocuticular lamella of the cavity wall. The lamella is heated and in turn heats and expands the enclosed air that then compresses a basal cuticular cap that displaces the tubular body of the sensor, triggering a dendritic impulse and transducing an action potential in the neuron. This process is possible because of the absorption by the chitinous lamella of the wavelengths of IR emitted by fires resulting in extraordinary IR wavelength discrimination, radiant energy sensitivity, and rapid response time. These properties distinguish the sensor from temperature sensors of insects, sometimes erroneously designated as IR sensors, that respond relatively slowly to broadband wavelength radiation.