Infrared detection without specialized infrared receptors in the bloodsucking bug Rhodnius prolixus

Bloodsucking bugs use infrared radiation (IR) for locating warm-blooded hosts and are able to differentiate between infrared and temperature (T) stimuli. This paper is concerned with the neuronal coding of IR in the bug Rhodnius prolixus. Data obtained are from the warm cells in the peg-in-pit sensilla (PSw cells) and in the tapered hairs (THw cells). Both warm cells responded to oscillating changes in air T and IR with oscillations in their discharge rates. The PSw cells produced stronger responses to T oscillations than the THw cells. Oscillations in IR did the reverse: they stimulated the latter more strongly than the former. The reversal in the relative excitability of the two warm cell types provides a criterion to distinguish between changes in T and IR. The existence of strongly responsive warm cells for one or the other stimulus in a paired comparison is the distinguishing feature of a "combinatory coding" mechanism. This mechanism enables the information provided by the difference or the ratio between the response magnitudes of both cell types to be utilized by the nervous system in the neural code for T and IR. These two coding parameters remained constant, although response strength changed when the oscillation period was altered. To discriminate between changes in T and IR, two things are important: which sensory cell responded to either stimulus and how strong was the response. The label warm or infrared cell may indicate its classification, but the functions are only given in the context of activity produced in parallel sensory cells.