Infrared detection withoutspecialized infrared receptors in the bloodsucking bug Rhodniusprolixus. J Neurophysiol 112: 1606–1615, 2014. First published June18, 2014; doi:10.1152/jn.00317.2014.—Bloodsucking bugs use infraredradiation (IR) for locating warm-blooded hosts and are able todifferentiate between infrared and temperature (T) stimuli. This paperis concerned with the neuronal coding of IR in the bug Rhodniusprolixus. Data obtained are from the warm cells in the peg-in-pitsensilla (PSw cells) and in the tapered hairs (THw cells). Both warmcells responded to oscillating changes in air T and IR with oscillationsin their discharge rates. The PSw cells produced stronger responses toT oscillations than the THw cells. Oscillations in IR did the reverse:they stimulated the latter more strongly than the former. The reversalin the relative excitability of the two warm cell types provides acriterion to distinguish between changes in T and IR. The existence ofstrongly responsive warm cells for one or the other stimulus in apaired comparison is the distinguishing feature of a “combinatorycoding” mechanism. This mechanism enables the information providedby the difference or the ratio between the response magnitudesof both cell types to be utilized by the nervous system in the neuralcode for T and IR. These two coding parameters remained constant,although response strength changed when the oscillation period wasaltered. To discriminate between changes in T and IR, two things areimportant: which sensory cell responded to either stimulus and howstrong was the response. The label warm or infrared cell may indicateits classification, but the functions are only given in the context ofactivity produced in parallel sensory cells.