This study gives the first electrophysiological evidence of hygroreceptors in carabids. Extracellular recordings from the antennal dome-shaped sensilla of the carabid beetle Pterostichus oblolongopunctatus (Coleoptera, Carabidae) clearly show the presence of moist and dry neuron antagonistically responding to humidity changes. The cold neuron of the same sensillum did not respond to changes in humidity. For the first time, we demonstrate that the binary system of two antagonistic hygroreceptor neurons discriminates differences between steady-state humidity levels more sensitively than either neuron separately. Another advantage of the binary system is that it guarantees immediate and strong phasic-tonic response to rapid humidity changes in either direction. In the hygrosensing system of carabids, this would allow detection of subtle step-changes in humidity with greater sensitivity than differences in steady-state values of humidity. Thus, construction of the hygrosensing system with opposing receptor neurons may allow insects to detect environmental humidity differences critical for their habitat and microhabitat selection, and survival with great precision. Copyright 2010 Elsevier Ltd. All rights reserved.