Humidity-dependent cold cells on the antenna of the stick insect

We present the first systematic study of the response of insect "cold cells" to a variation in the partial pressure of water vapor in ambient air. The cold cells on the antenna of the stick insect respond with an increase in activity when either the temperature or the partial pressure of water vapor is suddenly reduced. This double dependency does not in itself constitute bimodality because it could disappear with the proper choice of parameters involving temperature and humidity. In this study, we demonstrate that the evaporation of a small amount of water from the sensillum surface resulting from a drop in the water vapor pressure-leading to a transient drop in temperature and thus to a brief rise in impulse frequency-is the most plausible explanation for this bimodal response. We also show with an order-of-magnitude calculation that this mechanism is plausible and consistent with the amounts of water vapor potentially present on the sensillum. We hypothesize that a film of moisture collects on the hygroscopic sensillum surface at higher humidity and then tends to evaporate when humidity is lowered. The water might even be bound loosely within the cuticular wall, a situation conceivable in a sensillum that contains two hygroreceptive cells in addition to the cold cell.