1. Removal of both antennae of the mealworm beetle, Tenebrio molitor, completely abolishes the animal's reaction to humidity. Removal of both maxillary palps has no effect on the reaction.
2. The quantitative distribution of the five types of sensillae present along the eleven segments of the antenna is described. Two of these types occur also on the maxillary palps.
3. Progressive symmetrical amputation of antennal segments leads to a gradual reduction of the humidity reaction to zero; there is a reaction with four segments remaining on each side and no reaction when only three segments remain.
4. Asymmetrical amputation of the more basal segments shows that humidity receptors are nevertheless present on these and that a threshold number of sensillae must be left in action before a reaction occurs.
5. The experimental work, taken in combination with observations in the distribution of sensillae, shows that the pit peg organs are hygro-receptive. Either the peg organs or the bristles or both are hygro-receptive as well. It is, however, unlikely that the bristles are hygro-receptive.
6. Experimental work on the locomotory activity of this beetle shows that the conclusions are not invalidated by any general ill effects of amputation of antennae.
7. It is suggested that the hygro-receptors function hygroscopically.