Adult banana root borers were exposed to various relative humidity gradients in small circular arenas and their distributions in the dry and wet halves of the gradient were recorded. Different humidities were provided over water, graded solutions of KOH, or “Drierite” (anhydrous CaSO4). In general, males tended to aggregate in the lower humidities (except at the extremes of the humidity ranges), whereas females favored higher humidities. The weevils orient in humidity gradients by means of orthokinesis, klinokinesis, and klinotaxis; titubant reactions are also exhibited. Changes in behavior in humidity gradients of weevils from which various sense organs had been removed showed that hygroreceptors are present on the antennal clubs, the tip of the proboscis, and elsewhere. In males, sense organs on the antennal clubs mediate a dry response, those on the tip of the proboscis (mouthparts?) a wet response. Removal of the hygroreceptors from the antennae and proboscis did not eliminate a humidity reaction in males exposed to extreme humidity gradients (i.e., 15% to 100% and <5% to 100% r.h.), indicating the presence of additional hygroreceptors elsewhere on the insect's body that can mediate a reaction to humidity. In females, at least some receptors that mediated a wet response in the gradient between 51% to 100% r.h. are located on the antennal clubs and the tip of the proboscis; both sets of receptors had to be removed to eliminate the normal wet response. In very dry air the weevils walked abnormally and frequently fell onto their sides or backs; however, walking in dry air improved with time, which suggests that the receptors mediating this aberrant behavior become habituated. Ability to walk in dry air improved when Vaseline was applied to the leg joints; this suggests that receptors (proprioceptors?) adjacent to the leg articulations mediate the abnormal walking behavior in low humidities. Stimulation of the receptors may result from distortion of the cuticle as it dries; drying of the cuticle is prevented by Vaseline, and the receptors are not distorted, thus permitting the insects to walk more normally.