1. The behaviour of Peripatopsis moseleyi (Wood-Mason) towards humidity has been investigated. At 25.5°C. the animal is found to have a preferred relative humidity of about 98%
2. Humidity receptors mediating a reaction towards the wet occur over the general surface of the body. Receptors mediating a reaction towards the dry occur on the antennae. The receptors on the body surface probably depend for their functioning upon the rate of evaporation of water.
3. The reaction towards the wet is effected by an orthokinesis involving both speed and activity. This is enhanced by a tendency to turn more frequently in the wet than in the dry. There is further a directed reaction which depends upon the simultaneous comparison of the stimulation intensities at the anterior and posterior ends of the animal.
4. The reaction away from saturation is due to a speed and activity orthokinesis.
5. The existence of a reaction away from saturation together with the opposing reactions away from dryness result in a preferred humidity just below saturation.