1. Porcellio scaber exhibits hygrokinesis, being very active in dry air and almost motionless in nearly saturated air. With rising humidity a steadily increasing proportion of a batch of specimens is found to be inactive at any one moment.
2. P. scaber also shows hygrophobotactic behaviour.
3. The effect of both reactions is to collect the animals into moist air.
4. In a humidity gradient of given steepness the intensity of the humidity preference varies at different parts of the humidity range. There is no reaction at all if the gradient does not include a region below 65 per cent R.H. There is definite reaction to a difference of 6 per cent R.H. in approximately half-saturated air.
5. The humidity receptors, if they are confined to one area, apparently do not lie on the abdomen or on the head region.
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