1. The humidity reactions of Oniscus asellus, Porcellio scaber and Armadillidium vulgare have been analysed and compared.

2. The mechanism whereby the three species collect in moist air is twofold, consisting of (a) hygrokinesis, or decrease in activity and speed in moist air, and (b) of more frequent turnings in space, retaining them in the areas of greater humidity.

3. These mechanisms are most clearly expressed in Oniscus asellus and least in Armadillidium vulgare. This sequence may be correlated with the resistance to desiccation of the three species, which is greatest in Armadillidium vulgare, and greater in Porcellio scaber than in Oniscus asellus.

4. It is suggested that the humidity reactions of isopods are controlled by water loss by evaporation from the whole body.

5. A correlation between hygrokinesis and thigmokinesis was observed in Porcellio scaber.

6. There appears to be a reversal from negative to positive phototaxis in Oniscus asellus, correlated with the water loss by evaporation.

7. The humidity reactions, low thigmokinesis and negative phototaxis combine to retain the isopods in damp, dark habitats.

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