1. Groups of sixteen mites were starved for 48 hr. at 29% R.H. and then exposed for 18 or 24 hr. to one of nine humidities, from 0% to 93% R.H. They were weighed as groups before and after the treatments to determine total weight loss. Dry weights were also obtained to find water content and for the calculation of water and dry-weight losses. All work was done at 25° C.

2. Water loss, considered equivalent to total weight loss, was also obtained under several other conditions; and at all humidities it was found to be highest in mites killed in chloroform vapour while it was considerably less in those killed in HCN gas. Mites with spiracles kept open by air with 10% CO2 lost weight at rates midway between those for dead and those for living animals.

3. There is apparent regulation of body-water content as a percentage of the final weight over the whole humidity range.

4. Water loss is restricted by a CO2-sensitive mechanism, presumably the spiracles.

5. Active regulation of water loss by a cuticular mechanim was shown between 53% and 85% R.H., while at humidities below this, loss was actively restricted but not regulated.

6. It is postulated that both restriction and regulation are brought about by the same mechanism, which might be a form of active transport.

7. Uptake of water from unsaturated air was not found with any of the methods used.

8. Regulation such as was found here would help to maintain the internal environment of these mites as nearly constant as possible in the face of fluctuating humidities.

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