1. The rates of desiccation of three species of cockroaches (Periplaneta americana, Blatta orientalis and Blatella germanica) in dry air at various temperatures have been compared (Fig. 1). There are clear specific differences.

2. The rates of respiration of the three species are proportional to the weight to the power of 0 75-0 8.

3. In all three species, at 30° C. in dry air the animal loses about 6 mg. of water by evaporation for every 1 mg. of oxygen consumed. The smallest species (Blatella germanica), with the thinnest cuticle, loses least rapidly per unit surface area. These facts are consistent with the previous conclusion that the water passes through both the spiracles and the cuticle.

4. The upper limit of preferred temperature of Periplaneta americana and Blatella germanica is 33° C, while for Blatta orientalis it is 29° C. The lower limit is not so sharply defined and further work is required before it can be regarded as significant.

5. Certain suggestions are made concerning the ecological significance of the observations described.

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