1. Measurement of the rate of water loss in dry air from ten species of aquatic insects, representing six orders, has shown that the cuticles are in all cases more permeable than are those of typical terrestrial species. There is, however, a wide range of permeability within the group.

2. The rate of water loss increases exponentially with temperature. There is no ‘critical temperature’. The steepness of the exponential relating transpiration and temperature is greatest in the least permeable examples.

3. Chloroform treatment produces a marked increase in water loss, but abrasion by mineral dusts has very little effect.

4. Adult specimens of ‘Notonecta obliqua’ immersed in distilled water gain weight at a rate which suggests that the cuticle is comparatively permeable under these conditions also. The rate of water uptake is increased by chloroform treatment.

5. The results support the general theory advanced by Holdgate and Seal (1955), but no sound interpretation of the structural basis of cuticular permeability can yet be advanced.

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