1. The rate of water loss from the cuticle of Locusta is proportional to weight.
2. The rate of water loss from the cuticle at 30° C., 0% R.H. is 5.63±0.67 mg./ locust/hr., giving a permeability of 0.022 mg./cm.2/mm. Hg/hr.
3. The transition temperature at 46-48° C. is similar to that of Schistocerca (Beament, 1959) and probably indicates the existence of an oriented lipid monolayer in the epicuticle.
4. At relative humidities of 0-50 % the rate of water loss from whole locusts decreases with time. This phenomenon, which does not occur at 75%R.H., is partly due to the loss of adsorbed hydroscopic water during the initial period. A continuing apparent decrease in transpiration is shown to be a true permeability change.
5. The relationship between saturation deficit and rate of water loss at 30° C. is curvilinear, falling away at high saturation deficits. This results in a saving of water amounting to 1.5-2.5 mg./locust/hr. at 25 % R.H. and 2.7-4.0 mg./locust/hr. at 0% R.H. and will be biologically significant if not an artifact.
6. The anomalous relationship between saturation deficit and rate of water loss is caused by the permeability change occurring at low R.H. Three theories which may account for these phenomena are discussed.