1. In the collembolan, Podura aquatica, transpiration through the cuticle is high at all relative humidities. Above 90% relative humidity, cuticular permeability is abnormally high but no adequate explanation of this can be given.
2. Transpiration through the cuticle is high at all temperatures and there is no evidence of a critical temperature over the range 17-29° C.
3. Abrasion increases the transpiration rate, and at least partial recovery occurs within the instar. Moulting fully restores the transpiration rate to the normal level.
4. The hydrofuge nature of the cuticle strongly suggests the presence of an orientated lipid monolayer on its surface. Strong wax solvents render the cuticle wettable, whereas treatment with water under reduced pressure does not.
5. Consideration of the fine structure suggests that the hydrofuge layer is discontinuous and that it occurs on the tips of the minor tubercles. This is compatible with an overall high transpiration rate.
6. The occurrence of hydrophil areas of cuticle is correlated with a smooth cuticular surface.
7. The biological effects of the hydrofuge and hydrophil areas in the life of the insect are discussed.