Two layers in Periplaneta pronotum, endocuticle and combined epicuticle and exocuticle were functionally distinguished by their permeabilities and water affinities. A passive model combining the permeabilities of these layers and the water content of the endocuticle was designed to predict pronotal water contents in a variety of ambient activities at20°C.
The model predicted cuticle water contents with acceptable accuracy above ambient activities of 0.56 but overestimated them in drier conditions. Despite universally high water activities in the endocuticle, differences in water content related to ambient activity were predicted. Cuticle water contents below haemolymph equilibrated values, represented in the literature as evidence of active water regulation, were largely attributed to passive activity gradients combined with evaporation errors.