Water Content and Water Activity in the Cuticle of Terrestrial Isopods

Abstract

1. The water content of the cuticle of both desiccated and non-desiccated terrestrial isopods Porcellio scaber and Armadillidium vulgare was measured. The animals were desiccated for various times (up to 3 h) over silica gel and the mean water content of the cuticle was 54.0±0.78% for P. scaber and 52.7±1.11% for A. vulgare. There was no trend as regards the desiccation time, nor did the body weight affect the water content. 2. The water content of the cuticle remained virtually unchanged as long as the animal was alive in the desiccator. It dropped significantly after the animal had died after having lost some 30% of its body weight. 3. The cuticular water content of non-desiccated P. scaber tended to be slightly higher than that of desiccated ones. In A. vulgare no significant difference was observed between non-desiccated and desiccated specimens. 4. The water activity of the excised cuticle of the above two species and of Oniscus asellus and Cylisticus convexus was above that of the haemolymph and therefore not in osmotic equilibrium with it. The osmotic equilibrium points were below the osmotic pressures of the blood; the difference amounted from 1.5 to 2.8 atm. in different species. 5. The difference in water activity between blood and cuticle, the maintenance of water content with desiccation, and the drop in water level at death, all indicate the presence of an active mechanism regulating the cuticular water in terrestrial isopods.