The Permeability and Structure of the Cuticle of the Aquatic Larva of Sialis Lutaria

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Abstract
1. The permeability to water of the cuticle of Sialis larvae has been measured, using heavy water as tracer. The penetration was slow, the permeability constant being only 1.8 x 10-2 cm./hr. at 20° C. There was no obvious difference between the rate of water influx and outflux. The rate at which water penetrated into the tissues from the blood was much greater than through the cuticle. The Q10 for diffusion through the body surface was high, lying between 3.0 and 3.8. The osmotic uptake of water was calculated to be about 1% of the body weight per day at 10° C. 2. Drinking of water did not occur in normal larvae, but in larvae with the blood volume reduced, osmotic uptake of water through the gut did take place and the gut wall was much more permeable to water than the cuticle. A similar intake of water probably occurred during moulting. 3. The permeability of the cuticle to chloride was measured and also found to be of a low order (P = 1.04 x 10-4 cm./hr. at 17° C.). Sodium diffused out of the larva at the same rate as the chloride. 4. Histological examination of the cuticle showed that in the abdomen it was thin and consisted of a 7µ. thick endocuticle and a 1 µ epicuticle. Over the thorax it was thicker, and a polyphenol layer was present as the outer layer of the epicuticle. There was indirect evidence of the presence of a wax layer. 5. Wax was extracted from the cuticle, and the thickness of the layer from which it was derived was estimated by means of a monolayer technique. In the cuticle of the abdomen and gills the thickness averaged 0.1 µ. 6. The permeability to water of the cuticle was compared with that of terrestrial insects and was found to be much greater. This difference was not due to the thickness of the wax layer but probably to some physical properties of the wax. The cuticle of Sialis larvae showed no ‘critical temperature’ or sudden change in the permeability properties with temperature over the range of temperatures studied.