The Retention of Salt and the Regulation of the Non-Protein Nitrogen Fraction in the Blood of the Aquatic Larva, Sialis Lutaria

Abstract

1. As in the blood of other insects, chloride forms only a small proportion of the total dissolved substances in the blood of Sialis lutaria larvae. (Chloride 0.15-0.35% NaCl, osmotic pressure 0.80-1.10% NaCl.) 2. Only after more than 5 weeks of starvation in distilled or tap water was the blood chloride reduced to below 0.10% NaCl. With a few days' treatment in 1.0% NaCl it could be increased to 0.5% or higher. 3. The movements of chloride induced in this way were shown to occur through the external cuticle whose permeability to ions is thus very low. 4. There is no mechanism for the active uptake of ions from the external water, and the resistance to several weeks of starvation in glass-distilled water must be attributed solely to the very low permeability of the cuticle. 5. A large proportion of the non-chloride fraction of the total osmotic pressure is non-protein nitrogen, which is presumably amino-acid. 6. Large and rapid changes in blood chloride were induced by repeated blood extractions which were followed by restoration of most or all of the blood volume by the uptake of water. There was no corresponding alteration in the total osmotic pressure. 7. The concentration of non-protein nitrogen in the blood changes in such a way that the total osmotic pressure is not seriously affected by considerable dilution or concentration of the salts as indicated by chloride, which can be halved or doubled. 8. Estimations of protein nitrogen have suggested the possibility that changes in the non-protein nitrogen are made at the expense of the blood protein. 9. The mean normal protein nitrogen of the blood is approximately the same as that of mammals. But after 6 weeks of starvation it was reduced to about one-twentieth of the original, and in some cases to practically nil without apparent effect on the health of the larva. The initial level of non-protein nitrogen was, however, maintained throughout. 10. These results suggest that a compensatory regulation of the blood aminoacids is an important component of the osmo-regulatory mechanism.