Ionic Regulation and Water Balance in the Aquatic Larva of Sialis Lutaria

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Abstract
1. The electrolyte composition of the blood, tissues and excretory fluid of the aquatic larvae of Sialis lutaria has been measured, and the regulation of the concentrations of sodium, potassium and chloride in the blood studied in detail. 2. In the normal larvae these ions are not present in the excretory fluid. Potassium and, perhaps, sodium are reabsorbed in the rectum but chloride is never present in the rectum. 3. If these ions are present in the outside medium they are taken into the larvae through the gut. The blood concentration is regulated by the excretion of these ions via the rectal fluid. Potassium is rapidly excreted but chloride tends to be retained in the blood. Sodium is removed more rapidly than chloride. 4. Water enters the larvae by osmosis through the cuticle, but can also be absorbed through the gut by osmosis or together with sodium ions. The water intake is balanced by excretion of rectal fluid. The factors affecting the rate of water excretion have been studied. 5. The larvae are unable to survive in hypertonic saline solutions. This is due to their inability to make good osmotic water loss or to produce a hypertonic excretory fluid.