1. Two freshwater caddis larvae, L. stigma and A. nervosa, drink and regurgitate large quantities of salt water at frequent intervals. Drinking is not controlled, and larvae may drink an amount equivalent to 50% of the body weight per day. The gut wall is adversely affected by salt water and exosmosis occurs across the gut wall.

2. L. affinis larvae drink only small quantities of salt water. Drinking is strictly controlled, and the intake is roughly equivalent to 3-7% of the body weight per day over a wide range of external salt concentrations. The gut wall is not affected by high salt concentrations; regurgitation and exosmosis do not normally occur.

3. In the freshwater caddises the rate of rectal fluid production is approximately equivalent to 7% of the body weight per day. Rectal fluid is not produced at high external salt concentrations.

4. L. affinis larvae continue to produce rectal fluid at very high external salt concentrations. The daily output is probably roughly equivalent to the daily intake of salt water by drinking.

5. The osmoregulatory mechanism in L. affinis larvae and other salt-water insects is discussed. It is suggested that controlled drinking forms an important part of this mechanism, together with the ability of the gut wall to withstand exposure to high salt concentrations and the ability to elaborate rectal fluid hyper-osmotic to the external medium.

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