1. Limnephilus affinis larvae tolerate external salt concentrations up to at least 410 mM./l. NaCl (about 75% sea water) and survive for short periods in 470 mM./l. NaCl (about 85/ sea water).
2. The body wall is highly permeable to water, but relatively impermeable to sodium and chloride. Most of the sodium and chloride uptake from salt water occurs via the mouth.
3. The sodium and chloride levels in the haemolymph are powerfully regulated. Both are maintained strongly hypotonic against large external concentration gradients.
4. The Malpighian tubule-rectal system is very sensitive to changes in the haemolymph chloride level. The chloride concentration in the rectal fluid can be at least three times greater than the concentration in the haemolymph, and slightly greater than the concentration in the external medium.
5. The rectal fluid is hyper-osmotic to the haemolymph and to the medium at high external salt concentrations.
6. At external concentrations greater than about 200 mM./l. NaCl, water balance is maintained by regulating the haemolymph roughly iso-osmotic with the medium. This is partly achieved by increasing the non-electrolyte fraction in the haemolymph. A small quantity of osmotically free water is available to replace any osmotic loss. This can be obtained by drinking salt water and producing a concentrated solution of salts in the rectum.