1. A study has been made of the excretion and storage of ammonia by the aquatic larva of Sialis lutaria.

2. About 90% of the nitrogen excreted by the larva of Sialis during starvation was in the form of ammonia. The daily ammonia output averaged 10 µg. N/100 mg. wet weight.

3. Ammonia was found to be excreted into the hindgut, presumably via the Malpighian tubules. The concentration of ammonia in the hindgut fluid averaged 136 mg. N/100 ml.

4. Evidence was obtained that the tissue fluids are not maintained completely ammonia-free. Thus the total ammonia content of the body averaged 1.0 µg. N/100 mg. wet weight of tissue. The concentration of ammonia in the haemolymph averaged 0.50 mg. N/100 ml.

5. Evidence was obtained that the larval tissues are capable of ‘storing’ appreciable quantities of ammonia. Thus ammonia did not accumulate in the tissue fluids of larvae prevented from excreting for a period of days. Furthermore, it was found experimentally possible to raise the concentration of ammonia in the tissue fluids, the ammonia subsequently disappearing. The possible significance of this ‘storage’ mechanism was discussed.

6. The method used for raising the concentration of ammonia in the tissue fluids, by immersing the larva for some time in a solution of dilute ammonia, was considered in some detail, particularly with respect to toxic effects. When the concentration of ammonia in the haemolymph had reached a level in the region of 7.0 mg. N/100 ml. toxic symptoms started to appear.

Now at Zoology Department, University College of South Wales and Monmouthshire, Cardiff.