1. The investigation concerns the ingestion, excretion, and assimilation of dietary nitrogen by Tuberolachnus salignus (Gmelin) developing on Salix trees in various stages of leaf development.

2. Apterous and alate forms of T. salignus differ in their rates of development and nitrogen assimilation and in their behaviour.

3. The aphids assimilate considerably more nitrogen when developing on willows having a phloem sap rich in nitrogen than on willows having a phloem sap relatively poor in nitrogen.

4. For a given amount of nitrogen assimilated the aphids ingest a larger volume of a phloem sap poor in nitrogen than of a phloem sap rich in nitrogen.

5. Nitrogen balance experiments have shown that T. salignus nymphs assimilate only as much nitrogen as they are able to absorb from their food.

6. The evidence for and against the fixation of atmospheric nitrogen by aphids through the agency of their symbionts is briefly discussed. T. salignus do not supplement their dietary nitrogen supply by fixing atmospheric nitrogen.

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