A primary urine is formed in the Malpighian tubules, followed by movement to the rectum where selective resorption of ions and water can take place. These secretion and resorption processes are responsible for maintaining osmotic balance between the intracellular and extracellular compartments. In addition to maintaining osmotic balances, most organisms must also metabolize and eliminate the ammonia nitrogen that is formed by routine amino acid metabolism within cells. The normal dietary intake of protein by most animals is considerably different from ordinary protein needs. Typical diets provide amino acids in excess of the amounts required for synthesis of new protein, and often in quite different compositions.
Cells are nearly always in the processes of protein turnover.
This may not be an intuitively clear situation. We most often read about the nutritional importance of amino acids and proteins with respect to satisfying needs for essential amino acids. The point to be taken, however, is that dietary supplies of amino acids are rarely in optimal balance for protein synthesis within individual cells. Considerable levels of interconversions and other metabolic alterations of dietary amino acids follow from these imbalances between dietary supplies and instantaneous amino acid needs. As in regulation of salt balance, it might be thought that insects regulate amino acid balance at the level of the alimentary tract, then just excrete unnecessary amino acids.