1. 1.

    Previous work has demonstrated that fifth-instar nymphs of Locusta migratoria L. respond to differences in levels of dietary protein by altering intermeal interval but not meal size (Simpson & Abisgold, 1985): insects fed a diet with 14% protein (p) eat the same sized meals more frequently than those fed a diet with 28% protein (P). The physiological basis for this compensatory response is investigated.

  2. 2.

    Insects fed the P-diet had a significantly larger increase in blood osmolality during and after a meal than did those fed the p-diet.

  3. 3.

    Unexpectedly, this difference in blood osmolality did not result in a variation in the rate at which the fore-, mid- and hindgut emptied. Therefore a change in the rate of decline in negative feedback from gut stretch receptors does not underlie the alteration in interfeed interval.

  4. 4.

    40% of the difference in blood osmolality between p- and P-fed insects was attributable to changes in the blood concentration of free amino acids. Of the 16 free amino acids found, 11 occurred in significantly higher concentrations in the blood of P-fed insects.

  5. 5.

    There was no significant difference in the polypeptide and protein content of the blood of insects fed the p- or P-diet.

  6. 6.

    Increasing either blood osmolality or free amino acid concentration by injection delayed the next meal:injections that increased both had the greatest effect.

  7. 7.

    A mechanism is discussed whereby both blood osmolality and the concentration of various free amino acids regulate the time between meals, and thus compensatory feeding in response to changes in dietary protein.

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