The Effect of Dietary Protein Levels and Haemolymph Composition on the Sensitivity of the Maxillary Palp Chemoreceptors of Locusts

1. Previous work has shown that fifth-instar nymphs of Locusta migratoria (L.) compensate for a dilution of their dietary protein by reducing intermeat interval (Simpson & Abisgold, 1985). 2. The effect of dietary protein on intermeal interval is regulated, either directly or indirectly, by the osmolality and free amino acid content of the haemolymph (Abisgold & Simpson, 1987). 3. The possibility that levels of dietary protein and haemolymph composition affect the response of the maxillary palp gustatory receptors is investigated. 4. Insects fed a low-protein diet had a significantly greater receptor response (measured as the total number of spikes in the first second of stimulation of a sensillum) to stimulation with 0-0125 moll"' leucine in 0-05 moll"' NaCl, 0-05 mo i r 1 NaCl alone or 0025 moll" sucrose in 005 moll" NaCl than did insects fed a high-protein diet, although for both diets the response to sucrose was significantly lower than the response to the other two solutions. 5. Increasing the free amino acid profile of the haemolymph of a low-protein-fed locust up to that of a high-protein-fed locust by injection markedly reduced the response of the receptors to subsequent stimulation with a 0-0125 moll~' mix of eight of the 10 amino acids injected, but did not reduce the response to stimulation with 0-025 moll" sucrose in 0-05 moll" NaCl. This reduction was independent of the effect of injection on blood osmolality and was sustained for 50min after the injection. 6. The response to 0-05 moll" NaCl alone was influenced both by increases in blood amino acid levels and by osmolality, but the effect was less marked than the specific reduction in response to amino acid stimulation. 7. The possible significance of a reduction in receptor sensitivity on feeding behaviour and the relative roles of blood osmolality and free amino acid content are discussed.