COMPENSATORY WATER-INTAKE BY LOCUSTS (LOCUSTA-MIGRATORIA) - IMPLICATIONS FOR MECHANISMS REGULATING DRINK SIZE

Experiments were performed to test the effects of osmotic pressure of water (as manipulated by the addition of sodium chloride) on the size of drinking bouts by fifth stadium Locusta migratoria L. There was a p ositive relationship between saline concentration and volume ingested, suggesting that locusts compensate behaviourally for reduced capacity of ingested fluids to hydrate the haemolymph. This was not due to pha gostimulatory properties of saline, since a significantly greater prop ortion of insects rejected 1% saline than distilled water, demonstrati ng that sodium chloride at this level was phagodeterrent. Animals that drank saline were both more likely to accept a subsequent drink on di stilled water, and took on average larger second drinks, than those wh ich had previously drunk distilled water. This suggests that the addit ional volume ingested did not fully compensate for the reduced hydrati ng capacity of saline. These results, considered in conjunction with p revious experiments, imply a central role for haemolymph monitoring in the regulation of drinking bout size.