Local movements evoked by chemical stimulation of the hind leg in the locust Schistocerca gregaria

The behavioural responses of desert locusts, Schistocerca gregaria, to solu tions of four behaviourally relevant chemicals (sodium chloride, sucrose, n icotine hydrogen tartrate and lysine glutamate) applied as droplets to the hind tarsus were analysed. All responses following within Is of chemical st imulation were local leg avoidance reflexes, and the probability of eliciti ng such a response increased in a dose-dependent manner with increasing con centration for all the chemicals tested, Chemical identity, however, critic ally determined the concentration threshold at which the different chemical s became an effective stimulus. For example, a 2.5 mmol l(-1) concentration of the secondary plant metabolite nicotine hydrogen tartrate (NHT), a pote nt feeding deterrent to locusts, was sufficient to evoke avoidance response s in 50 % of cases, whilst for the nutrients, sucrose and lysine glutamate, 250-500 mmol l(-1) of the chemical was needed to induce avoidance behaviou r in 50 % of the locusts. NaCl was of intermediate effectiveness, with a 50 % response rate occurring at a concentration of approximately 50 mmol l(-1 ). The latency to the start of the response following stimulation was negat ively correlated with the concentration of NaCl, but for the other chemical s concentration had no effect on latency, The duration of the avoidance beh aviour decreased with increasing concentration for NaCl and more weakly for NHT, but not for the other chemicals. Adding a subthreshold concentration of sucrose to 50 mmol l(-1) NaCl decreased the incidence of response compar ed with 50 mmol l(-1) NaCl on its own, Experiments with other mixtures comb ining NaCl, sucrose and NHT indicate that the frequency and dynamics of the responses to chemical mixtures cannot be simply predicted from the respons es to their individual constituents.