Position-dependent Sensitivity and Density of Taste Receptors on the Locust Leg Underlies Behavioural Effectiveness of Chemosensory Stimulation

Chemical stimulation of contact chemoreceptors located on the legs of locusts evokes withdrawal movements of the leg. The likelihood of withdrawal depends on the site of stimulation, in addition to the identity and concentration of the chemical stimulus. A significantly higher percentage of locusts exhibit leg avoidance movements in response to stimulation of distal parts of the leg with any given chemical stimulus compared to proximal sites. Moreover, the percentage of locusts exhibiting avoidance movements is correlated with the density and sensitivity of chemoreceptors on different sites of an individual leg. The effectiveness of chemical stimulation also differs between the fore and hind legs, with NaCl evoking a higher probability of leg withdrawal movements on the foreleg. Moreover, sucrose was less effective than NaCl at evoking withdrawal movements of the foreleg, particularly at low concentrations. The gradients in behavioural responses can be partially attributed to differences in the responsiveness and density of the contact chemoreceptors. These results may reflect the different specialization of individual legs, with the forelegs particularly involved in food selection.