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Desert locusts show an extreme form of phenotypic plasticity, changing between a cryptic solitarious phase and a swarming gregarious phase that differ in many aspects of behaviour, physiology and appearance. Solitarious locusts show rapid behavioural phase change in response to tactile stimulation directed to the hind femora. Repeatedly touching as little as one quarter of the anterior (outer) surface area of a hind femur produced full behavioural gregarization within 4 h. Solitarious locusts have approximately 30% more mechanosensory trichoid sensilla on the hind femora than do gregarious locusts but have similar or fewer numbers of sensilla elsewhere on the legs. Tactile stimulation of a hind femur in solitarious locusts that had been restrained so that they could not move their legs failed to induce any behavioural gregarization. Patterned electrical stimulation of metathoracic nerve 5, which innervates the hind leg, however, produced full gregarization in restrained locusts. Our data show for the first time that the gregarizing signal combines both exteroceptive and proprioceptive components, which travel in both nerves 5B1 and 5B2, and provides us with a powerful experimental method with which to elicit and study neuronal plasticity in this system. Acetic acid odour, a strong chemosensory stimulus that activates the same local processing pathways as exteroceptive stimuli, failed to elicit behavioural gregarization, suggesting an early segregation in the central nervous system of the mechanosensory signals that leads to gregarization.