Graded limb targeting in an insect is caused by the shift of a single movement pattern

Grooming responses are movements of a multi-jointed limb that are targeted toward a stimulus site on the body. To be successful, they require a continuous transformation of stimulus location into a corresponding motor pattern or selection and blending of a subset of cardinal motor patterns. Tactile stimulation of one forewing of a locust elicits characteristic grooming movements of the ipsilateral hind leg. An initial targeted trajectory that moves the tarsus toward the site of stimulation is followed by a cyclic trajectory in the region of the stimulus. We have analyzed both components of this behavior to quantify the relative effects of somatotopic stimulus position and leg start posture on three parameters: initial movement direction, accuracy, and grooming distribution. Accuracy and grooming distribution were significantly affected by the stimulus location but were not influenced by the initial leg posture. Both cues systematically shifted the initial movement direction from the onset of the response. The subsequent cyclic component of grooming movements forms a behavioral continuum with no clustering in joint angle space. We therefore conclude that forewing grooming in locusts is generated by a single movement pattern that is continuously shifted by a sensory cue signaling position on the forewing surface. Both vertebrates and invertebrates can switch between distinct movement forms to groom different parts of their bodies. Our data provide the first evidence that invertebrates, like vertebrates, also have graded control of limb targeting within the somatosensory receptive field of a single form of motor response.