Sensory feedback in the control of mouthpart movements in the desert locust Schistocerca gregaria

ABSTRACT. When imposed movements were applied to one or both mandibles of the desert locust, Schistocerca gregaria , the other mouthparts moved in synchrony with the mandibles. This occurred in the presence or absence of food, and when the mandibles were driven at a higher or lower frequency than that seen during normal feeding. Electromyogram recordings from the mandibular closer muscles revealed bursts of activity at the same frequency as the imposed movement. This activity occurred during mandibular closing. Burst length was a function of driving wavelength. At low driving frequencies (less than 0.5 Hz), smaller bursts were seen prior to the longer closing burst; a series of similar small bursts was seen when the mandibles were held in the open position. When one mandible was driven, closer muscle activity was largely confined to that side. In the presence of food, however, activity was seen in both closer muscles. A possible mechanism for this is described. After destruction of the campaniform sensilla on the ventral surface of the mandibles, the bursts of activity in the mandibular closers, seen when the mandibles were held open, were replaced by continuous activity. This suggests that the function of these sensilla is to inhibit motor output to the closer muscles when the tension becomes high. When feeding on relatively incompressible food the closer muscle burst length increased, although chewing frequency did not alter. This effect was also produced by loading the mandibles artificially. A model for the feedback control of this behaviour is proposed.