1. The muscles involved in dorso-ventral and longitudinal ventilation in the pregenital segments of Schistocerca gregaria are described. Expiratory muscles are shown to be innervated by paired lateral nerves whereas the dorso-ventral inspiratory muscles are innervated by the unpaired median nerve system.
2. Normal pumping activity is brought about by alternating bursts of impulses in expiratory and inspiratory motor nerves. Inspiratory bursts are relatively invariant, whereas expiratory bursts show a positive correlation with ventilatory cycle length. The firing patterns of some units within the bursts are described.
3. In general anterior segments fire motor bursts earlier than posterior segments during well synchronized active ventilation, the metathoracic ganglion firing first. However, much variation is seen both within one locust and between different locusts. Burst-formation continues in isolated nerve cords.
4. Activity, phase-locked with expiration, has been recorded in the connectives. The evidence suggests that it occurs in a pair of co-ordinating interneurones which run from the metathoracic ganglion to the last abdominal ganglion and determine the initiation, duration and possibly the intensity of the expiratory motor bursts in each segment. A second parallel system may co-ordinate activity when the metathoracic co-ordinating interneurones are inactive. Inspiratory motor neurones are probably autoactive and the duration of their firing may normally be determined by the discharge phase of a metathoracic oscillator which acts by inhibiting the co-ordinating interneurones.