1. Three principal motor patterns have been distinguished in the nerves to the closer muscle of spiracle 1 of the locust. Pattern 1 brings about muscular contraction and spiracle closing; pattern 2 maintains the closure and may be brought about either by the activity of pacemakers located in each motoneurone or by synaptic activity or by a mixture of both. The possible origins of pattern 2 activity have been located by antidromic stimulation and by observations on the occurrence of impulse synchronization between the units of one segment and those of adjacent segments. In pattern 3, firing slows or stops and the spiracle opens.
2. In most individuals of Locusta, pacemaker activity predominates in pattern 2 firing, while in Schistocerca synaptic activity is more common. Different individuals of either species may remain consistent to one firing plan for long periods; their effect at normal frequencies at the muscle is indistinguishable.
3. Electrical stimulation of split bundles of the connectives has gone some way to distinguishing between the activity of possibly three command interneurones which run from the metathoracic ganglion and synapse with the spiracle motoneurones. These produce I:I excitation, acceleration of pacemaker activity or inhibition of the pacemakers, and in the intact insect are believed to bring about accurate synchronization of the spiracle movements with abdominal pumping.