1. Small mirrors are used to record the movements of the spiracle valves of Schistocerca gregaria, and some general observations are made on the synchronized movements of the spiracles with ventilation.

2. Spiracles 1-3 are shown to alter their positions of opening and closing in different carbon dioxide concentrations, within the pattern of synchronized movements.

3. Modifications of the amount of opening of spiracles 1 and 3 take place as a result of differential contractions of the openers. In spiracle 1 the energy is stored in an elastic system, and the opener does not necessarily make contractions every cycle. Oscilloscope recordings show these reactions, resulting from carbon-dioxide stimulation, are controlled entirely from within the central nervous system and do not depend on any sensory input.

4. The movements of spiracle 2 are controlled peripherally through the direct action of carbon dioxide on the muscle membrane (Hoyle, 1960) and also through a cuticular wide-opening device.

5. The maintained sensitivity of spiracle 1 and the increased sensitivity of spiracle 2 after they are uncoupled from ventilation are discussed.

6. Some remarks are made on the activity of the denervated spiracle 2, and the suggestion is made that it depends on a central core of thin fibres.

7. A central nervous mechanism is postulated which could account for the rhythmic cessation of impulses in spiracle and ventilatory nerves.

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