1. The rate of total transpiratory water loss from Locusta is proportional to weight and is not affected by activity within the limits possible in an enclosed box.
2. A trend for proportionately less water to be lost at low humidity than at high humidity probably involves active measures to control water loss from the tracheal system. The saving of water is 5 mg./locust/hr. at 0° R.H., 30° C.
3. Experiments involving exposure of locusts to CO2 of different concentrations show that little control over water loss is exerted by the spiracles except in so far as they may influence the type of ventilation. Hyperventilation, predominantly of the tidal type, doubles normal water loss.
4. Between 42 and 45° C. the ventilatory rate increases enormously with con-comitantly greater water loss.
5. Locusts pre-treated in dry air show a 23% reduction in abdominal ventilatory rate and a 25% reduction in water loss over locusts pre-treated in moist air.
6. Ventilatory movements of locusts under conditions of progressive desiccation show decreased rate and amplitude and an increased incidence of discontinuities, which will conserve water.
7. Ventilation and water loss are closely interdependent. The fact that ventilation can be controlled according to water reserves and the humidity of the air is important in water conservation.