1. A calorimetric method was designed which makes it possible to estimate the metabolic rate of a flying insect with an accuracy of ±10%.

2. For the first time, the metabolic rate of a flying animal has been related to the main aerodynamic component of force, the lift.

3. The metabolic rate of locusts increases almost linearly from 41 kcal. kg.-1 hr.-1 at 50% relative lift to 110 kcal. kg.-1 hr.-1 at 170%.

4. The results are in accordance with theoretical deductions and they show that the work necessary to accelerate the oscillating wings (inertia terms) is small compared with the aerodynamic work--probably three to five times smaller.

5. In flapping insects and small birds the metabolic rate per unit weight seems to be independent of size. The control of power is discussed.

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