Fig. 4. Changes in flight muscle mass-specific power requirements for flight and metabolic power with increasing numbers of thoracic spiracles participating in respiratory gas exchange (grey bars, A–D). Data were measured while the flies produced maximum aerodynamic flight forces (topmost 1% values of each flight sequence). Red data indicate power values estimated in unmanipulated flies (see text) at the corresponding flight force. , induced power requirements are the costs to generate an air downward momentum; , profile power requirements are the costs to overcome drag on the beating wings; , flight muscle mechanical power output that is equal to the sum of induced and profile power requirements assuming 100% elastic storage; and = metabolic power estimated from measurements of CO₂ release through the spiracle. For more explanations see Fig. 3 legend. Values are means ± s.d. (N=10 flies).