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Flight is an energetically costly form of locomotion, requiring metabolic rates as high as 100 times the resting rate (1). Most of the total energy required for flight is dissipated as heat in the flight musculature. For hovering animals, the remaining mechanical energy is divided into three components: induced power required to generate lift, profile power necessary to overcome drag on the wings, and inertial power required to accelerate and decelerate the wings during stroke reversal (2). Previous comparisons of total metabolic rate and estimated power output suggest that insects minimize the high cost of flight either by using highly efficient muscles or by recovering inertial power through elastic storage (3). It is not certain, however, which of these two strategies is actually used.
Female Drosophila hydei were tethered and flown in a flight arena that measured the frequency and amplitude of the wing stroke as well as the yaw torque produced by the animal (Fig. 1). (Figure 1 omitted). The fly and the torque transducer were enclosed in a respirometry chamber and surrounded by a visual panorama consisting of a dark stripe on a bright background. All experiments were done under closed-loop conditions such that the yaw torque produced by the fly was used to control the angular velocity of the stripe. Under these conditions, tethered flies actively modulated yaw torque through changes in wing-beat kinematics in order to fix the position of the stripe in the front portion of their visual field (4). In order to increase the variations in wingbeat amplitude and stroke amplitude, sinusoid and square wave voltage biases were added to the control signal in some experiments. Under these conditions, flies actively stabilized the stripe position by modulating flight kinematics in order to generate a compensatory torque opposite to the imposed bias. Additional modulation of wingbeat kinematics in some experiments was achieved by modulating the gain of the closed-loop feedback. These active modulations in wing stroke amplitude and frequency produced by these manipulations provided a useful and rigorous means for comparing respirometric and kinematic estimates of mechanical power.
A representative flight sequence is displayed in Fig. 2 and shows the simultaneous measurements of metabolic cost, stroke amplitude (summed from the two wings), wing-beat frequency, and yaw torque during the application...