Fig. 1. Oscillatory release of CO₂ during tethered flight in a single fruit fly Drosophila, flying in a flow-through respirometic chamber of a virtual-reality flight arena and exhibiting only small fluctuations in metabolic rate. (A) Location of spiracle openings on one side of Drosophila: sp1, mesothoracic spiracle; sp2, metathoracic spiracle; sp3–9, abdominal spiracles. (B) Muscle mass-specific mechanical power output of the asynchronous indirect flight muscles (IFM) in vivo was calculated from simultaneous measurements of aerodynamic production, flight force production, wing stroke amplitude and frequency (left scale, top trace). Bottom trace shows muscle mass-specific CO₂ release rate (right scale). In the bar below, blue indicates the oscillatory phase of gas release; gray, non-cyclic gas release. (C) Temporal distributions of oscillatory CO₂ release patterns (blue) of 12 fruit flies flown under three different experimental conditions (shown in the pictograms): (I) fruit flies vary metabolic rate in response to visual stimulation by external open-loop vertical motion of horizontal stripe patterns while simultaneously themselves stabilizing the azimuth position of a closed-loop vertical stripe using the relative difference in wing stroke amplitude (dark gray, left; see Materials and methods); (II) flight under visual-closed-loop conditions but in the absence of lift stimuli (yellow, middle); and (III) flight in absence of any moving visual objects (gray, right). Data show that oscillatory releases of CO₂ occur randomly and are essentially restricted to flight sequences without any visual stimulation of the surrounding panorama (stationary patterns).