Fig. 6. Significance of the spatial distribution of spiracle exchange areas for (A) IFM mechanical power output, (B) flight muscle efficiency, (C) lift coefficient, and (D) aerodynamic efficiency. Data show the relative difference () in performance between unmanipulated flies and animals in which up to 4 thoracic spiracles have been sealed during flight. The differences are scaled to the performance of the unmanipulated control group. Performance scores are plotted against total diffusive area of the animal's abdominal and thoracic spiracles that may participate in tracheal gas exchange. Due to the reduction of maximum flight force production with decreasing total spiracle opening area, data are calculated at 0.29 (493 µm² total spiracle area), 0.48 (3780 µm²), 0.74 (5424 µm²), 1.01 (7889 µm²) and 1.36 (10 355 µm²) relative flight force production for 0–4 thoracic spiracles open, respectively. A value of 1.0 normalized force means that the fly produces a flight force equal to body weight. Grey areas in the pictograms indicate maximum total spiracle opening area available for respiratory gas exchange and red lines indicate linear regression fits. See legend of Fig. 3 for number of tested flies and text for more explanations. Values are means ± s.d.