Fig. 4. Intratracheal pressure measured at the anterior spiracles (spiraclesI) and mesoscutellar air sac during steady flight in Manduca sexta and with increased wingbeat amplitude during forced flight. At the anterior spiracles, the mean pressure is negative, and pressure pulse amplitude increases with wingbeat amplitude, with mean pressure (white line) becoming more negative. At the mesoscutellar air sac, the mean pressure (white line) is positive and is less affected by the increased wingbeat amplitude. Wingbeat amplitude was measured as the amount of shade cast by the moving wing onto a laterally arranged silicon photocell. Maximum values of shading curves (the lowest voltage produced by the photocell) correspond to highest amplitudes. Sampling rate was 40kHz.