WALKER, J.A.; University of Southern Maine: How flies fly fast

The fruit fly, Drosophila melanogaster, can potentially increase flight speed by modulating wing beat frequency, the magnitude of wing displacement, the angle of the (stroke) plane along which the wing oscillates, the angle (or pitch) of a wing section relative to its flight trajectory, and the duration and timing of wing rotation about an axis from base to tip. We used a high-speed digital video camera (7000 hz, 256 x 128 pixel resolution) to film fruit flies flying freely in a small flight chamber. Supporting the current model of speed control in fruit flies, we found that the fruit fly decreases body angle and, importantly, increases stroke plane angle, with speed. But this “helicopter” model of longitudinal control is not sufficient. Concomitant with stroke plane angle changes are conspicuous modulations of both wing tip velocity and wing pitch. While the total cycle frequency of wing beats did not increase with flight speed, the upstroke (cycle-equivalent) frequency increased from 247 to 270 Hz between 0 and 80 cm/s while the downstroke (cycle-equivalent) frequency decreased from 238 to 211 Hz between speeds of 0 and 80 cm/s. From hovering to 80 cm/s, the upstroke pitch changed from 14 degrees to 70 degrees while the downstroke pitch changed from 41 degrees to –5 degrees. It is quite easy to show to a first approximation how the up and down stroke pitch and wing tip speed asymmetries control flight speed.