THE WAKE DYNAMICS AND FLIGHT FORCES OF THE FRUIT-FLY DROSOPHILA-MELANOGASTER

We have used flow visualizations and instantaneous force measurements of tethered fruit flies (Drosophila melanogaster) to study the dynamic s of force generation during flight. During each complete stroke cycle , the flies generate one single vortex loop consisting of vorticity sh ed during the downstroke and ventral flip. This gross pattern of wake structure in Drosophila is similar to those described for hovering bir ds and some other insects, The wake structure differed from those prev iously described, however, in that the vortex filaments shed during ve ntral stroke reversal did not fuse to complete a circular ring, but ra ther attached temporarily to the body to complete an inverted heart-sh aped vortex loop. The attached ventral filaments of the loop subsequen tly slide along the length of the body and eventually fuse at the tip of the abdomen. We found no evidence for the shedding of wing-tip vort icity during the upstroke, and argue that this is due to an extreme fo rm of the Wagner effect acting at that time. The how visualizations pr edicted that maximum flight forces would be generated during the downs troke and ventral reversal, with little or no force generated during t he upstroke. The instantaneous force measurements using laser-interfer ometry verified the periodic nature of force generation, Within each s troke cycle, there was one plateau ge of high force generation followe d by a period of low force, which roughly correlated with the upstroke and downstroke periods, However, the fluctuations in force lagged beh ind their expected occurrence within the wing-stroke cycle by approxim ately 1 ms or one-fifth of the complete stroke cycle. This temporal di screpancy exceeds the range of expected inaccuracies and artifacts in the measurements, and we tentatively discuss the potential retarding e ffects within the underlying fluid mechanics.