Course-Control, Metabolism and Wing Interference During Ultralong Tethered Flight in Drosophila Melanogaster

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Abstract
Tethered flight in a 3-day-old female Drosophila was sustained for 32.2 h with only short interruptions during uptake of sucrose solution. The course-control reactions derived from the difference of the wingbeat amplitudes on either side have been used to simulate the rotatory displacement of the surrounding landmarks during a comparable turn in free flight. Stabilization of a target in the preferred area of the visual field requires continuous visual attention. A rate of about 5 course-correcting manoeuvres per second was maintained throughout the experiment. Drosophila seems to be able to cover long distances in search of a favourable habitat. Flight-specific carbohydrate consumption is equivalent to a metabolic power input per body weight of about 18 W N−1. The tethered fly produces about 40 % of the lift required to sustain hovering flight. The resulting mechanochemical efficiency of about 0.04-0.07 is within the expected order of magnitude for flying insects. Expenditure of reserve substances may account for the difference between the comparatively low power input of about 7 WN−1 derived from carbohydrate uptake in the first hours of flight (Wigglesworth, 1949), and the actual metabolic turnover of about 21WN−1 derived from oxygen consumption during this period (Laurie-Ahlberg et al. 1985). Weis-Fogh's ‘clap and fling’, a widespread lift-generating process exploiting the aerodynamic wing interference at the dorsal end of the wingbeat, was in action throughout the flight. However, there were two significant modifications (as first conceived by Ellington, 1980): (1) during ‘clap’, there is a progress of wing contact from the leading to the trailing edge, which is likely to ‘squeeze’ a thrust-generating jet of air to the rear; (2) during ‘fling’, there is a progress of wing separation in the same direction, which is described as a ‘peel’ resembling the progressive separation of two plastic foils pulled apart against forces of mutual attraction. The wings of the test fly survived about 23 million such peels without damage. Increasing airspeed decreases the intensity of ‘clap and fling’ in Drosophila: results obtained in the wind tunnel show the transition to a ‘near clap and fling’, lacking mutual wing contact.