THE CHANGES IN POWER REQUIREMENTS AND MUSCLE EFFICIENCY DURING ELEVATED FORCE PRODUCTION IN THE FRUIT-FLY DROSOPHILA-MELANOGASTER

The limits of flight performance have been estimated in tethered Droso phila melanogaster by modulating power requirements in a 'virtual real ity' flight arena. At peak capacity, the flight muscles can sustain a mechanical power output of nearly 80 W kg(-1) muscle mass at 24 degree s C, which is sufficient to generate forces of approximately 150% of t he animal's weight. The increase in flight force above that required t o support body weight is accompanied by a rise in wing velocity, broug ht about by an increase in stroke amplitude and a decrease in stroke f requency. Inertial costs, although greater than either profile or indu ced power, would be minimal with even modest amounts of elastic storag e, and total mechanical power energy should be equivalent to aerodynam ic power alone. Because of the large profile drag expected at low Reyn olds numbers, the profile power was approximately twice the induced po wer at all levels of force generation. Thus, it is the cost of overcom ing drag, and not the production of lift, that is the primary requirem ent for flight in Drosophila melanogaster. By comparing the estimated mechanical power output with respirometrically measured total power in put, we determined that muscle efficiency rises with increasing force production to a maximum of 10%. This change in efficiency may reflect either increased crossbridge activation or a favorable strain regime d uring the production of peak forces.