Limits to insect flight performance are difficult to evaluate because the full range of aerodynamic capabilities cannot be easily elicited or controlled. Invasive experimental manipulations, such as tethering and weight addition, may adversely affect the biomechanics of the flight system as a whole. Because air density is a major determinant of aerodynamic force production, gas mixtures of variable density can be used to investigate insect flight performance non-invasively. Three species of orchid bee hovering in heliox (80 % He/20 % O2) exhibited dramatic increases in lift and power output relative to flight in normal air. Stroke amplitude increased significantly in heliox, while wingbeat frequency was unchanged; the Reynolds numbers of the wings decreased on average by 41 %. Although lift performance of airfoils generally degrades at lower Reynolds numbers, mean lift coefficients in heliox increased significantly relative to values for hovering in normal air. Mean muscle mass-specific power output for flight in heliox mixtures ranged from 130 to 160 W kg-1, substantially exceeding values determined from isolated asynchronous muscle preparations as well as limits postulated from the results of load-lifting experiments. The use of variable-density gas mixtures to examine animal flight performance is a simple yet powerful manipulation that will permit a new evaluation of both insect and vertebrate flight mechanics.