EXTRAORDINARY FLIGHT PERFORMANCE OF ORCHID BEES (APIDAE, EUGLOSSINI) HOVERING IN HELIOX (80-PERCENT HE 20-PERCENT O-2)/

Limits to insect flight performance are difficult to evaluate because the full range of aerodynamic capabilities cannot be easily elicited o r controlled. Invasive experimental manipulations, such as tethering a nd weight addition, may adversely affect the biomechanics of the fligh t system as a whole. Because air density is a major determinant of aer odynamic force production, gas mixtures of variable density can be use d to investigate insect flight performance non-invasively, Three speci es of orchid bee hovering in heliox (80% He/20% O-2) exhibited dramati c increases in lift and power output relative to flight in normal air. Stroke amplitude increased significantly in heliox, while wingbeat fr equency was unchanged; the Reynolds numbers of the wings decreased on average by 41%. Although lift performance of airfoils generally degrad es at lower Reynolds numbers, mean lift coefficients in heliox increas ed significantly relative to values for hovering in normal air. Mean m uscle mass-specific power output for flight in heliox mixtures ranged from 1.30 to 160 W kg(-1), substantially exceeding values determined f rom isolated asynchronous muscle preparations as well as limits postul ated 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 i nsect and vertebrate flight mechanics.