Flight Energetics and Aerodynamics

Using normoxic, variable-density mixtures of O2, N2 and He, we are able to elicit variation in hovering flight performance and, in some cases, maximal hovering performance and aerodynamic failure. We have shown for the carpenter bee Xylocopa varipuncta that this species is not isometric regarding thorax mass and wing area, both of which are disproportionately lower in heavier individuals. The minimal gas density necessary for hovering (MGD) increases with body mass and decreases with relative thoracic muscle mass. Wingbeat frequency and stroke amplitude during maximal hovering are significantly greater than in normodense hovering. Also, wingbeat frequency increases significantly with body mass during normodense hovering but is mass-independent during maximal hovering. Reserve capacity for wingbeat frequency and stroke amplitude decrease with increasing body mass, although reserve capacity in stroke amplitude (10–30%) exceeds that of wingbeat frequency (0–8%). Power production during normodense hovering is significantly less than during maximal hovering. Metabolic rates are significantly greater during maximal hovering than during normodense hovering and are inversely related to body mass during maximal and normodense hovering. Metabolic reserve capacity averages 34% and is independent of body mass. The allometry of power production, power reserve capacity and muscle efficiency indicate that larger individuals operate near the envelope of maximal performance even in normodense hovering due to smaller body mass-specific flight muscles and limited reserve capacities for kinematics and power production.

We are currently performing experiments in which we are measuring the kinematic response of honey bees and mosquitoes to aerodynamic challenges (such as natural loading and flight in hypodense atmospheres) using high-speed (6000 fps) digital video. For honey bees we have compared hovering flight in air (21% O2, 79% N2) to flight in heliox (21% O2, 79% He), a normoxic mixture with 1/3 the density of sea-level air. Relative to bees hovering in air, those in heliox have significantly greater wing stroke amplitude, with differences in both ventral and dorsal displacement. Wingbeat frequency does not change between treatments, but the increase in stroke amplitude results in a large increase in wingtip velocity and rotational velocity. Because aerodynamic forces scale with the square of velocity, the responses of honey bees to heliox likely enhances lift during both wing translation and rotation. During hovering in heliox, wings often contact at the dorsal stroke transition; however, it is unknown whether this phenomenon imparted a “clap-and-fling” mechanism of lift production, as has been suggested for smaller insects. Finally, the wings themselves are not planar throughout the entire stroke and were markedly deformed at the stroke transitions. It is unclear how such deformation affects lift or the applicability of traditional aerodynamic models.