The implications of speed, load carrying and vertical ascent on bumblebee (Bombus terrestris L.1758) flight energetics

Flight energetics are central to understanding behaviour, morphology and flight performance of insects. Mechanical power output is generated through contraction of insect flight muscles that consume metabolic energy. Different flight behaviours including forward, hovering, loaded and climbing flight incur different aerodynamic force requirements and incur different metabolic power requirements. The research presented in this thesis aims to provide an understanding of flight energetics under behaviours not previously studied as well as assessing whether methodology affects energy expenditure.
Respirometry was used to measure the rate of carbon dioxide production and therefore metabolic rate across a speed range during free and tethered flight conditions. A shallow U-shaped curve was observed for both flight groups, but mass-specific gross rate of carbon dioxide production during tethered flight was lower compared to free-flight. Trajectory of the metabolic power-speed relationship for both flight groups was similar. Tethered flight can therefore serve as a proxy for free-flight energetics across a speed range. Metabolic rate increased with increasing load size for any given flight speed. A shallow U-shaped trajectory was seen from unloaded free-flight with a significant divergence with increasing percentage load mass at higher flight speeds. Metabolic rate did not increase significantly with increasing vertical flight speed from hovering in either unloaded or loaded climbing flight conditions. Stroke amplitude varied significantly with vertical flight speed and percentage load mass under both unloaded and loaded flight conditions. Wingbeat frequency varied with vertical flight speed under loaded flight conditions but remained similar during unloaded flight and with increasing percentage load mass. These findings empirically show that climbing flight is associated with several different kinematic variations of stroke amplitude and wingbeat frequency across different loading conditions.
This research demonstrates the different metabolic requirements of different flight behaviours routinely used by bumblebees and their potential constrain an insect’s daily energetic budget. This research also presents the first study of climbing flight energetics for insects.