Body and wing morphology, flight metabolic rates, and wingbeat frequencies for 13 stingless bee species

Understanding the effect of body size on flight costs is critical for
development of models of aerodynamics and animal energetics. Prior scaling
studies that have shown that flight costs scale hypometrically have
focused primarily on larger (> 100 mg) insects and birds, but most
flying species are smaller. We studied the flight physiology of thirteen
stingless bee species over a large range of body sizes (1-115 mg).
Metabolic rate during hovering scaled hypermetrically (scaling slope =
2.11). Larger bees had warm thoraxes while small bees were nearly
ecothermic; however, even controlling for body temperature variation,
flight metabolic rate scaled hypermetrically across this clade. Despite
having a lower mass-specific metabolic rate during flight, smaller bees
could carry the same proportional load. Wingbeat frequency did not vary
with body size, in contrast to most studies that find wingbeat frequency
increases as body size decreases. Smaller stingless bees have greater
relative wing surface area which may help them reduce the energy
requirements needed to fly. Further, we hypothesize that the relatively
larger heads of smaller species may change their body pitch in flight.
Synthesizing across all flying insects, we demonstrate that the scaling of
flight metabolic rate changes from hypermetric to hypometric at
approximately 58 mg body mass with hypermetic scaling below (slope=1.2)
and hypometric scaling (slope=0.67) above 58 mg in body mass. The reduced
cost of flight likely provides selective advantages for the evolution of
small body size in insects. The biphasic scaling of flight metabolic rates
and wingbeat frequencies in insects supports the hypothesis that the
scaling of metabolic rate is closely related to the power requirements of
locomotion and cycle frequencies.