Allometric Scaling of Tracheal Morphology among Bumblebee Sisters (Apidae: Bombus): Compensation for Oxygen Limitation at Large Body Sizes?

The scaling of insect respiratory systems with body size has important implications for ecology and evolution of insects, ranging from allometry of aerobic performance to current and past environmental constraints on insect body size. Previous work examining scaling across beetle species and during ontogeny of grasshoppers suggests strong tracheal hypermetry, with larger insects investing relatively more volume in their tracheal systems. However, these approaches potentially confound body size differences with species or life stage differences in morphology or physiology. To circumvent these issues, we took advantage of large body size variation within bumblebee nests. Workers with 75% genetic similarity nevertheless varied in body size from under 50 mg to more than 250 mg, and queens exceeded 500 mg. We estimated respiratory volume, dimensions of pronotal spiracles, and body size of individual bumblebees (Bombus impatiens) from lab-reared nests. Among workers, tracheal system volume scaled hypermetrically with mass(2.6), whereas overall scaling of the cross-sectional area of the pronotal spiracular tracheae scaled isometrically with mass(0.67). Queen bumblebees had tracheal systems similar in size to those of workers 1/3 their body mass but had larger-than-expected cross-sectional area of spiracular tracheae. Given space constraints within the rigid exoskeleton, the strong hypermetry of the tracheal system among bumblebee workers may impose an upper limit to body size as the tracheal system occupied 25% of total body volume for the largest workers.