The idea that atmospheric oxygen has dictated the maximum body size of insects throughout their evolutionary history is deeply rooted in popular and scientific literature. In Nature, 30 years ago, the hypothesis was proposed that a limitation in oxygen diffusion at the level of the tracheoles limits the maximum body size of insects and that the increase in atmospheric oxygen concentration in the late Paleozoic allowed for insect gigantism. Here we challenge this hypothesis by showing that the relative space occupied by tracheoles in the flight muscle of insects (1) increases only 1.8-fold over a 10,000-fold increase in body mass (1,320 micrographs, 44 species, 10 orders), (2) is typically 1% or less in most species, and (3) that this observation holds true when we extend our analysis to the long-extinct giant Meganeuropsis permiana (approximately 100 g), according to research conducted by a team at the University of Pretoria. The small space requirement and the lack of a significant increase in tracheolar investment with body size, despite the clear evolutionary potential for it, provide compelling evidence that diffusive oxygen transport through the tracheolar-muscular system does not limit the maximum body size of extant or giant prehistoric insects.

NATURE