Giant Prehistoric Insects Didn’t Require High Oxygen After All, Study Finds

Three hundred million years ago, Earth was home to giant dragonfly-like insects with wingspans reaching up to 27 inches (70 cm). For decades, scientists believed their enormous size was made possible by atmospheric oxygen levels that were about 45% higher than today’s. However, a new study led by researchers from the University of Pretoria and Adelaide University has overturned this long-standing theory, revealing that oxygen levels alone did not constrain the size of these prehistoric insects.

The research, published in April 2026, used high-power electron microscopy to examine the tracheal systems of insects—the network of air-filled tubes that deliver oxygen directly to tissues. Scientists found that the space occupied by tracheoles, the tiny endpoints of this system where oxygen diffusion occurs, typically makes up only 1% or less of flight muscle volume, even in large species. This leaves ample room for the system to expand without being limited by oxygen availability.

“If atmospheric oxygen really sets a limit on the maximum body size of insects, then there ought to be evidence of compensation at the level of the tracheoles,” said Professor Edward (Ned) Snelling, an experimental physiologist at the University of Pretoria’s Faculty of Veterinary Science and lead author of the study. “There is some compensation occurring in larger insects, but it is trivial in the grand scheme of things.”

These findings suggest that other factors—such as body design constraints, predator avoidance, or ecological niches—may have played a more significant role in enabling gigantism among Carboniferous-period insects. Even as oxygen levels were indeed higher during that era, they may not have been the driving force behind insect size as previously thought.

The study challenges a hypothesis that has persisted since the 1990s, when a paper in Nature first linked peak atmospheric oxygen to the existence of giant insects like griffinflies. By demonstrating that insect respiratory systems have substantial reserve capacity, the research shifts the focus toward new explanations for why these ancient giants evolved—and eventually disappeared.

As scientists continue to explore the interplay between anatomy, environment, and evolution, this discovery underscores how long-held scientific assumptions can be revised with better technology and deeper investigation. The true reasons behind the rise and fall of Earth’s first giant sky dwellers remain, for now, a bigger mystery than ever.