Giant Prehistoric Insects Didn't Need High Oxygen Levels After All, Study Reveals
New research overturns decades-old theory about massive ancient dragonflies, showing oxygen alone can't explain their enormous size.
A new study has overturned a long-standing scientific explanation for why enormous insects once ruled ancient skies, revealing that high atmospheric oxygen levels alone cannot account for the massive dragonflies and griffinflies that lived 300 million years ago. The research, published in Nature, challenges the widely accepted theory that linked giant prehistoric insects to the elevated oxygen concentrations of the Carboniferous period.
During Earth's ancient past, when continents were joined in the supercontinent Pangaea and vast coal-swamp forests covered the landscape, the skies were dominated by insects of extraordinary size. Mayfly-like species reached wingspans of 17 inches, while dragonfly-like creatures called griffinflies stretched an astounding 27 inches across. These giant insects, first discovered as fossil impressions in Kansas sedimentary rock nearly a century ago, have fascinated scientists seeking to understand how such enormous flying insects could exist.
For decades, researchers believed these creatures could only survive because atmospheric oxygen levels during the Carboniferous period were about 45% higher than today. The prevailing theory, established in a influential 1995 Nature study, suggested that larger insects required more oxygen and that higher atmospheric concentrations made their size possible. This explanation was based on insect breathing systems, which rely on networks of air-filled tubes called tracheae rather than lungs.
However, the new research led by Edward Snelling of the University of Pretoria used high-power electron microscopy to examine how insect body size relates to the number of tracheoles in flight muscle. Their findings revealed that these tiny oxygen-delivery structures typically occupy only about 1% or less of flight muscle in most insect species. Even when applied to massive ancient griffinflies, this proportion remains small, suggesting that insect flight muscles are not limited by oxygen availability.
The study indicates that tracheoles take up so little space that insects could theoretically increase their number without major structural constraints, meaning oxygen levels alone cannot explain the size limits of ancient insects. This discovery forces scientists to search for alternative explanations for why these giants disappeared, potentially including changes in predation pressure, atmospheric composition beyond oxygen, or physical limits related to insect body structure and mechanics rather than respiratory constraints.
