Fossils show dragonfly-like Meganeura monyi and Meganeuropsis permiana grew wingspans of about 75cm during the Carboniferous and Permian periods.
Atmospheric oxygen near 30%—versus about 20% today—likely helped power such size despite insects relying on passive diffusion through narrow breathing tubes.
Researchers also point to oxygen’s toxicity, especially for aquatic larvae, with larger bodies potentially diluting its harmful effects.
A predator-light sky before birds emerged may have removed a key limit on size, leaving these insects at the top of the airborne food chain.
The combined explanations frame giant insects as a product of ancient atmospheric chemistry and ecology rather than a single evolutionary quirk.