Giant insects ruled the prehistoric skies during periods when Earth’s atmosphere was rich in oxygen. They reached their biggest sizes about 300 million years ago during the late Carboniferous and early Permian periods. This was the reign of the predatory griffinflies, giant dragonfly-like insects with wingspans of up to 28 inches (70 cm).

The leading theory attributes their large size to high oxygen concentrations in the atmosphere – over 30 percent compared to 21 percent today – which allowed giant insects to get enough oxygen through the tiny breathing tubes that insects use instead of lungs.

The scientists compiled a huge dataset of wing lengths from published records of fossil insects, then analyzed insect size in relation to oxygen levels over hundreds of millions of years of insect evolution. Their findings appear in the Proceedings of the National Academy of Sciences.

“Maximum insect size does track oxygen surprisingly well as it goes up and down for about 200 million years,” said lead author Dr Matthew Clapham, an assistant professor of Earth and planetary sciences at the University of California. “Then right around the end of the Jurassic and beginning of the Cretaceous period, about 150 million years ago, all of a sudden oxygen goes up but insect size goes down. And this coincides really strikingly with the evolution of birds.”

With predatory birds on the wing, the need for maneuverability became a driving force in the evolution of flying insects, favoring smaller body size.

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