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Explanation for why we don't see two-foot-long dragonflies anymore fails

March 29, 2026
•Ars technica
Explanation for why we don't see two-foot-long dragonflies anymore fails

Here's something that might surprise you — scientists have just debunked the long-held idea that giant ancient dragonflies were only possible because of super high oxygen levels. For decades, the 'oxygen constrain hypothesis' was the go-to explanation: as Earth's atmosphere lost oxygen, these enormous insects couldn’t survive anymore. But Jacek Krywko reports that recent research shows that’s not quite right. According to biologists like Edward Snelling, insect breathing isn’t like ours — no lungs, no closed circulatory system — so oxygen levels aren't the whole story. Instead, it turns out that other factors, like the insects’ body structure and flight mechanics, played a bigger role in limiting size. So what does this mean? Well, it’s a reminder that old theories can be wrong, and it’s always worth digging deeper. The big takeaway — size limits aren’t just about oxygen; it’s a mix of biology, physics, and evolution, and scientists are still unraveling how it all fits together.

Three-hundred million years ago, the skies of the late Palaeozoic era were buzzing with giant insects. Meganeuropsis permiana, a predatory insect resembling a modern-day dragonfly, had a wingspan of over 70 centimeters and weighed 100 grams. Biologists looked at these ancient behemoths and asked why bugs aren’t this big anymore. Thirty years ago, they came up with an answer known as the "oxygen constrain hypothesis."

For decades, we thought that any dragonflies the size of hawks needed highly oxygenated air to survive because insect breathing systems are less efficient than those of mammals, birds, or reptiles. As atmospheric oxygen levels dropped, there wasn’t enough to support giant bugs anymore. “It’s a simple, elegant explanation,” said Edward Snelling, a professor of veterinary science at the University of Pretoria. “But it’s wrong.”

Insect breathing

Unlike mammals, insects don't have a centralized pair of lungs and a closed circulatory system that delivers oxygen-rich blood to their tissues. “They breathe through internalized tubing called the tracheal system,” Snelling explained.

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Audio Transcript

Three-hundred million years ago, the skies of the late Palaeozoic era were buzzing with giant insects. Meganeuropsis permiana, a predatory insect resembling a modern-day dragonfly, had a wingspan of over 70 centimeters and weighed 100 grams. Biologists looked at these ancient behemoths and asked why bugs aren’t this big anymore. Thirty years ago, they came up with an answer known as the "oxygen constrain hypothesis."

For decades, we thought that any dragonflies the size of hawks needed highly oxygenated air to survive because insect breathing systems are less efficient than those of mammals, birds, or reptiles. As atmospheric oxygen levels dropped, there wasn’t enough to support giant bugs anymore. “It’s a simple, elegant explanation,” said Edward Snelling, a professor of veterinary science at the University of Pretoria. “But it’s wrong.”

Insect breathing

Unlike mammals, insects don't have a centralized pair of lungs and a closed circulatory system that delivers oxygen-rich blood to their tissues. “They breathe through internalized tubing called the tracheal system,” Snelling explained.

Read full article

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