The largest wings of any living insect belong to the Queen Alexandra birdwing butterfly and the atlas moth. They can span 10 to 12 inches across. But even these giants are puny compared to the insects of prehistory. Meganeura, for example, was a dragonfly that lived 300 million years ago and each of its wings was the length of my arm. Why do such behemoths no longer exist?

The prevailing theory, proposed around a century ago, is that the Earth’s atmosphere used to have much more oxygen—more than 30 per cent in the Permian, compared to just 20 today. This vital gas sets an upper limit on how big animals can be. The seething quantities of past eras