Three hundred million years ago, dragonfly-like creatures with wingspans stretching 70 centimeters patrolled the skies of a world nothing like our own. These griffinflies, as paleontologists call them, were the largest flying insects ever to exist, roughly five times the size of any dragonfly alive today. For three decades, scientists thought they knew why such animals could never return: modern air simply doesn’t contain enough oxygen to power them. New research published in Nature dismantles that explanation entirely. A team working across five years and more than a thousand microscope images has found that the respiratory structures of insect flight muscles occupy so little anatomical space that insects could, in principle, easily compensate for lower oxygen levels by growing more of them. The oxygen content of Earth’s ancient atmosphere, the theory goes, was not the reason griffinflies grew enormous. And it almost certainly is not the reason giant flying insects don’t exist today. That second question, what actually does limit insect size, now has no clean answer. The long-extinct griffinfly (left) dwarfs even today’s heavyweight …