Why Don't Insects Grow to the Size of a Dog Anymore?

Three hundred million years ago, a dragonfly-like hunter called Meganeura flew over the swamps of central France on wings about 70 centimeters across, as wide as a pigeon. On the ground, the giant millipede Arthropleura grew to around 2.5 meters. Today the biggest dragonfly wouldn't cover a sheet of printer paper. So what happened to the giants, and why has no insect come close since? In this video, we follow the clues from a French coal mine to a synchrotron beam near Chicago and an electron microscope in South Africa. We test the famous textbook answer, oxygen, see where it holds up, and look at the 2026 Nature study that knocked out one of its main pillars. And then the question turns around and points at you. 🪲 What You'll Discover Coal and oxygen: how buried Carboniferous forests may have pushed oxygen above 30 percent, compared with 21 percent today. No lungs: why insects breathe through a network of tubes, and why diffusion works brilliantly over tiny distances and poorly over long ones. Bigger in richer air: lab experiments in which flies, beetles and dragonflies grew larger in oxygen-rich air and smaller in thin air. Beetles that choke at the knees: X-ray images showing breathing tubes crowding the leg joints, with a predicted limit close to the size of today's largest beetle. The 2026 twist: a Nature study of 44 species finding that tracheoles take up about 1 percent of flight muscle, far too little to be the real ceiling. Birds, armor and molting: why insects stopped following oxygen after the rise of birds, and why a heavy exoskeleton that must be shed is its own trap. You and the air: how your lungs and blood beat the insect ceiling, and how thin air in Tibet shaped human genes. Tell me in the comments: what's the biggest insect you've ever met up close? And subscribe for more stories where the answer turns out to be about you. #insects #evolution #prehistoric #meganeura #biology