Why were prehistoric bugs so much bigger than modern insects?
During the Carboniferous period, dragonflies grew as large as hawks because of high oxygen levels.
Around 300 million years ago, Earth's atmosphere was 35% oxygen, much higher than today's 21%. Insects breathe through tiny tubes called tracheae rather than lungs. Since oxygen moves through these tubes via diffusion, the oxygen-rich air allowed these insects to grow to massive sizes, including the Meganeura, which had a 75-centimeter wingspan.
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The Carboniferous period, spanning from 359 to 299 million years ago, was characterized by vast swamp forests that produced immense amounts of oxygen. This biological boom caused atmospheric oxygen levels to peak at approximately 35%, whereas modern levels sit at just 21%. This environment supported 'island gigantism' in terrestrial invertebrates, most notably the Meganeura monyi, a predatory insect related to modern dragonflies.Insects rely on a tracheal system for respiration, which consists of a network of tubes that deliver oxygen directly to tissues via passive diffusion. Because diffusion is only effective over very short distances, the size of an insect is physically limited by how far oxygen can travel into its body. In the oxygen-rich Carboniferous air, oxygen could penetrate much deeper into larger tissues, removing the metabolic 'ceiling' on body size.Research led by scientists like Jon Harrison at Arizona State University has demonstrated this link by raising modern insects in hyperoxic environments, showing they can grow significantly larger in just a few generations. The Meganeura had a wingspan of up to 75 centimeters, or about 2.5 feet, making it one of the largest known flying insects in history. When oxygen levels eventually dropped during the Permian period, these giants could no longer sustain their massive bodies and went extinct.
Verified Fact
FP-0004536 · Feb 19, 2026