During the Carboniferous period, which lasted from about 359 to 299 million years ago, giant dragonflies and huge cockroaches were common. These prehistoric insects grew to impressive sizes, and one of the leading theories behind their gigantism is the abundance of oxygen in the atmosphere at that time. Here’s how it worked: 1. High Oxygen Levels: The rise of vast lowland swamp forests during the Carboniferous period led to atmospheric oxygen levels of around 30 percent—close to 50 percent higher than current levels. This rich oxygen environment allowed adult bugs to grow to ever larger sizes while still meeting their energy needs. 2. Larval Perspective: However, the new study suggests that it’s not just because oxygen affected the adults. Oxygen had a bigger effect on larvae. Insects like dragonflies and stoneflies have a larval stage where they live in water before becoming terrestrial adults. Larvae typically absorb oxygen directly through their skin, so they have little control over how much gas they take in. When oxygen levels were high, this passive absorption could lead to oxygen poisoning. To decrease this risk, growing bigger was advantageous because large larvae would absorb lower percentages of oxygen relative to their body sizes. 3. Regulating Oxygen Intake: Adult insects, on the other hand, can regulate their oxygen intake. They have valve-like holes called spiracles that allow them to adjust the amount of oxygen they absorb. While oxygen is crucial for life, it can be poisonous in large quantities. Humans exposed to excess oxygen can suffer cell damage, vision problems, difficulty breathing, nausea, and convulsions. Ancient insect larvae likely faced similar risks when oxygen levels were exceptionally high. 4. Dragonflies and Cockroaches: Not all insects responded the same way. For instance, dragonflies grew faster into bigger adults in an oxygen-rich environment (hyperoxia), while cockroaches grew slower and did not become larger adults. Overall, ten out of twelve kinds of insects studied decreased in size in lower oxygen atmospheres. The hollow tracheal tubes insects use to breathe played a crucial role in these responses. 5. Fossil Proxies: Understanding how modern insects respond to varying oxygen levels could help us use fossilized insects as proxies for ancient oxygen levels. By studying the plasticity of modern insects raised in different oxygen concentrations, researchers gain insights into the evolution of insects and the impact of oxygen on their size and growth.