If you don't like giant bugs, this is your last chance to turn back. Because we've known for quite some time that back in the Permian era (300 million years ago), bugs used to get big. Take Meganeura, for example. This giant dragonfly was a real monster with a wingspan exceeding 2 feet (70 centimeters). And then there's Arthropluera, a millipede that could easily reach 9 feet long (3 meters).

But what was it about the ancient world that made bugs get so big? A better question might be, why aren't they so big anymore? The answer most scientists agree on revolves around oxygen. In the Permian era, there was approximately 50 percent more oxygen in the atmosphere than there is today. Bugs breathe through tracheal air tubes, and the bigger they are, the bigger those tubes need to be to fuel their bodies with oxygen. But if there's more oxygen available, the tubes don't need to take up so much room, making it easier for them to get bigger and bigger. And that means '50s monster movies made into reality.

This all led biologists to the obvious question: What would happen if you raised modern insects in an oxygen-rich environment? The answer: You'd get giant bugs. In environments that mimicked the oxygen-rich Paleozoic atmosphere (about 31 percent oxygen to today's 21 percent), dragonflies grew ... wait for it ... half an inch bigger.

Okay, so it wasn't as dramatic as the giant dragonflies that lived back then, but the scientists were also fighting against a couple hundred million years of evolution. Just to put a nice little bow on the entire experiment, the dragonflies that were raised in a less oxygenated atmosphere grew much smaller than their cousins. The same thing happened with beetles. But there was one strange hiccup that points to some still-unanswered questions: Not only did cockroaches fail to grow any bigger in a more oxygenated environment, they also grew more slowly than normal. Just as everyone suspected, roaches are perfectly adapted to take over the planet once the apocalypse hits.

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For more about the era of giant fauna, check out the New York Times bestseller, "The Rise and Fall of the Dinosaurs: A New History of a Lost World" by Steve Brusatte. The audiobook is free with an Audible trial. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.