Harrison and his colleagues took the idea a step further. They hypothesized that high ambient oxygen could have permitted the existence of giant species. The demise of winged monsters and behemoth beetles 100 million years later may be explained partly by the simultaneous decrease in the air's oxygen content. Harrison says that the amount of available oxygen limits insect body size because of how the creatures' respiratory systems are made. Instead of lungs, insects breathe with a network of tiny tubes called tracheae.
Air enters the tubes through a row of holes along an insect's abdomen.
The air then diffuses down the blind-ended tracheae. The distance oxygen can travel down the tracheae depends on its concentration in the air. If atmospheric oxygen is doubled, theory says that it should be able to make it twice as far. According to Graham and Dudley, escalating Paleozoic oxygen levels may have helped speed oxygen transport in the longer tracheae of bigger insects. The environment itself could have opened the respiratory door for Paleozoic insects, allowing giant species to evolve.