~ George Orwell (1903-50)
Scientists have long wondered why giant bugs don't exist today. Unlike animals with backbones, insects deliver oxygen to their tissues directly and bloodlessly through a network of dead-end tracheal tubes. In bigger insects, this mode of oxygen transport becomes less efficient. In other words, a bottleneck occurs in insects' air pipes as they become humongous.
250 million years ago, in the Paleozoic Era, insects were able to overcome this bottleneck due to the Earth having a higher-oxygen atmosphere, and so there were dragonflies the size of hawks and millipedes longer than a human leg. But not today - thank God.