we know that insects, with the external body armor typical of arthropods, could never live at such size. Because of scaling properties and strength of the chitin making up the arthropod exoskeleton, a giant ant or preying mantis of even human size would collapse, its walking legs snapping. But there is another aspect of arthropod design that limits size—respiration. Insects, spiders, and scorpions appear to be size-limited by the degree to which oxygen can diffuse into the innermost regions of their bodies. Today, no insect is bigger than about 6 inches in body length (although arthropods in water can be and are bigger, since their weight is buoyed up by the watery medium they exist in). In the past, however, much larger land arthropods than this did exist—during the interval of the highest oxygen in Earth’s history, the subject of this chapter. Here we will explore how the highest oxygen in Earth’s history allowed some of the strangest—and largest—animals ever on Earth.
Here is what we might see were we to return to the world of 300 million years ago, when oxygen was at its highest in Earth’s history. The first noticeable characteristic is the color of the sky. It is a polluted yellow-brown, irrespective of weather; only in high winds does the air clear and then soon it muddies again. This is due to smoke from giant fires perpetually raging and new ones set alight with each lightning strike hitting the extensive forests of the temperate and tropical regions. But there is more than soot in the air; there is fine wind-blown dust as well, accumulating as loess, or glacial front deposits. The air is cold over much of the planet, for this is the time of one of the most extensive glaciations in Earth’s history, with ice caps at each pole and continental glaciers reaching icy fingers across the land as they snake downward out of the mountains onto the plains and river valleys. Where there are forests we find unending vistas of conifer-like trees, for the gymnosperms have by now evolved and have swept away many of the earlier dominant plants. No longer do tall but shallow-rooted primitive trees like Lepidodendron dot the landscape. It is much more familiar, or familiar at least to those of us who now live in the high