a 6-inch wingspan but far more commonly are much smaller. Other giants of the time included mayflies with 19-inch wingspans, a spider with 18-inch legs, and yard-long (or longer) millipedes and scorpions. A 3-foot-long scorpion could weigh 50 pounds and would be a formidable predator of all land animals, including the amphibians. But as we will see, the amphibians also evolved some giant species of their own.
So is it the biomechanics of legs that dictated (or limited) insect size? Apparently not. It is the insect respiratory system that dictates maximum size, it seems. All insects use a system of fine tubes called trachea. Air diffuses into the tissues from these tubes, and air is actively ventilated into the tubes. Either by the insect’s rhythmic expansion and contraction of the abdominal region, or by the insect’s flapping of its wings to create air currents around the tracheal opening, air is pulled into the canals. The tracheal system is fantastically efficient in either case. Flying insects achieve the highest metabolic rates of any animal, and experimental evidence shows that increasing oxygen to higher levels enables dragonflies to produce even higher metabolic rates. These studies showed that dragonflies are both metabolically limited and probably size limited as well by our current 21 percent oxygen levels.
Whether or not oxygen levels control arthropod size has been contentious. The best evidence that it does comes from studies of amphipods, small marine arthropods that are widely distributed in our world’s oceans and lakes. Gautier Chapelle and Lloyd Peck examined 2,000 specimens from a wide variety of habitats and discovered that bodies of water with higher dissolved oxygen content had larger amphipods. More direct experiments were conducted by Robert Dudley of Arizona State University, who grew fruit flies in elevated oxygen conditions and discovered that each successive generation was larger than the preceding when raised at 23 percent oxygen. In insects, at least, higher oxygen very quickly promotes larger size.
It was not only higher oxygen that allowed the existence of giant dragonflies. The actual air pressure is presumed to have been higher too. Oxygen partial pressures rose but not at the expense of other gases. The total gas pressure was higher than today, and the larger number of gas molecules in the atmosphere would have given more lift to the giants. Gas pressure is a function of the number of molecules in the air.