300 million years ago (mya), some of the largest arthropods roamed the earth. Giant insects included the Protodonata, relatives of the modern dragonflies. Some fossils have wingspans exceeding 70 cm. The Protodonata and other giant arthropods lived in the era of 31 percent oxygen but disappeared from the fossil record about 285 million years ago as oxygen levels declined. 240 mya, oxygen had declined to only 12 percent. Today, oxygen is 21 percent of the atmospheric concentration. Atmospheric oxygen concentration has long been considered a factor in the large size of those ancient insects. However, experimental evidence has been lacking.
A group at Arizona State University, led by John VandenBrooks, has some interesting experimental evidence on the effects of oxygen concentration on insect size. They reared a number of insects at different concentrations of oxygen- 31, 21 and 12 percent. Dragonflies reared at 31 percent oxygen were about 15 percent larger than those reared at 21 percent. This is clear evidence that high oxygen can lead to larger size.
They also found that the tracheal volume was lower in insects rear at higher oxygen levels. Tracheal volume is one of the limitations on insect size. As insect size increases, the volume of the trachea necessarily increases because the distance traversed is longer. At higher oxygen levels, the trachea have a smaller diameter and require less volume.
This research raises some very interesting questions and suggests a mechanism for atmospheric oxygen to influence size.