Shortly before Graham and Dudley's hypothesis was published, Harrison learned about the Paleozoic oxygen pulse from Dudley himself. The two were chatting at a national biology conference. He told me about the oxygen pulse over a beer. At the time, it was a shock to most of us to think that atmospheric oxygen concentrations could vary. If it did, it seems likely that there would be some fairly strong and interesting physiological consequences.
Scientists know that the flight muscle of an insect burns more oxygen than any other animal tissue.
The amount of oxygen supplied to an insect's muscles directly depends on the amount of oxygen in the air. Given these facts, it makes sense that giant insects would struggle to get by in a low oxygen atmosphere. However, Harrison points out that the presumption that the Paleozoic oxygen pulse actually caused the evolution of giant insects rests more on inference than evidence. There are no living giant insects, or fossils of their tracheae. A convincing test of the oxygen pulse hypothesis will depend on the weight of evidence from studies of several species.