ATMOSPHERIC OXYGEN AND THE EVOLUTION OF INSECT GIGANTISM

Most models estimate that over the last 500 million years atmospheric oxygen has varied from ~12% to 35%. Most strikingly, the giant insects of the late Paleozoic (i.e. dragonflies) existed when atmospheric oxygen was hyperoxic, supporting a role for oxygen in the evolution of insect body size. However, the fact that not all groups during this time period were giant (i.e. cockroaches) coupled with the paucity of the insect fossil record and the complex interactions between oxygen, organisms and communities makes it difficult to definitively accept or reject the historical oxygen-size link. Nevertheless, we have successfully reared dragonflies, cockroaches and a variety of other insect species under varying oxygen levels and the results of these studies do support a link between oxygen and the evolution of insect size: 1) dragonflies and other insect groups do develop and evolve larger body sizes in hyperoxia, while almost all insects develop smaller body sizes in hypoxia; yet cockroaches show no size difference when reared under hyperoxia, 2) insects developmentally and evolutionarily reduce their investment in the tracheal respiratory system when living in higher oxygen levels; suggesting there are significant costs associated with tracheal system structure and function and 3) larger insects invest more of their body in the tracheal system, potentially leading to greater effects of oxygen on large insects. These results provide several mechanisms by which the tracheal oxygen delivery system may be involved in the small size of modern insects and hyperoxia-enabled Paleozoic gigantism. When we begin to examine the fossil record closely, we see that certain groups have responded more strongly to oxygen variation. While taxa such as Protodonata and Paleodictyoptera have gigantic members, they are outliers to an overall pattern of oxygen-mediated body size change. On the other hand, Blattodea contain no giant representatives and demonstrate little effect on maximum body size, but do show shifts in average size correlated with changes in atmospheric oxygen levels. Here we examine the role of atmospheric oxygen in the evolution of insect body size and discuss the possibility of imaging fossil tracheae as a proxy for paleo-oxygen levels. This research was supported by NSF EAR 0746352 and DOD 3000654843 to JH and JVB.