There has been some controversy as to how insects of the Carboniferous period were able to grow so large.
Oxygen levels and atmospheric density. The way oxygen is diffused through the insect's body via its trachealbreathing system puts an upper limit on body size, which prehistoric insects seem to have well exceeded. It was originally proposed by that Meganeura was able to fly only because the atmosphere at that time contained more oxygen than the present 20 percent. This hypothesis was initially dismissed by fellow scientists, but has found approval more recently through further study into the relationship between gigantism and oxygen availability. If this hypothesis is correct, these insects would have been susceptible to falling oxygen levels and certainly could not survive in our modern atmosphere. Other research indicates that insects really do breathe, with "rapid cycles of tracheal compression and expansion". Recent analysis of the flight energetics of modern insects and birds suggests that both the oxygen levels and air density provide an upper bound on size. The presence of very large Meganeuridae with wing spans rivaling those of Meganeura during the Permian, when the oxygen content of the atmosphere was already much lower than in the Carboniferous, presented a problem to the oxygen-related explanations in the case of the giant dragonflies. However, despite the fact that Meganeurids had the largest-known wingspans, their bodies were not very heavy, being less massive than those of several livingColeoptera; therefore, they were not true giant insects, only being giant in comparison with their living relatives.
Lack of predators. Other explanations for the large size of Meganeurids compared to living relatives are warranted. suggested that the lack of aerial vertebrate predators allowed pterygote insects to evolve to maximum sizes during the Carboniferous and Permian periods, perhaps accelerated by an evolutionary "arms race" for increase in body size between plant-feeding Palaeodictyoptera and Meganisoptera as their predators.
Aquatic larvae stadium. Another theory suggests that insects that developed in water before becoming terrestrial as adults grew bigger as a way to protect themselves against the high levels of oxygen.