Studies have found that insects breathe through the stomatal system in their bodies. The stomata are connected to the trachea, and more and more layers of smaller and smaller pores are attached from top to bottom, thus sending oxygen to the whole body. At the current level of oxygen content in the atmosphere, the total length of the stomatal system has reached the limit. if the total length exceeds this limit, the oxygen supply capacity will be insufficient. 
In an environment with high oxygen content and high air pressure, the tested fruit flies lived to the fifth generation, and their body size increased by 20%
 it can be judged that the amount of oxygen can determine the size of insects.