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. 
During the Carboniferous period, the concentration of oxygen in the earth's atmosphere was as high as 35%, much higher than the current 21%. At that time, many giant insects lived on the earth, and the dragonfly wingspan was close to one meter
 it can be judged that the amount of oxygen can determine the size of insects.