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. 
Small locusts cannot survive in an environment with low oxygen content, especially in an environment with an oxygen concentration of less than 15%, while adult locusts can survive in an environment with 2% oxygen content
 it can be judged that the amount of oxygen can determine the size of insects.