Adult insects use oxygen at a high rate when they fly. They need it for the flight muscles, the most active tissue known in biology. The flight muscles use oxygen at a huge rate: 100 ccs of oxygen for every single cc of tissue per hour. With this system, the greatest diameter a muscle could have (and still consume oxygen at this rate) is about 0.5 cm. Even with special extra arrangements, insects cannot get larger than about 11 cm long. The largest insect bodies are about as big as a mouse.