It’s no secret that 300 million years ago, the largest insects were a lot bigger than they are now. The largest known insect that ever lived is an ancient griffenfly Meganeuropsis permiana. This creature belongs to the extinct order of griffinflies (Protodonata) – related to dragonflies - and measured an impressive 71 cm across, larger than most birds. There were a variety of other megainsects in prehistoric times, but these days, the biggest ones are considerably smaller. So what happened?
One fact which is widely considered to be relevant is the limitation of insect respiration. As insects don’t have lungs like us vertebrates, they rely upon oxygen entering their body through many tiny tubes. They can help it on its way with various tricks but in essence, if the tube is too long the oxygen can’t get down it far enough. So an insect that got too big would soon run out of oxygen and find it difficult to fly or run. So how did those huge ancient insects get around that limit? The answer is that they didn’t. In Permian times the atmosphere was more than 30 percent oxygen, compared with 21 percent today. So insects had more oxygen available, and could grow larger.