Did you know that three hundred million years ago the earth was swarming with giant insects? We're talking about five-foot-long millipedes, and dragonflies as big as hawks.
How did insects get so small today?
Well, there's less oxygen in the atmosphere today than there was in prehistoric times.
See, insects have breathing tubes, called tracheae, that run through their bodies. Oxygen enters the tubes through holes in the exoskeleton. The more oxygen an insect gets, the larger it can become. In the largest insects today, the tracheal tubes take up more space. The more space the tubes take, the less room there is for the other organs insects need to live. Therefore tracheae can only get so large, which limits how much oxygen an insect can get. That in turn limits its size.
Once upon a time, however, there was around fifty percent more oxygen in the atmosphere than there is today. The additional oxygen meant that an insect's tracheae could be relatively small compared to the rest of its body, and still channel lots of oxygen. All of that oxygen resulted in super-sized insects.