Don: Yael, did you know that three hundred million years ago the earth was swarming      with giant insects? I'm talking about five-foot-long millipedes and dragonflies as big      as hawks.

Yael: Yikes! That sounds cool, although I'm glad bugs aren't as gigantic today. But . . .      why not? How did insects get so small?

D: Because there's less oxygen in the atmosphere today than there was in prehistoric      times.

Y: Wait, I don't get it. Insects don't have lungs, right? So why would oxygen matter?

D: Insects may not have lungs, but they still depend on oxygen to live. See, insects have      breathing tubes, called tracheae, running through their bodies. Oxygen enters the      tubes through holes in the exoskeleton. And the more oxygen an insect gets, the      larger it can become. So in the largest insects today, the tracheal tubes take up more      space.

Y: So what's to stop insects from growing to prehistoric proportions?

D: Good question. The more space the tubes take, the less room there is for the other      organs insects need to live. So tracheae can only get so large, which limits how much      oxygen an insect can get.

Y: Which in turn limits its size. Okay, I get that. But I still don't understand why some      insects were once able to grow so large.

D: Because once upon a time there was around fifty percent more oxygen in the      atmosphere than there is today.

Y: Ahh . . . so more available oxygen meant that an insect's tracheae could be relatively      small compared to the rest of its body and still channel lots of oxygen.

D: Right. And all that oxygen resulted in super-sized insects.  

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