Great question!<p>We spoke to Jon Harrison about this (the scientist we interviewed who studies insect growth &amp; respiration). He says that the truth of the matter is we don&#x27;t know the answer, it&#x27;s still a hypothesis that the way insects breathe keep them small. There are other competing explanations, like that it&#x27;s insect&#x27;s exoskeletons that constrain their size, or ecological arguments (fewer predators means they can grow bigger). Also spiders don&#x27;t use trachea to breathe but are also small.<p>One piece of evidence favoring the oxygen hypothesis is research that shows that the fossil record on giant insect size correlates with the oxygen levels in the past, all the way up until birds evolved (and wiped out the ecological niche for giant flying insects!)<p><a href="http:&#x2F;&#x2F;blogs.discovermagazine.com&#x2F;notrocketscience&#x2F;2012&#x2F;06&#x2F;04&#x2F;giant-insects-disappeared-thanks-to-falling-oxygen-levels-and-agile-birds&#x2F;#.VkYYYWSrRhA" rel="nofollow">http:&#x2F;&#x2F;blogs.discovermagazine.com&#x2F;notrocketscience&#x2F;2012&#x2F;06&#x2F;0...</a><p>And I&#x27;m told there&#x27;s some more interesting work along these lines that&#x27;s yet to be published.. so hopefully we should know more soon.<p>Lastly, there&#x27;s all kinds of experimental work on breeding insects in high oxygen environments, but the results so far aren&#x27;t universal.. some kinds of insects grow bigger, other&#x27;s don&#x27;t. The idea being that oxygen can help the insect grow but can also do damage to cells (in a way similar to aging), so you may need a long while to evolve and adapt to higher oxygen levels.