Why don’t insects grow as big as dogs?

Chris Smith asked biologist Chris Pull from Royal Holloway University to expand on this question from Heather on Facebook. 

Chris P- The largest insects today are about 18cm. A few years ago, there was a stick insect in China which was actually 60cm long. Back in 500 million years ago there were insects the size of seagulls flying around. There was a…

Chris S - The size of Bobby Seagulls?

Chris P - Haha. There was a dragonfly which was flying around, and you had millipedes, which obviously aren’t an insect, but they go grow up to 2 metres long. The prevailing theory is that back then there was just a lot more oxygen in the atmosphere, and because they breath passively through the networks of air filled tubes, the larger you get, the harder it is for that defusion based respiration to work. It’s kind of like if you were to snorkel near the surface it’s easy for you to get enough air, but if you were sitting on the bottom of a swimming pool with a 3 metre long snorkel, you could imagine how much harder it would be to get enough oxygen and enough CO2 back out. That’s essentially why we think insects are limited because today the levels of oxygen are much lower than what they have been back in very ancient times.