Other invertebrates manage to grow up to 50cm e.g. crab and lobster.
Aren't they working under similar constraints as insects, could giant
bugs appear on a beach somewhere??
I've also heard both the "exoskeleton" and "oxygen passive diffusion"
arguments as to why insects didn't get any bigger.
Some people would also argue that in many ways a social insect colony
shares a lot of the properties of one large insect without the
problems of being a large insect in a single body.
You'd have to run your own evolution experiment (in a jar) for a very
long time!!!!
Simon T