Why did insects never evolve (or grow) to sizes more than about 10 to 12 inches long (and, even more strictly no more than an inch or two in diameter)?

According to what I've been taught in classes, an insects morphology limits its size. If an isect were to grow to be too large, the sheer weight of its exoskeleton could crush its body and kill it.

Thank you for your answer! :)

Insects can't grow any bigger than they are because they don't have lungs. They breathe using holes (spiracles) in their surface which lead to a network of tubes through which air is moved by the movement of the insect itself. When it's not moving the air only gets to the organs by diffusion. If the insect was any bigger, diffusion would be too slow to supply enough oxygen to the inner tissues so it would die.

Hope that helps!

I understand both the above answers to be correct.

However, I have often wondered: I am sure there are fossils of enormous insects, such a dragon-flies... anybody have info?

I once encountered (shudder) a Goliath Birdeating Spider which is the largest spider species on Earth.. 13 inches across!! But they don't like to get too close, because they are fragile. Looks like that's about the limit for an exoskeleton.

When I was in Texas I saw a praying mantis that was 17 inches long

Well you know what they say about the size of things in TX...

Core samples from both the arctic and Antarctic display changes in earths oxygen levels

The higher the o2 the larger the insect can grow.

Deinacrida heteracantha, of New Zealand had a reported weight of 71 grams

But the respetory system is very limited so even with higher oxygen levels they cannot attain body mass equal to mammals their respetory system cannot support it.

Their exeosheloton is capable of supporting larger body mass they evolved from crustations like the lobster.