I've had a great idea to leap over the giant boundary of respiration when making giant insects.The largest problem is that insects rely on diffusion through exterior holes (spiracles) to breathe, and the amount of diffusion relies on surface area, so a large insect wouldn't be able to move enough oxygen around its body because the volume of its interior mass would be much larger than surface area, and oxygen would need to travel further into the insect.(This is why the largest insects like meganeura had long, thin, tube-like bodies)
My idea is that since lungs (internal respiration) evolved from the gills of fish (external), perhaps the gills of aquatic nymphs like dragonfly larvae could evolve in much the same way.
As I've tagged, I'm looking for a reality check, as my knowledge of the movement from gills to lungs is not very sound. My question comes in two parts:
first, of course, is this idea plausible?
and secondly, how would the new internal structures affect the shape of the insect's body?
Edit: To clarify, I know that a new circulatory system would be needed along with the lungs, but that's a topic for a different question.
Also, I had another idea for the development of lungs or equivalent from the book-lungs of arachnids, so if you like you can touch on that in your answer.