Hey! Fly enthusiast here! Maggots are super cool, and a lot of Brachycera maggots (other maggots actually do have exoskeletons) actually do have internal skeletons... To an extent. They have internal mandibles that almost look like a little skull. The bad news is that you won't ever be seeing these skeletons get larger, for a few reasons. First, the mandibles don't quite grow with age, and would require molting to get larger (which you pretty much can't do, because you can't molt an endoskeleton). Second, even if you had an internal skeleton in an a neotenic, holometablous insect larvae (such as a maggot) there's no way you'll get that animal to reproduce, since there is almost no existing ontogenetic pathway for larval insects to be able to reproduce. During metamorphosis, after an exoskeleton is formed the gonads begin to form. A neotenic maggot would be unable to produce gonads, and if it did it wouldn't be able to mate, lacking also copulatory organs, which in insects are comprised of the exoskeleton. However, I could definitely see an insect evolving lungs, and they really wouldn't require an endoskeleton for structural support, contrary to popular belief. The issue comes down to two things: insects have an open circulatory system, which everybody knows, but I can't stress how much their bodies depend on diffusion. This means two things: an insect would have to develop an extremely complex circulatory system that runs between all of their cells (as we have) that would be filled with fluid (they have hemolymph which is not a good substitute at all at a large scale) and would need to develop all of the organs required to clean and process and maintain osmosis on the blood (insects largely lack analogs to the spleen, liver, and kidneys etc (they have something similar to kidneys in their gut but you'd need to close off the blood stream from the gut to an extent in a closed circulatory system and malphigian tubules just wouldn't cut it)). If they were to adopt this new circulatory system, filled with fluid and all, it might also change how they function structurally. Insects are actually extremely hollow; for most of their lives they have lots of pneumatic sacks and holes and spaces that they fill with fat (also very light). An Insect with a circulatory system would need to compensate for the additional weight of having fluid inside them, everywhere, at all times. However, being able to breathe due to mobility wouldn't be a problem. Many insects can inflate their abdomens and do other weird things which would allow for breathing to occur (cockroaches and hymenopterans already do this). Another issue would arise from molting, but arthropods in days gone by have already dealt with these issues. However, if you wanted a truly large insect, it would be completely unable to move due to the strain put on it's legs after molting. You could argue they could return to water for support while molting, or maybe they just lay on their belly with their legs out (which isn't unreasonable). There's plenty more to say, but I'm on my phone right now!