Depending on how picky you want to be about defining insects and spiders such that you include the rest of the arachnids, the things could get *huge.* But that was underwater. And while I've no doubt underwater critters *could* evolve high intelligence, their ability to create industry seems limited due to the inability to have fire.Spiders actually have lungs (well, some of them), but it isn't helping them much; they don't have the proper blood circulation system to make use of it. To put it simply, it would took too much efforts to turn spider into anything even remotely as efficient as fish or toad (not even talking about gecko or mice).
If they created reverse scuba systems that allowed them to spend extended periods on land, then it's just barely conceivable that they could eventually develop metallurgy and such. But that would seem a pretty unlikely series of events.
So in theory you could see Earth being invaded by sea scorpions wearing water-filled space suits. Likely their suits would be power armor to let them walk with all that added weight. Or maybe they've reduced it to a system that encapsulates their "book lungs", so they just wear a tank of water on their backs.
They'd be tough to beat to death with sticks and hard to stab with knives, but sledgehammers, drills, fire and good old fashioned firearms would make a mess of them.