So a little bit ago I asked a question that asked multiple questions so my new strategy is too ask multiple questions. First off this first question is going to revolve around the bugs themselves and their changes. So due to increased plant life oxygen is much more prevalent due to this fact most insects in this alternate earth have noncupled in size not all but most (meaning by nine times their original size). Now they would have changed anatomy to fit this new size (some insects wouldn't get too large because of the square cube law like goliath bird eating spiders but most would get pretty big) there changes would be leg placement exoskeleton thickness increased intelligence because of there increased brain sized (some may not need it though like ants) increased lifetimes so no more constant birth and thicker legs. (also flora would increase and size) mammals and birds in this world would stay around the same size but would be faster stronger and more endurant (this includes birds and reptiles). so in this new world of large bugs what niches would you think would be taken up by bugs and what changes would basic mammals,reptiles,and birds to deal with or eat these new giant insects. Also humans are during this story in medieval time.
LOOK OUT, MAMMALS:
Postulating that a large concentration of oxygen-producing plants means a warm, wet tropical climate globally, I think mammals would be in for a lot of troubles. During the Permian epoch, the proto-endothermic synapsid ancestors of mammals were the dominant life forms. After the Permian mass extinction (arguably the largest mass-extinction event on Earth) the warmer world with lots of open ecological niches allowed for dinosaurs to evolve. They did better with warm conditions and were exothermic, so they didn't need to expend as much energy just to live.
So if your climate change resulting in insect-friendly conditions opened up niches for insects, they would exploit a lot of the spots mammals hold today. The fewer limits to size the insects have, the more spots they'll take. The spots of smaller animals will go first. Cockroaches will displace rats, spiders and centipedes will displace small predators, and beetles and the like will displace small herbivores. Probably all the spots for birds would have a more efficient insect able to do the job better. Roles for big animals will still favor the mammals that are physically able to fill them better, only they will be better evolved to deal with big bugs.
But don't forget that mammals survived the dinosaurs, and reptiles lived even back in the carboniferous. The giant insects survived into the early Permian, but were pushed out by competing synapsids and climate change. You can justify almost anything, just by deciding the conditions favored X (where X is whatever fits your plot). A lot of birds in a niche can outcompete giant flies by eating away all the resources - or having a taste for giant maggots. Don't get too bogged down in details, because evolution is a sloppy, complex and fickle process with more variables than we can track. Have fun and good luck.
first off- evolution would take a long time to reach this point, a very long time. if humans are around in your story, they probably would've stopped it because who wants a two foot bee landing on your soda? second off, i'm not a biologist, so a different answer will almost definitely be better than mine.
anyways, assuming they evolve naturally to be bigger, birds, reptiles, etc would also evolve along side them(unless humans interfere) and have their adaptations ready at the same time/before the bugs got there. the bug eating animals would probably evolve to have stronger jaws/teeth/beaks to deal with the exoskeletons, and would have to adapt new hunting strategies if the bugs get too smart for them.
as for the niches, i think they'd take whatever was open, or whatever they could take over. think rats- they eat whatever, so they live wherever, doing whatever. there's enough different types of bugs you could put them anywhere eating anything(within reason, of course. can't have a bird eating bug...unless you really want one. )
if you want big bugs, you'll almost definitely get big bug eating animals, but big bugs also means big food needs, so you have to either scale down the bug population(quality before quantity) or increase the food source size, which doesn't seem as realistic. of course, you could always write in something about food waste, carnivorous bugs, eating sunlight, etc, etc, to deal with this, but then you'd basically have a bugpocalypse.
so, in short, big bugs = big bug eaters. bug eaters ready for bugs before or same time bugs get there, bigger bugs = bigger food source, bugs = rats, all this assuming humans stay out of it.
Edit- ah, humans. that changes things.
if humans want giant bugs(torture devices, terror agents, whatever the twisted mind could think of, etc) they might allow them to evolve. if the medieval times is already after they've become big, then the human society before then would need to have a good reason to let the bugs get big, or they probably would've (quite literally) squashed it before it even started. i don't know about everyone else, but i certainly would kill everything with a stinger just cause i hate wasps and stuff. in fact, i was stung by a yellow jacket earlier today, and i'm very glad it wasn't a foot long.
one good thing about big bugs tho- u ever heard of cricket meatballs? they'd probably have been changed(at least nowadays, not sure about medieval times) to be a big as possible, and maybe even replace meat cows. if people realize, hey we can eat these, they probably would keep some big, but not quite all. again,, getting big would take a very long time.
Many years ago, as I was leaving the movie house after an opening night showing of Jurassic Park, a friend of mine turned to me and asked, "Would lions, tigers and bears ever have evolved if raptors had not gone extinct?" That question has haunted my world building efforts ever since.
It is my belief that the presence of a small but communal apex predator would completely fill the top of every food chain that it could participate in. So lions and tigers who occupy warm reptile (and insect) friendly zones would be in for a challenge, but cold loving bears would probably be okay.
I believe that the insect-scale ecosystem is much more brutal than our larger scale ecology. Big cats have tough hides and sharp claws and teeth, but the major predators of the insect-scale have chitinous armor, ensnaring webs and mandibles! Pack sizes on our scale usually top out below a hundred while insect hives can number in the tens of thousands.
Your world wouldn't need any mammalian apex predators. That throne will be very efficiently filled by your super bugs!