so i'm thinking of a medieval society in a world with giant bugs. let's say the honeybee was about the size of a house mouse for reference (which makes the insects and arachnid nine times it's size). The reason would be due too increased oxygen from huge plants like in the past so most of the world is like a dense forest (or at least the world that is currently forrest). would any insects benefit humans and which insects would become worse or more deadly. would aphids be used in the same way as ants like cattle. would any insects be used for transportation or cargo shipment. what about for military purposes. (Here's a link to the sandbox version might be better to put it there)https://worldbuilding.meta.stackexchange.com/questions/7183/sandbox-for-proposed-questions
As a Food Source
Humans already enjoy eating many variety of larger arthropods including crabs, crawfish, and lobsters. That said, many people find eating large bugs "inconvenient" despite being both tasty and healthy because as the animal gets bigger, the shell becomes thicker and harder to get through so you are either stuck with something small enough that is easy to peel open with your hands like crawfish which is complicated by the fact you need to eat 100 or so of them to get full or large enough to only need 1-2 like crabs and lobsters, but take a lot of work to get into.
So when it comes to food, you'd probably see a larger range of big-bug meats showing up on the menu, but they would probably not replace mammalian and poultry meat as our easy-go-to meats.
Risk to Humans
Many of the qualities we think of as belonging to insects evolved as survival strategies related to being so small. Rapid reproduction cycles, powerful venom, and mindless aggression evolved as part of filling the niche of being tiny animals. When you take away being tiny as thier niche, the insects that evolve to be the size of larger animals will be competing to fill the same niches as larger animals. An animal that evolves to be larger needs to focus more on how the individual will survive to maturity than how to quickly produce enough babies to not all be killed off before the next generation; so, giant bugs will have fewer babies, spend more effort on child rearing, and be less prone to living in hives.
All this together means that bugs may push out a lot of the niches currently filled by smaller animals such as hawks, lizards, snakes, turtles, etc. But pushing out those niches will also mean evolving to resemble those niches; so, a sea-beatle would be not much more or less dangerous than the sea turtle it pushed out, and the viperpede would be not much more or less dangerous than the viper that it pushed out. So in the end, we would see more or less the same risks we see from wild life on our world, but they would just be wearing thier skeletons on the outside instead of with-in. This higher emphases on natural armor would have probably meant primitive man would have focused more on clubs and slings than spears and bows, but otherwise, our daily lives would not have been significantly impacted.
As Beasts of Burden
Bugs do not make good big creatures. The typical leg configuration of bugs using outstretched legs does not scale up well for supporting larger organisms because it puts the leg under extra shearing force just to stand up. Endoskeletons also create a mechanical advantage over exoskeletons. So despite what you may have learned in school, when you put a bug and a mammal at the same scale, the mammal is actually stronger when you factor in the square-cube rule. So, while you could get a spider the size of cat or wasp the size of a pigeon, you will generally not get a bug the size of a horse or oxe capable of supporting its own weight.
The one exception to this might be giant centipedes because they can spread thier weight out so efficiently that you might have a 20ft long bug the size of an anaconda that could be used for things like pulling a plow or carry lots of light weight stuff along its massive back, but it would still probably not be tall or fast enough to ride like a horse.
As Instruments of War
Large bugs don't make good horses, but they might make a really good alternative to war dogs. In theory, a man with a spear and shield is far more dangerous than a wardog, but by playing on man's natural fear of wolves, war dogs could often cause battle formations to break through fear alone. If there is one thing people instinctively fear more than large predators like snarling dogs, it is venous animals like spiders. Our instincts are such that animals do not even need to be venomous to elicit the same fear response, and just being bigger automatically makes something scarier. We see this with how people are terrified by tralachlas, even after you tell them they are not poisonous. So when you combine the size of a dog with the body plan of a spider, I think it's safe to say even the bravest of men would $h!+ themselves if they saw a battle line of those things skittering right at them.
The sclerotized chitin that makes up the rigid exoskeletons of many land-based arthropods is about as strong and light-weight as horn. While horn armor was historically used, it was not very common because it is much harder to get enough horn to make a full suit of armor than other materials. With the availability of larger arthropods, not only would sclerotized chitin be really common when compared to other materials, but depending on the bugs you take it from, it could come already more or less in the right shape. Giant centipedes or pill bugs could be hollowed out and used as loricata for your arms and abdomen with very little modification or bugs with more rigid bodies such as a beetles could be use as a cuairase, greaves, or helmets.
While not a replacement for some of the heavier armors like platemail, sclerotized chitin would probably be often seen throughout history in lue of many styles of lighter armors like linothorax, leather lamellar, and gambison.
Depends on your hand waving
First off, there are real world limitations to how much an insect can scale. Here's an excerpt from Reddit user Astrokiwi's answer to the question "If an ant was the size of a human, would it still be able to lift 10x it’s body weight?"
No. This is commonly referred to as the "square-cube law". Basically, the strength of something relates to its cross-sectional area - how thick it is. If you scale up something by a factor of 2, then the cross-sectional area of the arms/legs/etc increases by a factor of 4, i.e. the square of the scale factor. However, the weight of something increases with its volume. This increases as the cube of the scale factor. So if you double the scale, your object now weights 8 times as much. In practice, what this means is that small things can support themselves with very thin spindly legs, while large things need very solid legs directly underneath them.
You mention that you're using hand waving to embiggen your insects. How that happens is massively important to your story. For example, a real world ant can carry many times its own body weight, but as pointed out above, a really big ant could no longer carry even its own weight. If your hand waving maintains the lift-to-weight ratio, then a 1,000 pound ant could theoretically carry a dozen tons or more. On to the applications.
1. Beasts of burden
If you could find a way to train your insects, they could apply their massive strength to any number of activities. They could fly people across mountains, carry heavy loads, and dig mines. Of course, training is the tough part. Check out this answer for a detailed discussion on training ants. Insects don't respond to the same incentives as domestic mammals, so you might need more hand waving to make this happen.
Food would be a major use of gigantic insects. In the real world, there is a push to use insects as a low-impact alternative to traditional meat sources. For example, Exo Protein declares the cricket to be "The closest thing to a perfect protein source this planet has ever seen." A 2016 book goes further:
[I]nsects offer a very promising and likely much more environmentally sustainable yet vastly unexplored alternative to other animal-based livestock and indeed are attractive alternatives to many plant and other food and agricultural commodities as well. It is becoming clear that insects can be mass produced with much lower utilization levels of land, water, fertilizer, pesticide, feed, energy, and other resources. These efficiencies are also likely have benefits toward reducing climate change and the destruction of biodiversity and the natural environment.
In a medieval setting, people could either domesticate these insects on farms or hunt them in the wild.
One giant insect could destroy a tree every day. I imagine local flora would have evolved defenses against insect predation in order to survive. Still, these critters could do real damage to the environment and to local crops. Just think about the damage that locust can do to crops and scale it up.
Finally, gigantic insects could be used as weapons. Certain dangerous species could be bred under controlled conditions and released in enemy areas. This would be incredibly dangerous for both sides of a conflict. Perhaps you could release gigantic worker bees in hopes that they attack enemies. They wouldn't be able to reproduce and would therefore pose a minimal threat to your forces when you invaded months later (especially if you wait for winter before attacking). You could put a queen excluder on the hive to contain the queen.
first thing to noted when you upscale small creatures such as bugs -they will need more energy source to supply it metabolism which increased along with the mass/size, the bigger creature is the more energy they needed to survive/function
so if you having honeybee at size of house mouse, they will also need a lot of huge flower which could produce enough nectar to supply them and so on
that mean your world environment flora gonna be upscale as well
now to the question
which insects would become worse or more deadly.
Answer : Any, for example an ant can carry up to 10 and 50 times their own body weight (will mitigate down by up-scaling but still relatively strong: check comment below) now increased it size their jaw grip could come close or surpass certain predator's bite .also bug is also known for it sheer numbers even the upscale will reduce counts per nest from 300-2000 ants to 30-100 big ants is still a scary numbers if you think about it. and not to mentioned any with poison, up-scaling them = more poison produced and can be injected/used on victim = could reach/surpass lethal dose for human and so on
would aphids be used in the same way as ants like cattle.
Answer : yes,we are resourceful creatures, those aphid could even replace sheep depend on your setting b/c bug have faster life cycle than a sheep so they may reproduce or enter harvest-able phase more often than waiting sheep wool to grow back
would any insects be used for transportation or cargo shipment. what about for military purposes.
Answer : extended from the first question, absolutely yes, imagine a special trained cavalry riding beetle armed with spiked armor flew across battlefield or even dragonfly express travel across continent ladybug taxi
it all depend on your imagination