Taking inspiration from this clip, I'd say the scariest thing about these giant bugs is being stuck helpless to resist them, like when Andy Serkis is being devoured by those worms with everted esophagi. When you think about it, insects are basically like little machines of nature, blindly executing their genetic programming until they die. This would normally make them easy to deal with, because they aren't very bright, but the real terror comes when you find yourself in a situation where your options are so limited that you can't effectively outwit them. It's the same with zombies. How do films make zombies scary? By having them surround you, surprise you, or grab you and pin you down while they chew on your guts.
Think of each species of giant insect as occupying a particular terrain niche for hunting, such as dark caves, or shallow water, or treetops, or forest underbrush, or loose sand. Adventurers who suddenly find themselves in exactly that niche terrain will have an oh-shit moment as they realize they're pitching right to the bug's wheelhouse. They're standing in the kill zone. This is where you go to die. Just like in the King Kong scene when the flare dies out and the trench goes dark, allowing all the hundreds of bugs to come out of their holes to see what new meat just fell down from above... just like they've been doing for millions of years every time some animal mistimed its jump or some bird or bat fell out of the sky; and the bones that litter the trench floor are proof that this is a one-way trip.
So that example with the mosquitos herding people into bushes and then all of a sudden the last thing you see is this giant proboscis coming through the shrub, seeking the nearest bipedal heat source to rip into it and suck it dry... that can be really scary if it's described in the right way, like you've got nowhere to run, and your only option is to try to keep dodging this giant organic syringe as it repeatedly jabs at you (unless you want to flee the bush and get surrounded and ripped apart by the males).
An example of how I used giant spiders in the past: the party of adventurers were picking their way through a misty swamp, trying to stick to the dryest route, when all of a sudden someone succeeds on a spot check and notices that one of the big dead-looking trees next to them has a giant fucking spider attached to it, the same width as the trunk, and completely camouflaged with the bark. Then as you look around, you see there are like five or six or ten other trees that also have spiders clinging to the side of them, surrounding the party from all directions. And then they all start moving. Here the heroes are in a tough spot because they have to really watch their step to not get their boots stuck in the mud, while the spiders can hop around as they please, encircling the heroes and using the trees and the water to their advantage to frustrate range attackers, and then moving in for the kill when it looks like someone is exposed.
Another small idea for reimagining giant wasps... you'll notice that when Japanese wasps/hornets attack a hive of another species, they use their mandibles to behead the other insects. If wasps were scaled to the size of humans, they would certainly attempt the same tactics, first coming at you from above and then trying to latch onto your back and hold you in a grapple with all six legs around your torso (and the stinger between your legs). Once they've got you in a Pin (if we're talking D&D 3.5 mechanics), they can just snip your head from your shoulders (would count as either an automatic critical hit, or a coup-de-grace). But if you break out of the grapple, their instinctive reaction is to sting.
And then of course there's that famous species of wasp that uses its stinger to override a cockroach's brain function and cause it to crawl like a zombie back to the wasp's nest to be devoured by its hatchlings... (wis-based poison that provokes a Will save in addition to the poison's Fort save? Failing the will save leaves you Fascinated, and you blindly follow the creature that stung you, taking a single move action per turn). The stinger comes up under the base of the skull from behind, through the back of the neck... it would be long and thin, like a stiletto, so as to pierce the brain without tearing too big a hole and killing you outright. Since it's usually coming from the rear, the occipital lobe takes the initial injury, which would result in temporary blindness, or being dazzled. So your conscious mind doesn't even see where your feet are taking you.
Another thing you can take from nature is fungal ants; ants infected by a fungus that makes them blindly seek out other creatures to spread the spores to. So you'd want to keep these things out of melee range if you don't want to get afflicted with the fungal disease. But that's easier said than done, since their chitin is so thick they're like six-legged tanks, and even as you start hacking them apart they're still soldiering on like machines on auto-pilot, since the fungus has made them effectively undead anyway (they'll die on their own after 24-48 hours, but who can afford to wait that long?)
