As Aarthew III said at this point it would probably be more efficient to take a human/whatever else you want it to look like and "upgrade" it with an exoskeleton.
Given the size of the organism I doubt the exoskeleton will be sufficient, it would probably need an internal skeleton as well.
To avoid problems with size-changing organs it could have an internal buffer, the lungs expand into pockets of gas inside the body so that the exoskeleton need not move/deform. The displaced gas could compress or move to another part of the body that acts like a balloon (this would be the only part that would have to be exposed) or, if it is air, could simply be connected with the outside. In this latter case the exoskeleton would simply have the same size of the lungs (or other organ) at their biggest and an opening for the air to stream in and out.
edit: Another solution to the lung problem is having 2 lungs, one expands when the other retracts, the total volume of both lungs never changes. This would mean that the organism would need 2 mouths/noses though, otherwise it would breathe in the same air that it breathes out, which is already partly deprived of oxygen.
Keep in mind that such an exoskeleton (especially if designed to protect) would be very heavy, the resulting organism would need to be very massive compared to their prey/predators to be able to support it or have some other means to compensate (like some form of antigravity device incorporated in their skeleton. I once read a book in which there were stones which would grow like plants that produce a force opposing gravity, if the insectoid absorbs and incorporates tiny pebbles of a material like this into its shell it could support it with much less required strength).
Given the weigth of the exoskeleton such an organism wouldn't move very quickly, instead depending on size and brute force to take out their prey.
What their prey is depends on their size and other peculiar organisms that might live there.
A shell like that would be effective as protection against projectile weapons, if a creature that shoots projectiles exists that could be a potential prey. Optimally that would be a stationary plant that defends itself by shooting thorns (venomous ones, perhaps), allowing the insectoid to approach it under its protective shell.
The shell is also a good defense against insects that sting, if there is an area with an abundance of dangerous wasp-like insects (or anything else that sucks blood, deposits larvae in living tissue or attacks other animals for any reason, not necessarily one that exists in our universe) this could be one of the few animals that thrives there, but only if the holes in the shell (for breathing, eating and exchange of gas as described above) are smaller than the insects and/or can be closed.
edit: If no system to alleviate the shell can be introduced it will have to remain comparatively thin, which is fine if it exists for purely aesthetic purposes, but may not if it has a defensive purpose. If the shell is supposed to protect against predators as large as or larger than the insectoid it will be too heavy. Any predators deterred by the shell would have to be smaller than the insectoid (Half? Two thirds? Something like that), or perhaps weakened for other reasons (maybe the insectoid lives in an environment that's poisonous to the predators so they arrive already weakened). This is not an issue if the shell only needs to protect against insects as detailed in the later parts of my answer, as these can be defended against with a small shell, perhaps the internal skeleton could be hollowed out (like birds') since the exoskeleton already absorbs parts of the hits, to compensate for the weight of the shell.
