When most creatures die, what they leave behind is their skeleton and the rest of the soft tissue is broken down by microorganisms and things. That's the reason why if you were to exhume a grave for someone and opened it, you would see that skeleton. That's what's left behind in an insect but because their skeleton is on the outside of their body, it preserves their entire form perfectly. So, there's very little that needs to be done to preserve them.