When Kay is eaten by the enormous bug and blasts his way out, the bug blows in half. Immediately after, it comes back whole! I don't think it could be put back together.
It doesn't come back whole, if you watch it crawling up from behind (after it's been blown in half), you can see it is dragging itself with it's hands and is missing the lower half.
It would seem that isaac616 apparently never watched this part of the movie more than once. Otherwise, he/she would have easily seen that when the bug/alien/whatever comes back to life, it's crawling using what is left the upper half of it's body ONLY.
Suggestion to isaac616...You might want to double check your findings next time before posting any more (alleged) slip-ups.
This is no "semi slip-up". Earth bugs have the ability to have some limbs and sometimes whole body segments severed and still function (however improper it might seem).