Because all bugs are insects, but not all insects are bugs. http://dictionary.reference.com/help/faq/language/d27.html
Actually, this is a very confusing case (see links) in regards to the additional "c" where EP PT (which also covers Africa and Asia) used to have it, but since the new orthographic agreement that took effect in 2016 no longer does.
Confusing indeed! =[
So glad I am not the only buggy nerd on this site. :)
That said, bug in general often refers to arthropods which is artrópodes in PT or invertebrates which is, invertebrados
While bug, as in computer bug or, travel bug translates to, "bug" in PT.