@timhutton I think I'm getting confused because mandibles are a sort of mouthpart. But looking a bit more into this, I think that doesn't mean they're part of the mouth, but more like something that assists in the manipulation of food for the mouth.
Have you heard of the 'endless dispute', the popular name for the anthropod head problem? Zoologists have been debating this stuff, including the homology of anthropod appendages, forever.