Just a note: "cuprum" is Latin for copper, not bronze. The Latin for bronze is either "pyropus", "aes" or "aeneus". Not that it makes too much difference with regards to ranking (since both are typically just below silver), but if you're going to use Latin roots, I'd prefer if you got it right.
Anyway, here are my ideas:
Copper 1st stage: Cupwurm - from "cuprum" and "worm", with "worm" being deliberately misspelled because this thing is not a cup worm.
Bronze 1st stage: Pywormpus - from "pyropus" and "worm", with "worm" being integrated into that part of the name because a "pyro" root sounds too much like the name for a Fire-type.
2nd stage: Argefense - from "argentum" and "defense", because of the cocoon protecting the mon from the elements and other outside influences.
Personally, I'm not a fan of those names that use only "ag" for the meaning of silver. So many words begin with "ag" that claiming any connection to "argentum" is like saying "le" comes from the word "lemon". Similar thing with using only the "cu" or "cup" from "cuprum" - there are other words that begin with "cu", including the word "cup".
That said, I do like Pupargent, Argentomb and Cuprapilar.
<dogfish44> I got a suprise KFC ^.^
<Glacier> kinky fat chick?