New NAUSICAA DVD has Mark Hamill as Voice

Giant insect, actually, not a slug. iirc.

I heard someone one time talk about juding an anime costume contest, and someone dressed up as one of those bugs. He apparently won the contest hands down.

Nausicaa is fairly old, as mentioned. Its loosely based on the manga of the same name (5 volumes, which I own). I haven't seen the movie myself, but my understanding is that there's a lot of similarity between it and Mononoke.
You forget two things..

*Did you know that millipedes were eventually just strings of dozens of spiders? Evolution changed them into the single animals we know today.

*Did you know that those spiders with hair on them are mammals, and thus produce delicious (and unusually cold) milk. Spiders produce milk using the same glands that they produce silk threads with.