Butterflies are souls of the dead waiting to pass through Purgatory.
There is a very good reason that Sailor Heavy Metal Papillon, the Animamate with a massive set of butterfly wings, was found in a graveyard. Because butterflies are the result of a very distinct transformation, they are often associated with souls of the dead - what is believed to be the final stage of humanity. This is also a rather wide-spread belief, as examples can be found all over the world.
* In ancient Greek the word for butterfly is psyche which means "soul."
* A form of the Russian language refers to butterflies as dushichka which is derived from dusha ("soul").
* A small town in Mexico to which monarch butterflies migrate every year sees them as the returned souls of the dead. In fact, it is on the Day of the Dead (November 2nd) that they arrive in the town.
So Naoko clearly knew what she was doing when she designed the butterfly-based character of Sailor Heavy Metal Papillon. In the world of Sailormoon butterflies are said to be the remains of the stars, but it can still be assumed that those "stars" are living beings, for Papillon wonders what colour Sailormoon, Kakyuu, and Chibichibi's butterflies will be. When Sailor Heavy Metal Papillon talks of the uninterrupted funeral procession of butterflies, she is alluding to the countless planets and their inhabitants that have been wiped out by Galaxia.
But what about Japan, where Sailormoon comes from? In Japanese culture, butterflies are the representatives of something quite different: young maidens and marital bliss. So clearly the inspiration for the sexy final Animamate only came from overseas.Some information was found at Wikipedia: Butterfly, The Butterfly, and The Cultural Entomology Digest. Additional information was submitted by Tel'aikai, Psyche, and Fuu217.