Why are butterflies not affected by the Black Iron Sickness?


In Zetsuen No Tempest, there was this sickness that turned everything into
black iron, which is where its name came from. However, from what I can
remember, I am sure there was no explanation in the anime to this question.

**Why are butterflies not affected by the Black Iron Sickness?**

Was this ever revealed in the manga? I am positive that it is not mentioned in
the anime series, or on [Zetsuen No Tempest
Wiki](http://zetsuennotempest.wikia.com), but was there ever an explanation
for this? And if this was in the anime, would you please include a reference
to the episode?

Answers are appreciated. :)

