First, I really like these. Second, I think you should lose the spoilers. Monster stats don't need to be put in spoilers.
Making the butterflies immune to the effects of high winds is an interesting touch but you don't really explain how this works. Is it something special about their physiology or something else? I think you need a little fluff -- just a sentence or two -- that fleshes this out. Also, do individual butterflies have this ability or just when they are a swarm? It could be neat if only the swarm had this ability.