。
The caterpillar turned into a butterfly.
Literal
Caterpillar [subject-が] butterfly [to-に] changed.
変わる ('to change, to turn into') is intransitive; its transitive partner is 変える ('to change [something]'). に marks the resulting state or form. 蝶々 is the everyday reduplicated form of 蝶 ('butterfly') — Japanese children's vocabulary.