Many insects like butterflies and moths, have four separate stages of life cycle, each of which looks completely different and has a different purpose, in the life of the insect. A mother butterfly lays eggs on fresh green leaves or stems. After a few days, the shell of the egg breaks and a long worm like caterpillar comes out. It's also called larva. A larva feeds voraciously, and grows quickly in size. It may shed its skin a few times to enclose its rapidly growing body.
Once the larva is big enough, it stops eating and enters the transformation stage. This is called the chrysalis and sometimes also, pupa. During this stage, the caterpillar's tissues are broken down, and the adult insect's body is formed.
After sometime, the shell of the pupa breaks and an adult butterfly comes out. Adult butterflies become mature gradually. Female butterflies lay eggs after mating and the next generation is born. The life cycle starts again.