How does a caterpillar become a butterfly?

1. A female monarch lays a single tiny egg on a milkweed leaf. Milkweed is the only plant monarch caterpillars can eat, so she picks it carefully.
2. After a few days a tiny caterpillar hatches and eats its eggshell. Then it munches milkweed leaves all day. Its skin can't stretch forever, so it sheds it to grow bigger.
3. When it's fully grown, the caterpillar spins a silk pad, hangs upside down in a J shape and sheds its skin one last time. Underneath is a green chrysalis.
4. Inside the chrysalis, much of the caterpillar's body breaks down into a kind of soup. Tiny groups of cells that were waiting inside grow into wings, legs and antennae.
5. After about ten days the chrysalis turns clear, and you can see orange wings inside. The butterfly pushes out, pumps fluid into its crumpled wings and waits for them to harden.
6. The butterfly flies off to sip nectar from flowers. When it's grown up, it will lay eggs of its own, and the life cycle starts again.

Step through the close-ups to see how a caterpillar changes.

• The egg. The egg is about the size of a pinhead. Just before it hatches, you can see the caterpillar's black head through the shell.
• Hatching. The caterpillar chews a hole in the shell and crawls out. Its first meal is usually its own eggshell.
• Molting. A caterpillar's skin can't stretch very much. So it grows a new, looser skin underneath and wriggles out of the old one. This is called molting.
• Hanging in a J. When it's fully grown, the caterpillar spins a little pad of silk, grips it with its back legs and hangs down in a J shape for about a day.
• The chrysalis forms. The caterpillar's skin splits behind its head and slides up. Underneath is a soft green chrysalis that soon hardens, decorated with gold dots.
• Rebuilding inside. Inside the chrysalis, much of the caterpillar's body breaks down. Tiny groups of cells that were waiting all along grow into wings, legs, eyes and antennae.
• Coming out. When the chrysalis turns clear you can see the orange wings inside. The shell cracks open and the butterfly climbs out, hanging on with its legs.
• Wings pump up. The new butterfly pumps fluid from its fat body into its crumpled wings so they spread out. It hangs still for a few hours while they dry and harden.

Slide through the days. Watch the caterpillar grow and molt, and peek inside the chrysalis to see the wings forming.

Experiment: How does a caterpillar get so much bigger?

1. Start at day 0 and slide forward slowly.
2. Watch for the old skins left behind.
3. Keep going until the butterfly comes out.

What we found: A caterpillar grows inside a skin that can only stretch a little, so it sheds it four times. Each time it comes out bigger, until it's about as long as your finger.

Turn on JavaScript to try it yourself.

Drag the six cards into the circle in the right order, starting from the egg. Then press Check.

A butterfly's life has four very different stages. It starts as a tiny egg laid on a plant. Monarch butterflies always lay their eggs on milkweed, because that is the only food their caterpillars can eat.

The caterpillar that hatches is an eating machine. It munches leaves all day and grows fast. Its skin can only stretch a little, so it sheds its skin four times, coming out bigger each time. This is called molting.

When the caterpillar is fully grown, it hangs upside down in a J shape and sheds its skin one last time. Underneath is a chrysalis. The chrysalis looks still, but inside, the caterpillar's body is being taken apart and rebuilt. Groups of cells that were tucked away in the caterpillar grow into wings, legs, eyes and antennae.

After about ten days a butterfly climbs out. It pumps up its crumpled wings, lets them dry, and flies off to drink nectar. Later it lays eggs, and the cycle starts again. This whole change is called metamorphosis.

Key words

Metamorphosis

A big change in body shape as an animal grows up.

Larva

The young stage. For butterflies, it's the caterpillar.

Chrysalis (pupa)

The case where a caterpillar changes into a butterfly.

Molting

Shedding an old skin so the animal can grow bigger.

Imaginal discs

Groups of cells in a larva that grow into adult parts such as wings and legs.

Check your thinking

1. What hatches out of a butterfly egg?

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   A tiny caterpillar. The egg hatches into a tiny caterpillar, which is a butterfly larva.

2. Why does a caterpillar shed its skin?

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   Its skin can't stretch enough for it to grow bigger. A caterpillar sheds its tight skin and has a bigger, looser one underneath.

3. What happens inside a chrysalis?

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   The caterpillar's body is rebuilt into a butterfly. Much of the caterpillar's body breaks down, and groups of cells grow into wings, legs and antennae.

4. Which order is right?

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   Egg, caterpillar, chrysalis, butterfly. Egg, caterpillar, chrysalis, butterfly, and then the butterfly lays eggs again.

5. Why do monarchs lay their eggs on milkweed?

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   It's the only food monarch caterpillars can eat. Monarch caterpillars only eat milkweed, so their mothers lay eggs right on it.