Why Is The Very Hungry Caterpillar So Dang Hungry?

Because it's hoarding protein. Not just for itself, but for the butterfly it will become and every egg that butterfly will lay. And it's about to lose its mouth... as it wriggles out of its skin during metamorphosis.

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That caterpillar in your backyard is chewing through your best leaves for a good reason.

“Caterpillars have to store up incredible reserves of proteins,” said Carol Boggs, an ecologist at the University of South Carolina. “Nectar doesn’t have much protein. Most of the protein that goes to making eggs has to come from larval feeding.”

Caterpillars are the larval stage of a butterfly. Their complete transformation to pupa and then to butterfly is a strategy called holometaboly. Humans are in the minority among animals in that we don’t go through these very distinct, almost separate, lives. We start out as a smaller version of ourselves and grow bigger.

But from an evolutionary point of view, the way butterflies transform make sense.

“You have a larva that is an eating machine,” said Boggs. “It’s very well-suited to that. Then you’re turning it into a reproduction machine, the butterfly.”

Once it becomes a butterfly it will lose its mouth, grow a straw in its place and go on a liquid diet of sugary nectar and rotten fruit juices. Its main job will be to mate and lay eggs. Those eggs started to develop while it was a pupa, using protein that the caterpillar stored by gorging on leaves. We think of leaves as carbohydrates, but the nitrogen they contain makes them more than one quarter protein, said Boggs.

-- What are the stages of a butterfly?
Insects such as butterflies undergo a complete transformation, referred to by scientists as holometaboly. A holometabolous insect has a morphology in the juvenile state which is different from that in the adult and which undergoes a period of reorganization between the two, said Boggs. The four life stages are egg, larva (caterpillar), pupa (also known as chrysalis) and butterfly.

-- What if humans developed like butterflies?
“We’d go into a quiescent period when we developed different kind of eating organs and sensory organs,” said Boggs. “It would be as if we went into a pupa and developed straws as mouths and developed more elaborate morphology for smelling and developed wings. It brings up science fiction images.”