What actually happens when a caterpillar becomes a butterfly? Is it a Pokémon situation or more of a Cronenberg thing?

Asked by: James Colley, via Twitter

It’s more of a messy, Cronenberg-esque transformation than a tidy Pokémon one. When a caterpillar starts to change into a pupa (or ‘chrysalis’), its own digestive enzymes begin to break down most of its cells into a chemical soup. Some of the caterpillar’s structures do remain intact, however, such as the gut, the tracheal tubes (for breathing), and some of the central nervous system.

Also resistant to the transformation process are clumps of cells called ‘imaginal discs’, which are contained within the caterpillar’s body for its entire life. During metamorphosis, the imaginal cells divide rapidly.

At first, these are treated as invaders and attacked by the caterpillar’s immune system. But eventually the imaginal cells overwhelm the dwindling caterpillar cells and begin to use the raw materials around them to assemble new butterfly structures, such as wings, legs, eyes and antennae. These structures eventually connect up with the remaining caterpillar parts, and the newly assembled butterfly emerges.

The whole process can take just two weeks in some species.

Read more:

• Do caterpillars have a sex before they become butterflies?
• Why don’t butterflies fly in straight lines?

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