What really happens inside a chrysalis?

Inside a chrysalis the caterpillar's body is taken apart and rebuilt into an adult. It is not a simple growing-up. In insects with complete metamorphosis there are four stages: egg, larva, pupa and adult. The pupa is a resting stage that does not feed, and during it the larval structures are broken down while the adult's structures grow from imaginal discs.

A popular story says the caterpillar dissolves into soup. That is too strong: larvae do not completely liquefy inside the cocoon. What really happens is a rebuild in place, directed by hormones, above all juvenile hormone, prothoracicotropic hormone and ecdysone.

Why do it at all? Because a caterpillar and a butterfly have different jobs. Larval traits maximise feeding, growth and development, while adult traits serve dispersal, mating and egg laying. A caterpillar has no wings and has simple rather than compound eyes. A butterfly has to get around and find a mate.

The pupa pays for this with vulnerability. Pupae are usually immobile and largely defenceless, so species wrap them in a cocoon, camouflage them or bury them. The pupal stage can last weeks, months or even years depending on the species and temperature: in monarch butterflies it lasts eight to fifteen days.