What happens when a caterpillar pupates?

We put this question to Chris Jiggins, from the University of Cambridge... Chris J - Okay, so I'm Chris Jiggins from the Department of Zoology in Cambridge. Well obviously, an insect like a butterfly has two different life stages which have very different ecologies and very different requirements. So when you go from a caterpillar to a butterfly, you require a lot of different structures. So, in the pupa, some of the caterpillar structures are carried through into the adult and some of the adult structures are completely developed, from scratch almost, in the pupa. So, the wings for example are unique to the adult and actually, they are present in the caterpillar as little blobs of tissue called imaginal discs. If you know where to look, you can squeeze out a little blob of tissue from the caterpillar which is going to turn into the adult wing.

So in the pupa, some of those structures which are just little blobs of tissue would grow into the wings, and others are developed from the structures already in the caterpillar. So for example, the legs and the antennae; the adult legs grow from a little piece of tissue at the base of the caterpillar legs and the adult antennae in a similar way. Other structures, like the central nervous system, are largely maintained from the larva through to the adult, but all the connections are reconnected. So obviously, there are things like wing muscles which aren't present in the caterpillar which need to be connected up in the adult. There are structures like the malphigian tubules which are to do with excretion which are carried through from the larva into the adult.

So, like most biology, I suppose it's somewhere in between being a complete sludge in the pupa and just producing wings. It's actually a mix of some structures being retained and some being developed.

Diana - But are any bits of the caterpillar discarded?

Chris J - So, the structures that aren't required are basically broken down and the proteins re-used in producing the adult structure. So there are obviously muscles associated with the legs which are completely reconstituted in the adult. So yeah, the protein sludge is not a bad analogy for some parts of what goes on actually.

Diana - Virtually, nothing is wasted by the caterpillar, as some parts of it are retained, and others do indeed become a protein in mush. And did you know that some chrysalides are able to make noises from inside their hard shell in order to scare off predators!