Gloria Hollier, of Tawa, asks :-

We have been enjoying watching the Monarch caterpillars and subsequent butterflies this season on our large swan plant. What actually happens inside a chrysalis?

Phil Lester, a biologist at Victoria University of Wellington, responded.

It is a wonderful process of insect metamorphosis. Insects, like monarch butterfly caterpillars, will completely transform into something that looks like an entirely different organism.

It is a process known as pupation. A caterpillar will stop eating, spin a cocoon in a safe place, and dissolve or disintegrate almost all its body effectively into soup.

But not all its body is turned into soup. There are specific, small groups of cells within this soup that are called “imaginal disks” that are not dissolved.

These imaginal disks or groups of cells are formed when the monarch butterfly is just an egg. There are groups of these cells that will form all parts of the adult body, including the wings, legs, eyes and mouthparts. They are present in the caterpillar but aren’t really active and often are not expressed in any way. These disks survive the disintegration process that turns the caterpillar into soup during pupation.

Within the developing pupae the imaginal disks start to grow and organise the cells around them. A wing in a fly might start with just 50 cells in an imaginal disk, but then grow to 50,000 cells. The soup is used as both a source of raw material and energy to fuel cell division.

It’s a fantastic and really amazing process. It would be like you or I going to sleep one night and waking up as a fish: effectively the adult is able to move through a completely different environment and eat different food from the juvenile.

Not all insects pupate and turn their body into soup like this. Baby aphids, for example, look like small adult aphids. There are a range of reasons that insects might pupate. For example, some scientists think that this process reduces competition between adults and juveniles for food. It also clearly helps with dispersion and movement of the adult to new food sources.