the very hungry caterpillar hits puberty
The cells in the imaginal discs are like stem cells without commitment issues. From early on in the caterpillar's life, each one of them is locked into becoming a particular bit of butterfly anatomy. And they're all just sitting there, waiting to get the go ahead to start making butterfly parts.
But to build a butterfly you have to break down a caterpillar.
During the week or two spent in its chrysalis (pupation) the caterpillar gradually digests all of its own tissue, releasing the nutrients that all those imaginal discs then use to grow into butterfly wings, legs, feelers and the rest. It's the ultimate in recycling makeovers, and it's due to some interdependent hormonal changes that make puberty look like a doddle.
The hormonal tango
From little grubs butterflies grow
The thing that drives caterpillars (and other flying insect larvae) to stop feeding their faces, settle down somewhere safe, and pupate, is the hormone ecdysone. It's the same hormone that causes the larvae to moult each time they outgrow their current skin. The reason this final moult into a butterfly is so different from the earlier ones is because the level of another hormone — juvenile hormone — is suddenly lower.
Juvenile hormone is the great controller of metamorphosis, by delaying it until the caterpillar has moulted and grown enough to produce a decent-sized butterfly. It works by blocking the genes in the imaginal discs, keeping those wannabe butterfly cells in a holding pattern. So while juvenile hormone is being pumped out of tiny glands behind the brain, all the caterpillar can do is feed, grow and — when instructed by ecdysone — moult. (It's so good at preventing larvae from maturing that a lot of insecticides have been based on artificial juvenile hormone).
But juvenile hormone isn't just a suppressor of butterfly development, it's essential for the caterpillar's own cells to stay alive.
The cells that make up the caterpillar's muscles, gut and salivary glands are destined to end up as spare parts for the greater-butterfly-good. Each cell is poised to self-destruct during metamorphosis by activating some of its own enzymes, called caspases.
Like digestive enzymes, caspases tear through the cell's proteins, releasing prime butterfly-making material. (This process is called apoptosis, or programmed cell death, and it's the same thing that happens to about 50 million cells in your body every day to make sure you don't double in size every time your cells multiply). At any given time juvenile hormone is the only thing stopping all those caterpillar cells from ending it all.
Like other flying insects that undergo complete metamorphosis, caterpillars go through five regular moults during their mindless hungry lives, upsizing their outer skin each time.
When the caterpillar is big enough, usually after the fifth moult, the level of juvenile hormone drops off. With less juvenile hormone around, instead of inducing a regular moult the ecdysone now drives the caterpillar to pupate. Once the caterpillar is safely ensconced in some kind of silky hideaway, juvenile hormone stops being made altogether.
Without juvenile hormone to suppress the imaginal discs, or to keep the regular cells from topping themselves, the two trademarks of metamorphosis kick into full swing. And in the kind of beautifully orchestrated way that only nature or really top-shelf creators can manage, the demise of caterpillar and creation of butterfly happen side-by-side.
http://www.abc.net.au/science/articles/2011/12/07/3384014.htm?site=science/basics