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Aug 24

Big Question: How Does Metamorphosis Work?

As children, we all learn stories about certain very hungry caterpillars that eat and eat and eat until, one day, they spin a cocoon and eventually emerge as a beautiful butterfly. Indeed, the transformation of a small, wriggling caterpillar into a graceful butterfly is a perfect metaphor for change. But the process has become such a well-known fact of life that we often don’t pause to think about how they actually accomplish this remarkable change.

A caterpillar, of course, is markedly different from a moth or butterfly. It is an eating machine, and nothing else. It has a mouth, some basic antennae and several pairs of legs, but that’s about it. Its simple design is to keep body expenses to an absolute minimum. It has no sexual equipment because it will not breed in this form; it has no mechanisms to send out signals, nor any complicated organs to receive them, because it has no need to find a mate; and it has no wings for dispersal since its mother already went to the trouble of laying her egg on a suitable food plant. Its sac-like body is designed only to deal with the almost continuous supply of food brought in from the jaws, whilst small, stumpy legs move the caterpillar from one mouthful to another.

800px-Nymphalidae_-_Danaus_plexippus_Caterpillar

Photo: Hectonichus

But one day, after eating an awful lot and growing considerably in size, the caterpillar will either spin itself a silken cocoon (if it will become a moth), or grow a specially hardened and protective skin called a chrysalis (if it’s destined to become a butterfly), turning into what is known as a pupa. From the outside, it may appear as though life has been completely suspended, but inside a remarkable natural transformation – metamorphosis – is taking place within.

The Ultimate Makeover

To understand how metamorphosis works, we must return briefly to the start of a caterpillar’s existence. When the caterpillar first started to develop inside its egg, its cells were split into two different groups. Some built the body of what would eventually hatch out as a caterpillar. Those cells that were primed to become genitals, wings and sophisticated sense organs, meanwhile, stopped dividing after just a few hours, thanks mainly to a constant wash of juvenile hormones. They remained dormant and inactive throughout the caterpillar stage.

But now, weeks later, within the pupa, their time has come. As the cells of the caterpillar die and disintegrate into a kind of soup (if you cut into the pupa at this point, ooze would just leak out), the dormant cells suddenly start to divide again to build a new body that is completely different to the old. As it does so, these cells nourish themselves on the soup of dissolved caterpillar – the animal is effectively recycling itself. The process of dismembering one’s body and then reassembling it from scratch can take as long as a few months or just under a week depending on the species.

When the butterfly or moth emerges, it probably has little more than a month to live. It no longer grows or develops. The fuel that it now gathers – nectar rather than leaves – is used only to sustain its flight as it searches for mates.

To Change or Not to Change

Metamorphosis seems strange to us because we are used to slow, sustained growth, from baby to adult, with no major or dramatic changes along the way. To our human-centric minds, the complete and sudden transformation from a worm-like creature to fluttering airborne butterfly is alien to us. But caterpillars are hardly the only insects – or indeed animals – to metamorphose.

In fact, around 65% of all animals on the planet undergo metamorphosis, including amphibians, crustaceans and jellyfish. In the insect world alone, beetles, flies, ants, bees, wasps, fleas and many others exhibit complete metamorphosis, from grub to adult, whereas many others – cockroaches, grasshoppers, aphids, termites, earwigs, dragonflies and so on – show incomplete metamorphosis, whereby the changes are less obvious and usually entail growing wings and functional reproductive organs as they mature. The image below shows the life cycle of a cockroach and its incomplete metamorphosis.

Metamorphosis, as you can see, is far from uncommon, but the caterpillar to butterfly change remains undoubtedly the most well-known. It is strange that we regard the butterfly as a beautiful and graceful creature that conveniently helps pollinate our plants, whilst we often see caterpillars as annoying pests that ruin our gardens – even though they are the same animal. But caterpillars are unfairly judged. As stand-up comedian, author and philosopher George Carlin once said, ‘The caterpillar does all the work, but the butterfly gets all the publicity’.

3 comments

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  1. Luke

    Interesting! When you say that “65% of all animals on the planet undergo metamorphosis”, though, do you mean species or individuals?

  2. unclebulgaria1

    Sorry for being vague. By 65%, I mean species. I don’t know the percentage of individuals, but since nematode worms (which generally don’t exhibit metamorphosis) account for about 80% of all individual animals on the planet, I expect that number to be quite a lot lower. Hope that clears it up!

    1. Luke

      Yes indeed, thanks. I didn’t know that about nematode worms, either.

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