Some fascinating insights into this evolutionary magic trick.
By Elizabeth Jane
When you look at an elegant butterfly flitting across the landscape, flapping its delicate wings, resting briefly to feed on nectar from flowers, it’s almost impossible to imagine the real story of its evolution- that it has only recently wriggled out of a pupa- a mummy-like sarcophagus where it underwent an unbelievable transformation from being a caterpillar. And that as a caterpillar, it may have moulted upto five times, shedding its continuously expanding outer body, as a gluttonous crawling insect, decimating host plants, to fatten up, and reach its optimum size. And before taking the shape and form of a caterpillar, it was a tiny maggot-like grub that emerged from a miniscule egg, laid by a butterfly just like the one you’re looking at.
As an amateur entomologist and nature photographer, I have over the years observed various creatures around me and marvelled at them, without really being able to either fathom the evolutionary link or understand the reasons and purposes of their development stages.
I found answers to many of these mysteries in a Science Weekly podcast of The Guardian, curated by Natalie Grover. She explores a range of aspects about that fascinating thing called ‘Metamorphosis’- where a creature remodels itself between life stages; one of the most astounding and bizarre feats of biology. While we might presume this is a relatively rare phenomenon, she says, it’s surprisingly common, as explained by experts- Stuart Reynolds, a professor at the University of Bath and Anjali Goswami, a research leader in life sciences at the Natural History Museum, London, and honorary professor in palaeobiology in the Department of Genetics, Evolution and Environment at University College London.
While I share pictures of some wonderful caterpillars from my neighbourhood, I thought, I’d also share some of the insights from the podcast, to put their evolutionary story in context.
So, what are caterpillars? Why do animals bother undertaking this huge transformational change, called metamorphosis? And how do they rebuild their bodies from one form to another?
Prof Stuart Reynolds explains that metamorphosis is something people have been fascinated with ever since Aristotle. The way we usually use the word is to describe “an abrupt and quite quickly accomplished change in the shape of the animal concerned and the fact that the two different forms have very different behaviours and habits. It’s as if they occupied different niches” but he clarifies that “actually there are two different kinds of metamorphoses that insects undertake. There are some insects like grasshoppers that hatch out looking like little adults except that they don’t have wings, but there’s another kind of metamorphosis, which a majority of insects undertake. The difference between the immature form and the adult is very great. The immature insects look like little grubs or maggots or caterpillars and they’re totally different from the adult. The difference in shape is so great that you have to have a stage interposed between them and we usually call this a pupa or a chrysalis. During the time that an insect is a pupa, it’s going to have to undergo very great changes in its body form.”
In a nutshell, what is the typical journey from egg to adult in one of these extreme cases?
Prof Reynolds explains, “The insect lays an egg, the creature that develops inside that egg turns into a larva, like a caterpillar. The caterpillar is highly adapted to eating and growing. In fact it eats like crazy. It does absolutely nothing else. It periodically has to replace the hard exoskeleton on the outside because its inextensible. It can’t be stretched anymore. So the insect actually grows a new cuticle- a new skin inside the old one. And then the old one splits and the new caterpillar comes out. And we go through a series of moults like this. Maybe five moults- is pretty typical for a butterfly caterpillar and then somehow, the caterpillar knows that it’s big enough. And it stops eating. It stops growing and instead of moulting into another caterpillar, it moults into something that looks a bit like an adult but actually isn’t very mobile. We call this a pupa. And at this time a great deal of restructuring has to take place. The pupa has things that look a bit like wings but they aren’t flappable. They wouldn’t be able to fly with them. It has things that look like adult legs but it couldn’t walk on them. And lots of other structures have to be reorganised. New muscles have to be grown. Bits of nervous system have to be put in place, so the adult insect will be able to know where it is and what it’s doing. All of this time, during the life of the pupa, there’s a great deal of breaking down of the old tissues that has to go on, and it takes almost as long as the caterpillar spent growing in size to turn the fully fed caterpillar into a proper adult and that is the pupal stage. Sometimes the pupa lives inside a silken cocoon, sometimes it lives under the ground, but the changes that go on at that time are pretty similar… and eventually the pupa is now ready to emerge as an adult. So, it moults again and what comes out is an adult that we would easily recognise as being a proper butterfly. And as an adult, it has a completely different lifestyle. Instead of being concentrated on growing, it is interested really only in sex!”
Not a bad life at all, for a creature most of us view dispassionately, as gorgeous, fairy-like winged beings, just passing through…
So, how does the turning point present itself to a caterpillar, to decide that today is the day, that it will change itself completely and irreversibly?
Turns out that the answer is not all that clear. But, Prof. Reynolds says, this could partly owe to the fact that caterpillars have “stretch receptors inside its body that say- oh, you’re getting a bit fat now. And so, they know that they’re big enough. But actually when you think about it, the way that the stretch receptor works, is that it’s stretched between two bits of the outside of the body. And every time the insect moults, they’re going to be reseting their ideas of how big is big. So somehow, the insect has to know how many times it’s already moulted as well, and we really don’t have a very good idea about that. So the basic idea is that the insect knows how big it is and then it decides it’s going to moult. And what it does then, I think is utterly fascinating. It turns out the insect has a hormone, which it stops releasing when it’s big enough. It’s called the juvenile hormone. The role of this hormone is to stop metamorphosis from happening. As you can imagine quite a lot of people since the 1930’s have heard about the juvenile hormone and thought it would be great if you could put it in a bottle and sell it. But of course it’s a hormone that’s really only found in these jointed-limb animals- the arthropods, that is basically crustaceans and insects.”
Natalie Grover says that for insects, once this gatekeeper- the juvenile hormone stops being produced the process of metamorphosis can begin. For the most extreme cases like a caterpillar turning into a butterfly, in the pupal stage the insect needs to disassemble itself and rebuild a totally different form- reallocating materials and turning them into new legs or muscles or wings. She asked Prof. Reynolds about the role of genetics in this biological reprogramming.
According to Prof. Stuart Reynolds, when insect biologists first got interested in this at the start of the 20th century, “it was speculated that an insect having three markedly different body forms- larva, pupa and adult, might actually need three whole separate sets of instructions on how to form those three different kinds of bodies. The idea was you’d have three separate genomes, all combined into one, because that’s what genes do. They tell the body, how it should develop, what activity should be going on. So the idea wasn’t a completely stupid one. But actually it turns out to be wrong. It turns out, that the larva, the pupa and the adult- they’re using the same set of instructions to make their respective bodies. If you think about this, in a simple human analogy, if you’re a building firm, you employ the same set of builders and the same kinds of building tools, to make a house or a factory or a skyscraper. The techniques are all the same. The blueprints are a bit different- they just tell you what to do and when to do it. So, the tools that you actually need to make the larva, the pupa and the adult are all the same. What really is different is that there’s a master gene in each case, that then feeds down onto a very complex interacting system of feedbacks that gives you the use of all of these different tools and instructions.”
Anjali Goswami says, metamorphosis is surprisingly widespread, and not just in insects. It is “much more common than most people realise. Something like half of the animals actually go through metamorphosis. Some of the most famous examples are the amphibians- the frog’s life-cycle is a really classic example of metamorphosis- They’re very much like a fish in their earlier phases and then they transform into an animal that has limbs and in many cases it’s on land and honestly if you found these different forms and didn’t actually witness that they actually transformed into each-other, then I think it would be really hard to match up larval forms to their adult forms. It’s a problem when we think about fossils where we don’t actually capture that transition. So, it’s extremely common in the animal kingdom but it’s also really extreme in terms of the amounts of change that happen between their larval forms and their adult forms’, she says.
This begs the question about why animals undergo this change? Why even bother going through this huge transformation?
Goswami says, “Why metamorphosis is so common is a really interesting question. You’d think that it would be really uncommon because it’s so difficult. The idea for why it is so common is because of competition. The competition specifically between the young of a species and its adult stages. So, you can imagine if the juveniles or the babies are living and eating in exactly the same way as the adult forms, there could be a lot of competition between them for the same resources. Obviously, it really disadvantages the young, but by having a completely different lifestyle it means that they can specialise for a very different life, diet, habitat- it removes the competition between different life stages of the same species.”
Of course, this implies a downside, “to be one organism and then transform into essentially another organism. You have to basically rebuild your body from scratch twice, and that’s really energetically expensive. Our natural inclination is to think, that’s a pretty crazy thing to do”, Anjali Goswami says.
As far as eliminating competition, Anjali and her team investigated if metamorphosis could also play a role in evolution, using salamander skulls from the natural history museum collection, Anjali and her team measured skull shapes of a 148 species to learn about how salamanders evolved, depending on their developmental stages.
Using micro CT scanners to create 3D scans of various salamander species, Anjali Goswami and her team made some amazing discoveries. She says her research was “able to show with our salamander data set that, species that are direct developers- that is the ones that have lost that metamorphic phase- they all cluster together… and the ones that do go through metamorphosis also cluster together and show similar shapes in skulls. So just by looking at skull shapes we get really strong signals of how these different animals develop, which is really interesting… it’s really difficult to tell an aquatic salamander from one that lives in the trees, from one that lives in the ground. We have a much better chance of guessing or figuring out how they developed just from the shape of the skull.”
Turns out metamorphosis offers a clear evolutionary advantage to animals: “In fact, what we found is that the species that are metamorphic and also the species that are pedomorphic, meaning they stay in their larval form- evolve much faster and show much more diversity than the ones that are direct developers- so when they lose that metamorphic phase altogether. So that’s really interesting and surprising how much slower these direct developers are evolving, compared to the metamorphic forms.”
Finding that different parts of the skull don’t always evolve in the same way, and particularly for species that undergo metamorphosis, demonstrated to Anjali and her team that this transformational change was having a significant impact on their evolution and their diversity.
“In terms of how metamorphosis might be shaping the evolution of different species – there’s a lot of potential there in terms of why that could be. Why it could be a promoting factor. One is that by separating life stages into different bins, maybe you can specialise more as an adult into a certain habitat, because you’re not limited by what your young form has to do. You can basically, completely change what you’re doing and that could give you a lot more flexibility. And one thing we were able to show is, when you look at different parts of the skull- you can break it up into different bones that form the skull- and within the metamorphic species, we are able to show that different parts of the skull are able to evolve more independently. So you might have certain things that are influencing the evolution of parts that are related to the mouth, that are evolving really independently of the parts that are involved in say the brain case or the attachment to the neck. The species that are metamorphic seem to show more independence in the evolution of skull parts than the ones that don’t go through metamorphosis. And maybe that means that those metamorphic species are just evolutionarily more flexible, and that’s interesting because you can use that to understand, well, as the environment is changing rather quickly these days, as we go through this biodiversity crisis- are there some species that can do better than others? Basically how quickly they can evolve, how adaptable they may be.
The issue of a biodiversity crisis, set into the context of climate change and destruction of habitats caused by human activity creates a whole lot of questions about the future of creatures that might adapt and those that might perish. Anjali Goswami is not very optimistic about the pace at which species might manage to adapt and survive, given the velocity of habitat destruction unleashed by humans. She says, “metamorphic species have essentially two completely different lifestyles that need to have a habitat preserved, and as we go through this period where due to human activity we are decimating a lot of habitat and really getting rid of areas where animals live, things that really need complex or multiple habitats might have a harder time than things that just have a simple or single habitat. Now, I don’t know if that’s the case but it’s a hypothesis that I can think about and test. Whether or not, even though these metamorphic species seem to evolve more quickly in the past, whether these extreme changes that are going on right now, because of human activity, are quite different from the changes that happened in the past, in terms of the environment, and by of course fragmenting and destroying large areas of natural habitat, it would have quite a severe impact on metamorphic species. Hopefully they’ll be ok, being adaptable, as they are metamorphic. But, I’m not sure that’s going to be the case.”
Given the miracle, that the process of metamorphosis is, and the mysteries of nature that the human mind is always grappling to unravel, it might be worth our while to pause and ponder over these fantastic phenomena, especially at a time that we’ve been brought to our knees by microscopic germs. I hope you enjoy my pictures of a few caterpillars from my neighbourhood for now.
You may listen to the Science Weekly podcast- ‘How do animals undergo metamorphosis and why’, here:
About the author
Elizabeth Jane combines over 14 years of experience in a range of roles across media sectors. She has been a journalist for the bulk of her career, covering news from the field as a reporter, while also offering analytical perspectives on developments from the studio, as a Senior Anchor. She has worked with India’s national broadcaster DD News in Delhi, with the BBC World Service in London, TV 18 Network and the India Today Group.
Before moving to India’s news-capital, New Delhi, Elizabeth spent more than five formative years in the Maximum City, Mumbai, where she assisted Prahlad Kakar on making ad-films and also worked on TV serials and documentaries with directors like Vikas Desai and Ajay Kartik. At Banyan Tree Communications, she worked as a Senior News Editor/ presenter, handling political news in the South-Asian region for an internet radio station and also wrote pilot scripts for fiction formats on radio.
Elizabeth gained a Masters Degree in Communication Studies from Pune University after graduating in Psychology, English Literature and Journalism, from Mount Carmel College, Bangalore.
Recently, following a sabbatical to raise her child, she turned to flexitime work options. She worked as Senior Editor with Trans Asia News Service, a portal offering news and analysis from an Asian perspective, where she interviewed the Dalai Lama.
Most recently, she brought to bear her collective experience to manage the Communications and Media management services for a Tata Trusts initiative called Social Alpha. At this Bangalore-based startup specialising in the ‘social business’ sector, her work entailed content creation, copywriting, PR and media relations, as well as communications strategy for internal and external stakeholders & events.
Elizabeth is an animal-lover, is passionate about plants, enjoys photography, and is keenly interested in health & fitness.