The science behind metamorphosis

It's morphing time: Discover the transformative power of metamorphosis and the range of species that undergo it

As children we are taught the simple transformations of some species; the iconic blossoming of a butterfly and the tail-shedding cycle of tadpoles, for example. Known as metamorphosis, this process completely changes an animal’s anatomy. However, this transformation is far from simple and spreads across a wide range of species.

At first glance, it’s easy to come to the conclusion that a caterpillar and butterfly could be identified as two completely different species. English physician William Harvey did just that in 1651, describing metamorphosis as a process whereby free-living embryos had escaped eggs, which provided little nutritional value. He also suggested that what we now know is the pupa stage was in fact a second egg from which a new species was reborn. Dutch biologist Jan Swammerdam later discredited Harvey’s theory in 1669 when he realised that the larva, pupa and adult stages all belonged to a single species.

There are two different types of metamorphosis: complete and incomplete. The differences between the two isn’t whether or not a tadpole becomes a complete frog versus one that still has its tail; it relates to the species’ level of anatomical change.

Complete metamorphosis occurs in those that completely change their physical characteristics, for example, a caterpillar changing into butterfly. On the other hand, incomplete metamorphosis results in only some changes, such as those seen in crickets, where the larval stage doesn’t involve the development of wings but otherwise does look similar to its adult counterpart.

Shedding skin for wings

Insects are the most diverse class of animals on the planet, made even more diverse if you consider their change in forms, an occurrence that some undergo more than once in their lives. Some species start out in water as aquatic larvae, such as dragonflies, while others munch their way through vegetation on land. Many stay in their infant environment, but others decide to ditch walking or swimming and take to the skies. So how do insects shed their skin for wings?

Insect larvae carry a cellular bag of tricks within their bodies in order to carry out complete metamorphosis. Known as imaginal discs, these sac-like epithelial structures are the driving force for insect transformation. Once a caterpillar or ladybird larva has finished a series of moulting (where it has shed its skin multiple times) it enters the pupa stage in a chrysalis. While snuggled up in its new home, digestive enzymes break down part the of larva’s cellular structure with the exception of the imaginal discs. This creates a kind of chunky insect soup, with the imaginal discs playing the role of pieces of diced vegetables. During this process the discs begin to form the external structures of the soon-to-be butterfly. Working from the outside in, these structures will continue to form organs, wing veins and eyes.

Incomplete metamorphosis doesn’t involve such an intense transformation. Crickets start out as nymphs rather than larvae, and instead of becoming a pupa they undergo several series of moulting, a process known as ecdysis. A nymph’s exoskeleton will become too tight and, prompted by the juvenile hormone ecdysone, the nymph will form new skin and step out of the old one. The wings also develop at this stage, after which the nymph grows to its adult size. 

Trading tails for legs

The defining feature of amphibians is their ability to live both in water and on land, and much like insects, many amphibious species start out their lives in the water. However, unlike insects, when amphibians undergo the process of complete metamorphosis, there is a distinct
lack of a chrysalis or cocoon to shelter a metamorphic soup.    

In order to trade their tails for legs, most amphibians rely upon hormones to trigger the chain reaction of limb loss and limb growth while still swimming around. Thyroid hormones (TH) and prolactin hormones (PRL) are the predominant biological chemicals that control the process of metamorphosis. The two work together in a balancing act. TH is the agent of change and ultimately causes the gene expression that results in a frog’s transformation, while PRL works as a blocker to TH. As a frog begins its life as a tadpole, the ratio of TH and PRL levels are low. As they move through the stages of metamorphosis, PRL levels will decrease as TH levels increase, allowing the frog’s anatomy to change over time. This accumulation of chemicals results in amphibians making the necessary internal changes needed to survive for life on dry land.

Gills that formed in the larval/tadpole stages are now slowly absorbed and replaced with lungs. The intestinal structures in a larva are much longer than those of its future form; during metamorphosis this length is shortened. Larvae feed on plant matter, which takes longer to digest than the insects that adults eat, therefore the intestines are longer in tadpoles than in adult frogs. The same is true in other amphibian species, such as newts and salamanders.

Studies have revealed that there are other forces that can trigger these biological transformations, in particular environmental ones. Ponds naturally dry up as the seasons change, and as this natural process begins it acts like an eviction notice for the salamander larvae, for example. In artificial recreations of these conditions, scientists found that as the oxygen and water levels of a dying pond decreased, larvae were prompted to hurry up and grow some legs and lungs, and sure enough they did. 

Transforming below the tides

Metamorphosis isn’t just seen in the bugs and frogs of the world but in many marine creatures too. Jellyfish undergo the process of biological transformation, similar to metamorphosis in insects. They begin their lives as a stalk-like polyp. Attached to the seafloor, over time the polyp will break down into segments and form tiny jellyfish in a process called strobilation. These baby jellyfish (ephyrae) will gradually grow into adults (medusa). In one particular species, known as the immortal jellyfish, this process can be reversed, allowing adults to revert back to their juvenile stage, essentially giving them ever-lasting life.

Another terrific marine transformer is the sea slug (nudibranch). Not only weird and wonderful in both their vibrant colour and odd physical forms, their microscopic transformations to become adults are also awe-inspiring.

Beginning life as tiny organisms called veligers, these mini sea slugs reside in their own microscopic shells, which they shed during metamorphosis and after eating a lot of plankton. In order for these sea slugs to develop into the oddities of the seafloor, their internal and external physicalities are both transformed.

Changing for survival

It’s clear that these animal transformers have a fascinating lifecycle, but why do they feel the need to change it all? Fossil records for insect metamorphosis date back 280–300 million years, and it’s suggested that complete metamorphosis evolved from incomplete metamorphosis over time. The predominant theory as to why these transformations occurred in the first place was due to the need to reduce competition.

It has been proposed that creatures that could change form did so to ensure their survival when competing for resources. A tadpole or dragonfly larva will take its food from the water, whereas their future selves get their food from above the surface. Having offspring that live in a different environment, or demand different resources, eliminates the competition between juveniles and adults, thus extending the chances of survival for both.

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This article was originally published in How It Works issue 110

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