‘My soul is wrought to sing of forms transformed to bodies new and strange! Immortal Gods, inspire my heart, for ye have changed yourselves and all things you have changed!” (Ovid, “Metamorphoses,” Book I)
Early in life, I fell in love with metamorphosis when I collected caterpillars and placed them in jars with their preferred leafy food, then watched them transform into pupae and finally into butterflies.
In high school biology I discovered that their magic shape-shifting lay deep in their evolutionary development, which allowed the immature and mature stages to occupy different ecological niches dining on different foods. This allowed one form of the insect to gorge on abundant, ephemeral food, fattening up before that next stage of its life began.
In those same years, I also discovered the singularity of the Latin language when, in Mr. Hatch’s Latin class, we struggled to make sense of Ovid’s “Metamorphoses.”
Ovid’s origin tales are far closer to those of pre-industrial societies than to the rationalist branch of my Western heritage, namely scientific explanations. Ovid describes scores of metamorphoses — bones become rocks, rocks become people, tears become streams, people become all manner of other living things — coral reefs, lions, lizards, snakes, seals, wolves, birds (woodpeckers, jackdaws, doves), plants (narcissus, hyacinth, reeds, mint) and myriad trees (myrrh, oak, laurel, willow and the like). He provided perfectly valid explanations for shape-shifting that are anything but scientific.
Much of my adult life, however, has been spent in pursuit of knowledge from the natural sciences, and here, I’d like to focus on some recent research on insect metamorphosis — how a crawly, lumpy thing turns into an elegant flying thing after spending time cooped up in a self-made shell.
Some 80%t of insects undergo this transformation, and they represent more than half of all animals.
“Complete metamorphosis” is the term used for the transformation of the immature form of an animal (a caterpillar) into a resting form (a pupa), then into an adult (a butterfly).
Let’s first investigate how this works: The cells of insects, as of all life forms, are programmed to perform certain functions and to change or die on a set schedule. Hormones chiefly regulate the schedule, and, in insects, the principal hormone that signals metamorphosis is called the juvenile hormone, or the JH. As long as it is present in the cells, the larva will simply continue to grow, cast off its skin, and grow again. At the fifth caterpillar stage in most butterflies, however, the JH, prompted by other hormones, disappears, signaling the caterpillar to construct its chrysalis.
Imagine what goes on inside the chrysalis. If you break one open, you’ll find a kind of soup, but this soup is no bunch of undifferentiated cells. It turns out that much of the larva’s tissues and organs are re-specified, such as the skin, much of the nervous system and many muscles. That is, these organs and tissues perform the same functions in both larva and butterfly.
Then there is the process of programmed cell death and creation of replacement cells for functions that butterflies need, such as tongues for nectar feeding that replace chompers for chewing leaves.
Even more wondrous is the remodeling of some organs, such as the gut, which transforms from a leaf- to a nectar-digester. What a magnificent system that rearranges body parts and functions, not so unlike Ovid’s fantastical metamorphoses after all. And scientists are just beginning to understand it.
Now to the question of the evolutionary origins of complete metamorphosis:
Most researchers argue that the caterpillar evolved from the butterfly over a long period of time starting 400 million years ago. There is no agreement, however, on its evolutionary origins. Perhaps complete metamorphosis developed from the two-stage “incomplete” metamorphosis that most aquatic insects, cockroaches and grasshoppers undergo, which lacks the pupal stage. (As an example of incomplete metamorphosis, grasshopper nymphs look very much like smaller versions of the adults they become through progressive molts.) Or perhaps complete metamorphosis had its own parallel evolution. There are even arguments as to whether the pupal stage developed from the adult or from the larval stage of the insect.
There is no question, however, that complete metamorphosis has produced a prodigious class of animals that have spread across the globe over eons of time. According to a 2019 journal article by Free University Berlin Professor Jens Rolff and colleagues, “Insects are of outstanding importance for biodiversity, for ecosystem services, as vectors of disease and as a source of protein.”
My decades of European education have provided a foundation to understand metamorphoses. Such is my knowledge as an American with indigenous origins in Europe. What metamorphosis stories do the Indigenous people of Turtle Island, of North America tell, I wonder? Perhaps this is a subject for a Native author to write in a future Earth Matters column.
“All things are in a state of flux, and everything is brought into being with a changing nature. The spirit wanders, comes now here, now there, and occupies whatever frame it pleases. From beasts it passes into human bodies, and from our bodies into beasts, but never perishes.” (Ovid, “Metamorphoses,” Book 15)