Metamorphosis—A Symphony of Miracles

by Dr. Gordon Wilson on April 1, 2014; last featured February 1, 2015
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Since the dawn of time, people have marveled at the caterpillar’s visible transformation into a butterfly. New technology reveals a symphony of unseen changes that are just as miraculous.

Everyone enjoys a good music performance. To most enthusiasts, the performance just happens, and they revel in the sound. Few appreciate all that goes on behind the scenes. Those who do, however, appreciate the music all the more.

That’s true of wonders in nature, too. Every spring we see a few fat caterpillars chomping away on leaves, and then a few weeks later fluttering butterflies fill our backyards. Thanks to modern technology, we can watch this wonder inside the comfort of our own homes—on high-definition TV. Nature programs show the best bits in super-slow motion, all accompanied by an epic soundtrack. It’s amazing, but they’re only touching the surface.

Every choreographed step underscores the wisdom and power of our Creator.

It gets even better. With the aid of modern DNA analysis and MRIs, we’ve begun to unravel the secret wonders that make metamorphosis possible. Every choreographed step underscores the wisdom and power of our Creator.

Let’s start with the caterpillar. It’s a miracle all by itself. This undulating, worm-like creature is an eating machine, born with specialized tools to grab hold of food and chow down. Along with its six stumpy legs, it has several pairs of fleshy, cup-like stubs, called “prolegs” (yes, that’s the name for them). These extra limbs are ringed with crochet-like hooks that help grip surfaces. The chewing mouthparts then cut and devour gluttonous quantities of leaves.

All the information to assemble this creature was in place long before it hatched. Indeed, the miracles began even earlier. For many butterfly species, the caterpillar eats only a specific type of plant. Its mother has to know just where to deposit the egg to provide the best food for her young.

Chrysalis

© CathyKeifer | iStockphoto.com

Imagine what must transpire inside a chrysalis—a complete body makeover. In addition to sprouting wings, several major structural changes occur. Six stubby legs transform into long, slender ones; leaf-munching jaws are replaced by a nectar-sucking straw (the proboscis); simple eyes become compound ones; and the aviator needs a whole new breathing system, new innards, a new thorax, and a new abdomen.

Upon hatching, it clambers out of the egg and—without further instructions—starts its eating schedule. First on the menu is the egg shell. After that, the caterpillar devotes most of its larval life to “pigging out” on plants. By the time it’s finished, the caterpillar can grow over three thousand times its original size. That’s equivalent to a six-pound (2.7 kg) baby growing into a nine-ton (8.2 m. ton) Godzilla . . . in several weeks or so.

Needless to say, that kind of growth spurt creates some challenges, especially for a creature that wears its skeleton on the outside. But the caterpillar is prepared for the growing pains.

Most insect exoskeletons are fairly rigid, like a suit of armor, but a caterpillar’s exoskeleton is more spandex-like and allows for considerable growth. Though stretchy, it eventually reaches its limit. Every so often, because of its rapid growth, it needs to shed, or molt. Its shameless weight gain maxes out the stretchiness.

Before it gets rid of its exoskeleton, a new one needs to be laid down beneath the old one. This outfit has wrinkles, like a child's baggy britches that have “room to grow.” But unlike the storebought variety, this new set of clothes is made out of chemicals secreted under the old exoskeleton.

During the molt, the youngster tears out of the old outfit by inflating and wriggling. Then it enters another period of bingeing at the salad bar until the outfit is maxed out and it must molt again.

These periods between molts are called instars. Each species has a typical number of instars before it needs to molt into something very different . . . a new body plan, technically known as a chrysalis or butterfly pupa. This is not the adult butterfly, and it is not a caterpillar.

Here’s where things get really interesting, and scientists continue to discover more of the intricacies of the chemical commands that make this possible.

Molting is very complex, but here are the basics. Certain specialized cells in the caterpillar’s brain patiently wait until the body sends it the right combination of signals, indicating that it’s time to molt. When that moment comes, these brain cells release a hormone called PTTH.1

This chemical then drifts rearward with the flow of the caterpillar’s blood. Eventually it arrives at a specialized organ in the thorax,2 where it prompts the release of another hormone called ecdysone. The ecdysone then goes to the skin where it triggers ecdysis, a fancy name for molting.

The caterpillar’s skin is composed of a one-cell-thick layer beneath the stretchy exoskeleton. This cell layer is responsible for making the exoskeleton. The skin has different genetic instructions for each stage of life: the larva, pupa, and adult. The skin just needs the right chemical cues to know which type of exoskeleton it needs to build.

During early molts, the brain sends out a chemical called juvenile hormone (JH). This hormone tells the skin to keep producing another caterpillar exoskeleton like the last one (the larva stage). JH is the status quo hormone. When ecdysone says, “Molt,” JH says, “Do the same thing; just make it bigger.”

But eventually, the eating frenzy must end. God designed this insect with a loftier purpose. Someday this squiggly, stumpy-legged, leaf-loving worm must rebuild itself—using stored-up materials—into a beautiful flying creature that pollinates countless kinds of flowers the world over.

When the brain detects that the time is right, an amazing series of events happens. During the caterpillar’s last instar, the brain is programmed to release ecdysone as usual, but with a smaller amount of JH. This prevents molting into supersized caterpillars and activates the molt into a pupa.

The pupal stage is a stepping stone toward the ultimate goal of manufacturing the aerodynamic body of an adult butterfly. During this stage, one final exciting molt occurs. But this time, JH levels are very, very low. When a molt occurs with almost no JH, the skin (epidermis) of the pupa knows to switch to a completely different game plan. This new combination of chemical signals (ecdysone with almost no JH) is a command to make a radically different exoskeleton, making the pupa into an adult.

A butterfly pupa is a very passive creature from all outward appearances. It doesn’t eat or crawl. Instead, it just sits there, attached to the plant by a tuft of silk, doing apparently nothing. But it is far from being slothful.

In just a few days, each of the pieces of the former life are carefully torn apart and rearranged.

With the help of MRIs, we can now watch how, in just a few days, each of the pieces of the former life are carefully torn apart and rearranged. A few larval innards survive the remodel, such as some muscles and nerves, while others are completely new.

A huge remodel is going on inside. Somehow, six stubby legs must transform into long, slender ones. Leaf-munching jaws must be replaced by a nectar-sucking straw (the proboscis); simple eyes must become compound ones; and, yes, the creature must sprout four wings, which it never had before. (By the way, each species of butterfly creates its own unique color patterns by making myriads of minute scales of different colors in unique arrangements. Even the shapes of the wings differ from species to species.) An adult butterfly also requires a whole new breathing system, new innards, a new thorax, and a new abdomen.

The main behind-the-scenes workhorses are tiny packets of cells, called imaginal discs. The body begins forming scores of them on a set schedule. The discs appear throughout development in very specific places just inside the skin. They form throughout the caterpillar—wherever new body parts will be needed—before the molt into a pupa.

These imaginal discs get really busy after the pupal exoskeleton is completed. They begin to produce many cells that form totally new structures, which never existed before in the larva. The imaginal disc cells become connected to the existing skin. Some of the larval skin remains intact, but it is now under new orders to produce a radically different adult exoskeleton.

Different discs follow distinct growth patterns, like members of a marching band moving in very different ways but accomplishing the drum major's overall marching orders. Where are these orders found? These cells “read” the parts of their genetic blueprints appropriate for that section of the adult exoskeleton. (Previously and in the same way, they “read” different parts of their genetic blueprints to build the larval and the pupal exoskeleton).

At the macro level the different body parts are produced. The disc cells multiply, conform to a certain shape and make the appropriate body part with its corresponding exoskeleton. One disc makes a wing, while other discs team up to form the adult head and thorax, and so on.

The remodel can be loosely compared to a drastic home remodel. During this “quiet” pupal stage, much of the caterpillar’s digestive tract, muscles, and nervous system are demolished like a house being gutted down to the studs. Meanwhile, new adult counterparts are made. The old nervous system undergoes drastic reorganization (the nerve cells that aren’t destroyed are rearranged) resulting in a very different nervous system. This would correspond to the rewiring of the house.

The comparison to a remodel, however, eventually breaks down. After the demolition of a house, all the debris is hauled to the dump, and new building materials brought in. Not so in the pupa. The pupa doesn’t eat, so all parts must come from the materials accumulated during the larval stages. The liquefied caterpillar parts provide the building materials for the adult parts.

The imaginal discs must also make a new, much more complicated, exoskeleton. The new exoskeleton includes more than just the basic body wall. It forms the outward-projecting body parts and some of the inward-projecting body parts. The former include legs, antennae, siphoning mouthparts, and scale-covered wings. The latter include first and last parts of the digestive tract (foregut and hindgut) and the complex new breathing system that will power flight.

For thousands of years, the exoskeleton has blocked our curious eyes from seeing this amazing transformation. With the modern blessing of MRI, scientists are continuing to unveil more mysteries of metamorphosis. King Solomon would have loved to see the show.

When the skin has finally finished constructing the adult exoskeleton, the last molt takes place. The adult inflates itself until it splits the pupal exoskeleton along a pre-formed seam. A body with an entirely different shape and behavior crawls out of the slit. That’s what we see on TV.

Blood is pumped into the wing veins so that they unfurl to full size. Once everything is ready, the exoskeleton hardens (or stays flexible) in all the right places. The transformed creature flaps its glorious wings and takes to the air.

It’s always mind-blowing to look deeper into God’s workmanship at the nano-scale.

It is human nature to get caught up in the spectacle and fail to appreciate the intricacies necessary to make it happen. We glory in the fact that God knit us together in our mother’s womb, but most of us aren’t very eager to study embryology to see just how glorious it actually is. Why? It’s work! It might be extra work, but it’s always mind-blowing to look deeper into God’s workmanship at the nano-scale. Plus, it gives us yet another reason to praise Him and share His glory with others.

Next time we marvel at beautiful things, we need to stop and appreciate the Creator’s attention to unseen details, which He lavished on all His creation.

A long time ago science was usually a hobby; rarely a profession. In fact, many naturalists were clergymen. They intently studied the Word of God and nature, knowing that both are treasure troves of God’s boundless glory. These wonders would make even King Solomon reel in awe. “Consider the lilies of the field [or butterflies for that matter] . . . even Solomon in all his glory was not arrayed like one of these” (Matt. 6:28–29, bracketed portion added). “Great are the works of the Lord, pondered by all who have pleasure in them” (Psalm 111:2). The next time you see a butterfly flutter onto a flower, be still, ponder, and worship its Creator.

Miraculous Life Cycle

Egg The Egg: All the information to assemble this creature was in place long before it hatched. Its nectar-eating mother knew just where to deposit the egg. (For many species, the caterpillar survives on only one specific type of plant.)
Caterpillar Caterpillar: The caterpillar is born with specialized tools to grab food and chow down. It will grow up to three thousand times its original size, so sometimes it needs to shed its outer skeleton. That means laying down a new outfit under the old one—wrinkles and all.
Caterpiller to Pupa Pupa (Chrysalis): Eventually the caterpillar molts into a new body plan (a chrysalis). At the end of this stage, one final molt occurs. With the help of MRIs, we can watch how the pieces of the former life are carefully torn apart and rearranged.
Pupa to Butterfly Butterfly: Once the adult body is built, the butterfly will inflate until it splits the old exoskeleton. Then it pumps blood into the wings so they unfurl. After a short wait to let the new exoskeleton harden, the transformed creature takes to the air.
Dr. Gordon Wilson, Senior Fellow of Natural History at New Saint Andrews College, earned his PhD from George Mason University in environmental science and public policy. He holds a master of science degree in entomology from the University of Idaho.

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Metamorphosis

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Footnotes

  1. Prothoracicotropic Hormone
  2. Prothoracic Gland

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