By Mary Hoff
Caterpillars turn into butterflies. Tadpoles turn into frogs. Maggots turn into flies. Grubs turn into beetles. In fact, many animals start out looking like one kind of creature and end up like something else entirely. To make the change, they go through a shape-shifting process known as metamorphosis.
Metamorphosis (the morph part comes from the Greek word for form) is a dramatic change in what an animal looks like and how its body works that takes place as it changes from an immature form (often called a larva) into an adult. Animals that undergo metamorphosis include insects, amphibians, mollusks, crustaceans, and some fish.
Metamorphosis allows young and adults of the same species to occupy different niches. A niche is how a living thing fits into the world around it, including what it eats and the resources it uses. In species that undergo metamorphosis, young and adults don't have to compete for food, space, and other things they need to survive. Tadpoles eat algae, while toads eat insects. Mosquito larvae live in water, while adults live on land.
Metamorphosis also lets animals have specialized body parts to use at different times in their lives. Larvae often have strong mouthparts and a powerful digestive tract, so they can eat a lot of food and grow quickly.
Some adult insects don't eat at all. But they have well-developed organs for producing young. Some have wings that allow them to move over long distances in search of a mate. Many also have a keen sense of smell that helps them find mates.
Watch pocketbook mussel display
As the young of animals that undergo metamorphosis grow, certain cues tell them it's time to make a big change. Inside their cells some bits of DNA wake up, while other bits that had been awake go to sleep. The newly awakened bits of DNA provide instructions the cells use to make the adult animal.
How do you know when it's time to get up in the morning? Maybe the sun wakes you. Or you hear the sound of an alarm clock. Or perhaps your body listens to the clock inside you and wakes up on its own.
In the same way, young animals receive many cues that can signal when it's time to undergo metamorphosis. Cues include amount of daylight, temperature, humidity, chemicals in their bodies, and chemicals around them.
Without a sound, a young green darner dragonfly, called a nymph, crawls from the water and onto a rock at the edge of a lake. The nymph pauses with its back end still in the water. For the past three months, the wingless nymph has been living underwater. After spending its life getting oxygen through gills, as a fish does, the nymph is breathing air for the first time.
Underwater, the nymph had been capturing insects and fish for food with the huge, clawlike structure that is its lower jaw. But in recent days, the nymph has been changing form inside. Tiny bumps on its back look like they could be wings. Under the nymph's skin, an adult dragonfly is waiting to get out.
Rip! In the middle of the night, a split starts to form on the back of the nymph's head, just like the split you might make in a pair of pants that's too tight. With a wiggle and a stretch, the adult beneath the skin pops its head and front legs out. It arches backward away from the nymph skin and hangs there as its soft skin dries and hardens.
When its legs are stiff, the adult uses them to pull the rest of its body out of the shell. Its wings lie like crinkled lumps on its back. Like pumping air into a beach ball, the green darner inflates its wings to full size by pumping insect blood, called hemolymph, into them. When its wings are dry, it flies off. Leaving its aquatic days behind, the green darner is now a denizen of the sky.
The woolly bear caterpillar starts its life as an egg. The egg hatches into a tiny, fuzzy larva, or caterpillar. The caterpillar has one big job in life: to eat. It eats just about any kind of plant it can get its little legs on. When winter comes, it curls up into a ball. In spring, the warm air tells the caterpillar that it's time to become an adult moth.
The caterpillar finds a hidden spot. It starts to make a cocoon for itself out of its own hair, along with silk it releases from spinnerets located on its head, near its mouth.
Underneath its caterpillar skin, the woolly bear has been growing another skin, called a pupal skin. Then it sheds the caterpillar skin and hardens into a kidney bean-colored lump.
Tucked tightly into its cocoon, the caterpillar begins to dissolve, except for a few cell clusters called imaginal buds. At first, it is as mushy as a fruit smoothie. Then those few cells start to grow and multiply. They differentiate -- form various organs and structures, such as legs, eyes, and a digestive tract.
After a couple of weeks the cocoon opens. An Isabella tiger moth emerges, with wings all shriveled. But soon the moth's wings expand and are ready to fly.
Like other frogs, spring peepers start their lives as jellylike eggs attached to underwater plants. The eggs hatch into tadpoles, fishlike creatures that live in water and breathe through gills.
As it grows, the tadpole's body starts to make a chemical called thyroxine. The thyroxine tells the tadpole's insides that it's time to undergo metamorphosis and become an adult frog.
Unlike a caterpillar, a tadpole keeps moving as it undergoes metamorphosis. As it swims around, little legs start to sprout. Its beak disappears, and it grows teeth and a tongue. Its intestines get shorter and more complex in preparation for switching from a diet of algae to a diet of insects and other animals.
The tadpole's gills disappear, and air-breathing lungs grow. The chemicals in its blood change in a way that suits a land-hopping, air-breathing animal instead of a water-swimming one. Its tail shrinks, and the long nerves that control tail movement die.
After about three months, the tadpole has completely transformed into an inch-long frog. With its powerful legs, it hops out of the water and begins a new phase of life on land.
An adult American eel is a long, tube-shaped fish. But when eels first hatch from their eggs in the Atlantic Ocean, they look more like see-through willow leaves. The name of the larva at this stage, leptocephalus, means "thin head."
Leptocephali feed on small aquatic organisms as they float along with ocean currents. After about six months of swimming and drifting, the leptocephalus approaches North America.
When the leptocephalus is almost to the continental shelf, chemicals inside its body tell it to start undergoing metamorphosis, and it becomes what is called a glass eel. It looks like a translucent mini-eel as its intestines, gills, and other internal organs start to change to their adult form. It gets shorter, and it fattens into a snakelike shape.
When a young eel enters freshwater it is called an elver. As it becomes an adult, its skin will turn to a dark blue-gray color. Adult eels that live in Minnesota were once leptocephali drifting in the ocean! They swam 2,000 miles up the Mississippi River to get here.
On a gravelly bed of the St. Croix River, a female plain pocketbook mussel relaxes the muscle that holds the two halves of her shell together. Her mantle flaps -- soft, slimy pieces of tissue -- stick out into the water. One end is long and loose and waves in the current like the tail of a swimming fish. The other end has a dark spot, like a fish's eye. She rhythmically pulses her mantle flaps to simulate a swimming minnow. Before long a hungry bass comes by. The bass tries to eat what it thinks is a minnow. As it does, the mother mussel releases thousands of larvae into the water. Some of the mussel larvae, called glochidia, get sucked up into the fish's mouth. When they get to the fish's gills, they attach and tissue grows over them.
The glochidia develop a strong foot, the beginnings of gills, and digestive organs. After two to six weeks, the glochidia have metamorphosed into juveniles and move out of the fish's tissue. The juveniles are easy prey for insects, crustaceans, worms, and fish. Scientists think some may temporarily burrow into the riverbed to stay safe. Eventually they rise to the river bottom and begin filtering water for food. Unless an animal moves them or they get washed away by currents, this is where they will spend the rest of their days -- until they too lure a fish and the cycle begins again.