Metamorphosis is the process in which an animal undergoes a noticeable changes in its physical appearance as it moves from one stage of its life to another.

Animals that commonly undergo metamorphosis include insects and amphibians.

Some species of fish also undergo metamorphosis.

Insect Metamorphosis

Almost all species of insect experience metamorphosis.

An insect may undergo incomplete metamorphosis or complete metamorphosis.

In incomplete metamorphosis, the insect goes through three stages: egg, nymph and adult.

A nymph looks like a small adult, but has no wings or working reproductive organs.

The nymph molts (replaces its exoskeleton with a new one) a number of times, growing larger with each molt, until finally, it becomes an adult.

Once it is an adult, it no longer molts.

Incomplete metamorphosis is also known as hemimetaboly. Insects that undergo incomplete metamorphosis are called hemimetabolous insects.

Insects that undergo complete metamorphosis undergo four stages: egg, larva, pupa and adult.

A larva, such as a caterpillar, looks nothing like the adult insect it will become. It usually looks like a worm. It spends eats time eating and growing.

Larvae molt several times and grow larger each time.

Eventually, a larva will create a cocoon around itself. While it is in its cocoon, the insect is known as a pupa. During the pupal stage, the insect changes into its adult form.

In butterflies and moths, the pupa is known as a chrysalis.

Finally, the insect becomes an adult and leaves the cocoon.

Complete metamorphosis is also called holometaboly. Insects that undergo complete metamorphosis are holometabolous insects.

An insect in its adult stage of development is sometimes called an imago.

Amphibian Metamorphosis

Amphibians go from an egg stage, to a larval (juvenile) stage in which they live in and breathe water, and then move on to an adult stage in which breathe air and can live on land.

All amphibians lay their eggs in the water.

The larvae of frogs and toads are known as tadpoles.

A tadpole looks like a fish, with a tail and no legs.

It breathes through gills.

Later on, the tadpole develops lungs and legs. It may lose its tail.

The larvae of newts and salamanders also live in water and breathe through gills. Some of them may have legs.

As they metamorphosize into the adult stage, they lose their gills and begin to breathe through lungs.  They grow legs if they do not already have them.  If they do have legs, their legs grow larger.

Fish Metamorphosis

Some species of fish undergo a period of metamorphosis in which the young, or larvae, change into a completely different adult form.

Flatfish, such as flounder, undergo a striking metamorphosis.

Young flatfish look like ordinary fish.

They live near the ocean’s surface.

When they reach adulthood, flatfish move to the bottom of the ocean.

Their bodies change so that they become well-adapted to their new habitat.

During metamorphosis, a flatfish's body changes shape and becomes flat.

One of the flatfish's eyes migrates to the opposite side of its body, so that both eyes are on the same side of the body.

This way, the flatfish can lie with one side on the bottom of the ocean floor, with both eyes looking upward.

Salmon begin their lives in freshwater streams but live in the ocean when they are adults.

During metamorphosis, they undergo a change in body chemistry that enables them to live in salt water.

They change color during metamorphosis as well.

The salmon must return to fresh water in order to spawn (breed).

Fish that live in the ocean as adults but spawn in fresh water are anadromous.

Lampreys go through a period of metamorphosis.

Juvenile lampreys, which are known as ammocoetes, look very different from adult lampreys.

Unlike adult lampreys, ammocoetes have no teeth and no suckers.

Some species of lamprey are anadromous, like salmon. They spawn in fresh water and live in the ocean as adults.

During metamorphosis, their body chemistries must change so that they can survive in saltwater.

The European eel is catadromous. This means that it breeds in the ocean but spends most of its life in fresh water.

European eels lay their eggs in the Sargasso Sea.

The larvae, which are known as leptocephali (singular is leptocephalus), have flat, leaf-shaped bodies.

They live in the sea, drifting along with the current, for as many as three years.

Eventually, the Gulf Stream carries them to the European coast, where they metamorphosize into glass eels.

Glass eels are shaped like adult eels, but are still transparent.

The eels then migrate up freshwater streams. They begin to develop pigmentation, and become known as elvers.

As they grow, the eels' pigmentation becomes even darker, and they become yellow eels or brown eels.

After many years, the eels become sexually mature and their scales become silvery or coppery in color. The silver eels, as they are now called, migrate back to the sea to spawn.

The larvae of African lungfish have external gills. During metamorphosis, their gills become reabsorbed into their bodies.