Metamorphosis

Metamorphosis is a special change that happens in an animal's body as it grows. Many animals, like insects, fish, amphibians, and mollusks, go through this change. It often changes how they get food or behave.

Some animals change a lot between stages of their life. This is called complete metamorphosis. Other animals change only a little. This is called incomplete metamorphosis. During these changes, young animals called larvae grow up to become adults. This process helps animals live better in different parts of nature.

Hormonal control

In insects, growth and changes in body shape are controlled by special chemicals called hormones. These chemicals are made by endocrine glands near the front of the body. A hormone from the insect's brain tells other glands to release another hormone, ecdysone. This causes the insect to shed its outer shell, called ecdysis. Another hormone, juvenile hormone, helps decide if the insect looks more like a baby or an adult after it sheds its shell.

Experiments with firebugs show how these hormones can change the number of stages an insect goes through before becoming an adult. In animals like fish and amphibians, changes in body shape during growth are also influenced by special chemicals related to iodine.

Insects

Insects change in three ways: no change, incomplete change, and complete change. With no change, called ametaboly, the young look almost the same as the adults. With incomplete change, called hemimetaboly, the young, known as nymphs, look similar to adults but are smaller and lack wings and other adult features. These nymphs grow through stages called instars, molting (shedding their skin) between each stage.

With complete change, called holometaboly, the young, known as larvae, look very different from adults. They go through a resting stage called a pupa (or chrysalis in butterflies) before emerging as adults. For example, a butterfly starts as a caterpillar (larva), forms a chrysalis (pupa), and then becomes a butterfly (adult). Temperature can affect how fast these changes happen.

Chordata

Amphioxus

In cephalochordata, body changes happen because of a special substance. This might be an old feature of all chordates.

Fish

Some fish, like bony fish (Osteichthyes) and jawless fish (Agnatha), change their body shape. This change is often managed by a hormone from the thyroid.

Examples include the lamprey. The salmon changes from living in freshwater to saltwater. Many flatfish begin with one eye on each side of their body, but as they grow, one eye moves to join the other on top of the head.

The European eel goes through several changes, from its early stage to the glass eel, then to the elver, and finally to the adult migrating phase.

Most other bony fish start from an egg to small, unmoving larvae, then to moving larvae that must find their own food, and finally to the juvenile stage where they start to look like adults.

Amphibians

In amphibians, eggs are laid in water, and the young live in water. Frogs, toads, and newts hatch as larvae with gills, but later grow lungs.

Metamorphosis in amphibians is managed by special hormones in the blood. Tadpoles have unique features like tooth ridges and fins that disappear when they change into adults.

Frogs and toads

In frogs and toads, the young hatch with external gills that are quickly covered, and lungs form soon after. Front legs grow under the gill sac, and hind legs appear a few days later. Tadpoles eat plants and have long, spiral-shaped guts, but as they change, their guts shorten to help them eat insects.

Quick changes happen when their lifestyle changes completely. The spiral-shaped mouth and gut disappear, the animal grows a big jaw, and the gills vanish. Eyes and legs grow fast, and a tongue forms. All these changes can happen in about a day. The tail disappears a few days later.

Salamanders

Salamander development varies; some change a lot from water larvae to land adults, while others, like the axolotl, keep their larval features and never become land adults.

Newts

In newts, body changes happen because of where they live, not because of food. Newts' gills are not covered by a sac and disappear just before they leave the water. Adults can move faster on land than in water. Newts often live in water in spring and summer and on land in winter. Hormones help them adjust to living in water or on land.

Caecilians

Some caecilians, like Ichthyophis, change from water larvae to burrowing adults, losing some features. Other caecilians stay burrowing and do not change much like frogs.