Animal Life Cycles and Metamorphosis

An animal life cycle is the series of stages from fertilized egg to reproducing adult, and metamorphosis is a dramatic change of body form during that cycle. In brief: some animals, such as birds and mammals, hatch or are born looking like small adults, while others pass through larval stages that look very different. A butterfly goes through four stages (egg, larva, pupa, adult) in complete metamorphosis, while a grasshopper goes through three (egg, nymph, adult) in incomplete metamorphosis.

Metamorphosis allows young and adult animals to use different food and habitats, which reduces competition between them. This guide describes simple and complex life cycles, explains complete and incomplete metamorphosis in insects, covers metamorphosis in amphibians, compares the types in a table and works through a monarch butterfly example.

What are animal life cycles?

A life cycle describes how an animal develops, grows, reproduces and eventually dies, and how the next generation begins. Most animals start as a fertilized egg. The egg develops into an embryo, which becomes a young animal that grows to maturity.

The cycle can be written as a sequence of stages that returns to its starting point, because the adults produce eggs. The length of the cycle varies from a few days in some insects to many decades in large animals such as elephants.

What is direct development?

In direct development, the young animal looks like a smaller version of the adult. It grows larger and becomes sexually mature, but it does not change its general body form.

Mammals, birds, reptiles and many fish develop this way. A kitten has the same body plan as a cat, and a chick resembles a small hen apart from its down feathers. Spiders and scorpions also develop directly, and the young molt as they grow.

What is metamorphosis?

Metamorphosis is a marked change in body structure, and often in lifestyle, between the larval and adult stages. The larva is specialized for feeding and growth, while the adult is specialized for reproduction and dispersal.

Hormones control the process. In insects, a high level of juvenile hormone keeps the animal in larval form, and a drop in this hormone together with ecdysone, the molting hormone, triggers the change. In amphibians, thyroid hormones trigger metamorphosis.

What is complete metamorphosis?

In complete metamorphosis there are four stages. Butterflies, moths, beetles, flies, bees and ants develop this way.

1. Egg: The female lays eggs, usually on a food plant for the young.
2. Larva: The larva, such as a caterpillar, grub or maggot, eats a great deal and grows, molting several times.
3. Pupa: The larva stops feeding and forms a pupa, such as a chrysalis or a cocoon. Inside it, most larval tissues are reorganized into adult structures.
4. Adult: The adult emerges, usually with wings, and reproduces.

Larva and adult often eat different foods. A caterpillar chews leaves, while the adult butterfly sips nectar. This reduces competition between the young and the adults of the same species.

What is incomplete metamorphosis?

Incomplete metamorphosis has three stages: egg, nymph and adult. There is no pupa. Grasshoppers, dragonflies, cockroaches and true bugs develop this way.

A nymph looks like a small wingless version of the adult. It molts several times, and at each molt it gets larger and its wing buds develop. After the final molt, it becomes a winged adult. Dragonfly nymphs live in water and breathe with gills, and the adults fly in the air, so the change in habitat can be large even without a pupal stage.

How do amphibians undergo metamorphosis?

Most frogs and toads lay eggs in water. The eggs hatch into tadpoles with gills, a tail and no legs, and tadpoles often eat algae. Over weeks or months, hind legs and then front legs develop, the lungs form, the gills disappear and the tail is absorbed.

The adult frog is a carnivore that lives on land and in water. Some salamanders keep their gills and stay in water for life, and some frogs skip the tadpole stage and hatch as tiny frogs, so metamorphosis is not universal in this group.

Types of development compared

Worked example: the monarch butterfly

The monarch butterfly shows complete metamorphosis well. The typical timeline in warm weather is about a month from egg to adult, although it varies with temperature.

1. Egg: A female lays a single tiny egg on a milkweed leaf. It hatches after roughly four days.
2. Larva: The caterpillar eats milkweed leaves for about two weeks and grows several thousand times heavier. It molts four times, which gives five growth stages called instars.
3. Pupa: The caterpillar attaches to a twig and sheds its skin for the last time to reveal a green chrysalis. About ten days later, the pattern of the wings can be seen through the wall.
4. Adult: The butterfly emerges, pumps fluid into its wings to expand them and flies after they dry.

The stages occupy different parts of the cycle. Growth takes place almost entirely in the larval stage, the pupa reorganizes the body and the adult is responsible for reproduction and dispersal. Counting the molts is a good way to check understanding: four molts between five instars, with a fifth molt to the pupa.

How was metamorphosis discovered?

Ancient observers noted that caterpillars become butterflies, but some people thought that maggots arose spontaneously from rotting meat. In 1668 Francesco Redi showed that flies lay the eggs that become maggots.

In the late seventeenth century, Maria Sibylla Merian recorded the development of insects and their food plants in detail. Jan Swammerdam dissected insects and showed that the pupa already contains the structures of the adult. Hormone control was worked out in the twentieth century.

Which mistakes are common?

Mistake 1: Treating the pupa as a resting stage

The pupa looks still, but inside it the body is being rebuilt. It is a time of intense activity.

Mistake 2: Calling a nymph a larva

Nymphs resemble small adults and have no pupal stage. Larvae look very different from adults and are followed by a pupa.

Mistake 3: Thinking that all insects go through four stages

Many insects, such as grasshoppers and cockroaches, have three stages. Some primitive wingless insects, like silverfish, develop directly.

Mistake 4: Placing the stages in the wrong order in a cycle diagram

The order is fixed: egg, larva, pupa, adult. An exam answer should name the stage and say what happens in it.