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Different Ways of Growing
When animals are born or when they hatch from their eggs they often do not look like their parents. As an animal grows it becomes bigger and it changes its shape. Some animals, such as insects and amphibians, change their shape considerably as they grow.
The growth and development of an insect : The cabbage white butterfly
This is a species of butterfly which is very common throughout Europe and most of Asia. The male and female cabbage white butterflies mate in the month of May and the female lays her eggs on the underside of the leaves of cabbage plants. The eggs hatch about one week later. The offspring is called a caterpillar and it looks quite different from its parents.
The caterpillar's body is quite simple. It is long and soft but divided up equally into segments. The caterpillar has a head with mouthparts that are designed for chewing as it feeds upon cabbage leaves. The caterpillar's eyes are quite simple, they can detect light and darkness but not much else. The body of the caterpillar behind its head is divided into segments. The first three segments carry three pairs of "true" legs but the caterpillar also has five pairs of sucker-like prolegs further along its body. After a month of feeding and growing the caterpillar leaves the cabbage plant to find a sheltered place. Here it turns into a chrysalis or pupa. The pupa is a resting stage when all the tissues in the caterpillar's body are rearranged.
After two weeks the pupa hatches into an adult butterfly which is quite different from the caterpillar. The body of the adult is clearly divided into three parts : the head, the thorax and the abdomen. On the thorax are found three pairs of legs (the adult does not have any prolegs) and two pairs of wings. So the adult butterfly can fly as well as walk. The adult butterfly feeds by sucking nectar from flowers so its mouthparts are long and tube-shaped. The adult also has very large eyes to see with and a pair of long antennae to smell the air. Finally, the adult butterfly can reproduce because inside its abdomen are testes if it is a male or ovaries if it is a female.
This dramatic change that we observe in the life cycle of the cabbage white butterfly is characteristic of many insects, many other invertebrates and some vertebrates such as the amphibians. This change is called metamorphosis, which comes from the Greek words meta "to change" and morphos "shape". In general the immature forms are called larvae (sing. larva). (The larvae of some animals may have special names. The larvae of butterflies and moths are called caterpillars and the larvae of frogs are called tadpoles.) The reason for metamorphosis is because the larvae have a different diet or live in a different place to the adult. For example, the caterpillars of butterflies feed upon the leaves of plants but the adults suck nectar from flowers. The frog tadpole swims in water but the adult frog can walk on land.