N. J. Berril
The crawling caterpillar becomes a moth or a butterfly. The tadpole swimming in shallow water changes into a frog or a toad. Many other animals also undergo a change in form and appearance during the development from egg to adult. This change is called metamorphosis.
The word "metamorphosis" comes from a Greek word meaning "change of form." During the distinctive stages of metamorphosis, more than just the shape and appearance of an animal changes. So does its way of life. For example, the young creature that emerges from a starfish egg is much too small to live and feed in the same way as the adult starfish. The young starfish, or larva, swims near the surface of the sea, feeding on microscopic plants until it has grown to many times its original size. Then it sinks to the seafloor and remains there while it changes into a little crawling starfish. On the seafloor, it feeds on mussels, clams, and other shelled animals.
Metamorphosis is said to be complete when there is a clear distinction between the stages of development. Animals such as frogs and butterflies exhibit complete metamorphosis.
The Life Cycle of a Frog
A frog lives most of its life out of water as a four-legged hopping creature, but it lays its eggs in water. The larva that hatches from each egg must therefore be able to breathe in the water, as a fish does. It must also be able to take its food from the water. The frog larva, called a tadpole, has gills like a fish and swims by means of its long, thick tail. It feeds on water plants and grows to many times its original size. Small legs begin to grow. The tadpole's body thickens and begins to look more like that of a little frog.
When the tadpole reaches a certain size, metamorphosis takes place. Quite suddenly, during the course of a day or so, the legs grow larger and the tail starts to shrink. The gills stop working, and lungs, which have been developing inside the tadpole's body, begin to work.
At this time, unless the frog crawls out of the water, it will drown. It has changed from a fishlike animal that breathes underwater to an air-breathing land animal. It no longer eats plants. It feeds on worms and insects.
The Life Cycle of a Butterfly
Most insects undergo metamorphosis. Insects that fly are able to do so only when they are fully grown. Until then they must live and feed in a way different from that of the adult. The greatest change in insects is seen in butterflies and moths. The small moth or butterfly egg usually hatches into a crawling larva, or caterpillar. The caterpillar feeds until it grows to a certain size. Then it spins a covering around itself called a cocoon, or it may burrow into the ground or hide behind loose bark. The caterpillar goes into a resting stage and is now called a pupa. Inside the skin of the pupa, a big change takes place. The larva gradually changes into an adult.
The butterfly or moth that emerges from the pupa is practically a new creature. The metamorphosis of butterflies and moths, with its four distinct forms, is more complete than that of any other animal group.
Primitive insects, such as dragonflies, undergo incomplete metamorphosis. While complete metamorphosis is a relatively quick transformation with distinct stages, incomplete metamorphosis involves a gradual transformation. When the young hatch from the eggs, they closely resemble the adult form. They go through a series of stages, looking progressively more like adults with each stage.
The Life Cycle of a Dragonfly
Dragonflies may take two to three years to develop from an egg to an adult. The young that emerges from the egg is called a nymph. The nymph lives underwater. It breathes in the same way that a fish does — by using gills. It also uses gills to help it move about in the water. Jets of water, ejected from the gills, propel the nymph about. As the nymph grows, it gets too big for its tough external covering, or skin. In order for it to continue growing, it must shed its skin, a process called molting. Through a series of molts, the dragonfly nymph approaches adult size and structure.
Because the adult dragonfly lives out of the water, the final molt must take place above the water's surface. The mature nymph crawls up a stem and clings tightly. When the skin splits, the winged adult emerges. It crawls free of its nymphal skin and remains on the stem until its soft wings are stiff and fully expanded. Usually within two hours, it is ready to fly.