Metamorphosis

Metamorphosis << `meht` uh MAWR fuh sihs >> is a Greek word that means transformation. Biologists use this word to describe the extreme changes in form and appearance that occur in certain species between the growing phase of life and the mature adult phase.

Such animals as cats, dogs, and horses—like human beings—are similar in form and structure to the adult when they are born. They differ chiefly in size from the mature animals. As these animals mature into adults, they grow larger, but they keep the same limbs, organs, and basic form. This kind of growth is known as direct development. When such animals as ants, butterflies, and sea urchins emerge from an egg, they appear to be different from their parents. They may have different limbs, organs, and body structures from adults. In many of these young animals, major changes in structure and appearance must occur before they become adults. This kind of growth is known as indirect development. Metamorphosis is a kind of indirect development.

The changes that occur in the life cycle of a butterfly or a moth are among the most striking examples of metamorphosis. Because butterflies and moths pass through four distinct stages, scientists consider them as examples of complete metamorphosis. The first stage is the egg in which the embryo forms. Eggs of butterflies and many moths are deposited on plants that will provide food for the next stage, called the larva.

The larva

of a butterfly or a moth is also known as a caterpillar. Caterpillars may be hairy, spiny, or smooth-skinned. They may be a single color or have striking color patterns. Caterpillars differ from the adult in having suckerlike prolegs on the abdomen and chewing mouthparts. The adult lacks prolegs and has sucking mouthparts. In addition, caterpillars have no wings.

Caterpillars grow rapidly, molting (shedding) their outer skins several times. After about a month in the larval stage, a butterfly caterpillar deposits a pad of silk on a twig or other support and attaches itself there. Then it molts once more to enter the third stage of metamorphosis, the pupa. Many moths form a silken cocoon before molting to the pupal stage. Other moths pupate in underground cells or in plant stems.

The pupa.

During the pupal stage, the developing butterfly or moth is inactive. The butterfly pupa, also called a chrysalis, is protected only by its abdomen. Within the pupal case, larval structures are replaced by those of the adult. Usually there is another period of inactivity, known as diapause, in which even these changes cease. The pupal stage is ideal for passing periods of environmental extremes, such as drought or winter. The pupal stage may last only a few days, or several months, depending on climate and species.

The adult.

The adult insect, also called the imago, emerges by pushing against its pupal casing, which splits open. The adult crawls onto a twig or other support and pumps blood into its shrunken wings until they are full-size and strong. It then flies away to feed on liquid food, such as flower nectar or tree sap, and carry out its reproductive functions.

Other examples.

Beetles, flies, bees, wasps, ants, and many other insects exhibit complete metamorphosis. Some insects, including grasshoppers and cockroaches, have only three stages: egg, nymph (larva), and adult. This type of metamorphosis, which lacks the pupal stage, is called incomplete metamorphosis.

Metamorphosis occurs in other animal groups as well. Among vertebrates (animals that have backbones), frogs are the best-known example. They lay eggs in water, and the eggs hatch into legless tadpoles with tails and gills. A tadpole gradually develops into a four-legged frog, as lungs replace the gills and the tail disappears. Unlike tadpoles, which must live in water, frogs can survive both in and out of water.