Metamorphosis

Metamorphosis comes from a Greek word meaning "transformation," and is the term biologists use to describe the extreme changes that some organisms go through when they pass from an egg to a adult. A caterpillar turning into a butterfly is an example of complete metamorphosis. Metamorphosis often gives an organism some type of competitive advantage and usually occurs in organisms with short life spans like amphibians, some fish, and various invertebrates (animals without a backbone), especially insects.

Metamorphosis means much more than a physical change, however. In the process called complete metamorphosis, major differences occur not only in outward appearance but in an organism's internal organs and processes as well. Sometimes, an adult or mature organism has an entirely different set of cells and organs compared to what it had in an earlier stage of its development. Many organisms look different as adults compared to what they looked like when just born or very young. For example, a crowing adult rooster does not resemble the chirping, yellow ball of fluff that it was as a chick. This is not an example of metamorphosis, but simply a difference in size and an elaboration of certain characteristics. For metamorphosis to occur, an organism must go through at least three stages of development during which it changes radically both inside and out.

Metamorphosis does not happen in the life cycles of what are called "higher animals," such as dogs, cats, or human beings. Rather, metamorphosis occurs only in certain "lower animals" like ants, butterflies, sea urchins, and frogs. During the most dramatic example of complete metamorphosis, an organism passes through four distinct stages of development and ends up being a completely different type of organism.

The butterfly is probably the best-known example of an organism whose life cycle undergoes complete metamorphosis. In its embryonic or egg stage, it is deposited as an egg on a green plant that will serve as its food when it hatches. Its second stage is its feeding time and is called its larva stage. A larva is an insect that is in its wormlike stage. This stage begins when the egg hatches. In the case of a butterfly, what emerges from the egg is a caterpillar. It may be hairy or smooth and have distinct markings or little color according to what species of butterfly its parent was. At this point in its life, the larva or caterpillar has legs, and chewing mouthparts but no wings, and is best described as an eating machine. It looks more like a worm than an insect that will fly. The caterpillar eats and continues to grow and go through several molts (the shedding of its outer skin). As it grows, its body parts continue to be rearranged and modified and it finally matures into an adult caterpillar. At this point, it is ready to enter the third stage of complete metamorphosis called its pupa stage or cocoon stage. When ready, the larva attaches itself to a branch or a twig and forms a protective covering around itself. Butterflies form a hard, shiny shell called a chrysalis that hangs suspended from a twig, while moths spin their coverings out of silk and wrap themselves almost flat against the twig. It is during this pupal or resting stage that the larva changes into the adult butterfly it will eventually become. Inside the chrysalis or cocoon, most of the larva's cells are broken down and new tissues and organs begin to develop. Eventually, an entirely new and different organism is created using the raw materials left by the old one. When this transformation is finished, the chrysalis or cocoon breaks open and the adult butterfly emerges. After its shrunken wings stretch and fill with blood, they soon are strong enough for the butterfly to fly away. This is its final stage—called its adult stage. As an adult, the insect will eventually reproduce and lay its eggs, beginning the cycle and metamorphosis once more.

The often startling aspect about metamorphosis is how suddenly and dramatically one form of life is changed into another that seems completely different. A butterfly in its larva stage crawls about and eats leaves, while as an adult it flies from flower to flower and sips nectar. The same transformation occurs for some amphibians. An example is frog eggs that develop into swimming tadpoles and breathe with gills like a fish. After they change into an adult, nearly every organ has changed. The tadpoles' tails are absorbed into the legs of a frog and they breathe with lungs. For all organisms that go through metamorphosis (beetles, flies, ants, bees, and wasps as well as butterflies and frogs), their hormones begin the process and keep it going.

Unlike butterflies and frogs, some insects go through only a three-stage process (egg, nymph or larva, and adult) called incomplete metamorphosis. In this abbreviated version, the pupa stage is omitted and the changes are more gradual. Grasshoppers, crickets, cockroaches, and termites go through incomplete metamorphosis. Even by undergoing only incomplete metamorphosis, it is believed that species that live entirely different lives as a young organism and as an adult have an advantage over those who do not. For example, a species that is slow-moving and limited in range in one stage of its life (such as a caterpillar) can suddenly move around quickly as an adult and can therefore lay its eggs far away in a more advantageous habitat. Another advantage is that overall competition is reduced, since at one stage an organism is seeking one certain type of food, and in a later stage it eats something entirely different.

[See alsoAmphibians; Insects; Larva; Life Cycle ]