Metamorphosis
Metamorphosis
Metamorphosis is the transition in overall body pattern that occurs during the life history of some animals following birth or hatching. Two well known examples are the development of caterpillars into butterflies and tadpoles into frogs.
Metamorphosis is considered an indirect form of development, in that a metamorphic animal passes through morphologically distinct stages before reaching the adult form. In contrast, humans and many other animals undergo direct development, in that the young and old resemble one another, except in size and sexual maturity. Metamorphosis occurs in at least 17 phyla of the animal kingdom, including Porifera (sponges), Cnidaria (jellyfish and others), Platyhelminthes (flat-worms), Mollusca (mollusks), Annelida (segmented worms), Arthropoda (insects and others), Echinodermata (sea urchins and others), and Chordata (vertebrates and others). Although the term “metamorphosis” is generally not applied to plants, many plants have a developmental life cycle, called the alternation of generations, which is also characterized by a dramatic change in overall body pattern.
General features
In general, cells in the different parts of a multicellular organism all have the same genes, although only some of these genes are expressed (translated into proteins) in any given cell. At the molecular level, highly regulated temporal and spatial changes in gene expression causes metamorphosis in all animals. Thus, in the case of a butterfly, a very simple model of metamorphosis is that one family of genes is expressed in the larva (caterpillar), a second family of genes in the pupa, and a third family of genes in the imago (adult). Such a model provides a framework for studies of metamorphosis, although there is clearly much more to metamorphosis than implied by this simple model.
Metamorphosis is associated with adaptive changes in the way an organism interacts with its environment, and this may be why it evolved independently in so many different phyla of the animal
kingdom. For example, adult amphibians (Chordata phylum) often eat very different foods than their larvae. Thus, adults and larvae do not compete for food, a limiting resource in many environments. A second example of the adaptive significance of metamorphosis is in barnacles (Arthropoda phylum). Adult barnacles are sessile, but the larvae are free-swimming. Thus, the dispersal of larvae gives adults the opportunity to colonize new habitats, where the local environment might be more favorable.
Environmental cues often trigger hormonal changes in an animal that lead to metamorphosis. For example, many insects enter a dormant stage of development during the winter and often will not metamorphose unless exposed to low temperatures. Light is another important environmental cue which triggers metamorphosis. In one well known case, the length of the light period in a light-dark cycle controls metamorphosis of fruit fly pupae into adults.
Insects
Some of the best known cases of metamorphosis occur in insects, a class of the Arthropoda phylum. There are about a half million known species of insects, and great diversity in the way different insects develop. According to one classification scheme based on metamorphosis, insects are classified as Ametabola, Hemimetabola, or Holometabola.
The Ametabola do not undergo metamorphosis. This is an evolutionary primitive condition and is exemplified by insects such as the bristletails and springtails. During development, these insects increase in size, but do not undergo distinct changes in form. In general, the Ametabola do not have wings.
The Hemimetabola undergo gradual metamorphosis. This is exemplified by insects such as the dragonflies, termites, roaches, and grasshoppers. In the Hemimetabola, a form called the nymph hatches from the egg. Nymphs lack wings, but have compound eyes and otherwise resemble the adult form, except they are smaller. The wings of the Hemimetabola grow gradually during a series of molts, developmental periods in which the cuticular exoskeleton is shed, allowing for growth.
The Holometabola undergo complete metamorphosis. This is exemplified by insects such as moths, butterflies, wasps, and flies. In the Holometabola, a worm-like larva with short legs, no wings, and simple eyes, hatches from the egg. As in the Hemimetabola, the larva increases in size through a series of molts. Eventually, the larva develops into a pupa inside a cocoon. The pupa is often considered a resting stage and can often survive in unfavorable environments. Eventually, the pupa metamorphoses into an adult. In this process, the pupa resorbs larval organs and uses them as nutrients while special groups of cells, called imaginal discs, form and reshape the insect. The adult typically has wings, compound eyes, legs, antennae, and sexual organs.
Hormones
Hormones have an important role in insect metamorphosis. In many species, two classes of hormones, molting hormones (made by the prothoracic glands) and juvenile hormones (made by the corpora allata) act together to control metamorphosis. Each regulates the expression of different genes, so that a change in their relative concentrations causes metamorphosis, the development of different body patterns.
The two well known molting hormones are ecdysone and 20-hydroxyecdysone. These control the molting of larvae prior to metamorphosis and their level remains relatively constant during development. There are three known juvenile hormones. These gradually decrease in concentration during the many stages of larval molting. When their concentration falls below a critical level, the larva transforms into a pupa. Then, production of juvenile hormones ceases and the pupa metamorphoses into an adult.
Interestingly, the leaves of some trees, such as hemlock, make a chemical similar to the juvenile hormones of insects. When larvae (caterpillars) feed on these leaves, they cannot metamorphose into the adult form. Apparently, this is a chemical defense mechanism some trees use to prevent leaf-feeding insects from reaching sexual maturity.
Amphibians
Metamorphosis has also been extensively studied in amphibians, a class of vertebrates which includes frogs, toads, and salamanders. “Amphibian” means dual (amphi-) life form (-bian) and refers to the typical life history of these animals, in which an aquatic larva metamorphoses into a terrestrial adult. The reptiles, such as turtles, lizards, and snakes, is another class of vertebrates whose species superficially resemble adult amphibians, but do not undergo metamorphosis.
Metamorphosis differs in the many different amphibian species. In frog development, the eggs hatch and give rise to tadpoles, small aquatic larvae that have external gills and are mainly vegetarian. As the tadpole grows, internal gills and limbs form. Several significant changes occur during metamorphosis into the adult, including growth of a large mouth and tongue, loss of gills, formation of lungs, growth of the front legs, and resorption of the tail. Numerous biochemical changes accompany these morphological changes, such as synthesis of a new visual pigment in the eyes and a new oxygen-binding hemoglobin protein in the blood. The adult is mainly insectivorous and partly terrestrial.
Interestingly, the sexually mature adults of some amphibians, such as the axolotl, have a larval morphology. The retention of larval or juvenile characteristics in adulthood is defined as neoteny. Neoteny is apparently caused by a genetic mechanism which uncouples development of body cells and the development of the sexual organs. Although neoteny is most apparent in amphibians, because they are normally metamorphic, changes in developmental timing may underlie the evolution of many species, including humans.
KEY TERMS
Alternation of generations— General feature of the life cycle of many plants, characterized by the occurrence of multiple reproductive forms which often have very different overall body patterns.
Gene— A discrete unit of inheritance, represented by a portion of DNA located on a chromosome. The gene is a code for the production of a specific kind of protein or RNA molecule, and therefore for a specific inherited characteristic.
Gene expression— Molecular process in which a gene is transcribed into a specific RNA (ribonucleic acid), which is then translated into a specific protein.
Hormone— Chemical regulator of physiology, growth, or development which is typically synthesized in one region of the body and active in another and is typically active in low concentrations.
Imago— Adult form of an insect that develops from a larva and often has wings.
Larva— Immature form (wormlike in insects; fishlike in amphibians) of a metamorphic animal which develops from the embryo and differs radically from the adult.
Molting— Shedding of the outer layer of an animal, such as the cuticle during growth of insect larvae.
Neoteny— Retention of larval or juvenile characteristics in a sexually mature adult.
Hormones
As in insects, a complex interaction of hormones in the amphibian larva precipitates metamorphosis. Ultimately, two major classes of hormones act together to control amphibian metamorphosis: the thyroid hormones (made by the thyroid gland) and prolactin (made by the pituitary gland). Thyroid hormones function somewhat like the molting hormones of insects, in that an increase of their concentration relative to prolactin leads to metamorphosis of the larva into the adult. Prolactin functions somewhat like the juvenile hormones of insects, in that it tempers the action of the thyroid hormones. In most species, thyroid hormones increase dramatically in concentration during metamorphosis and this stimulates resorption of certain larval organs and differentiation of new adult organs.
Developmental biologists often investigate amphibian metamorphosis by experimentally manipulating hormone levels. For example, injection of thyroxine into a young larva can induce metamorphosis, although the injection must be at an appropriate stage of larval development and injection of high levels can lead to developmental abnormalities. If the thyroid gland is removed from a larva, it will not metamorphose into the adult form; moreover, a larva without a thyroid will metamorphose into an adult if thyroid tissue is implanted.
The relative ease with which these and other experimental manipulations of hormone levels can alter metamorphosis indicates that hormones have a profound effect on development. It also indicates that the endocrine system is relatively malleable. These two features suggest that natural selection may dramatically affect the course of animal evolution by altering the endocrine system.
Resources
BOOKS
Carde, Ring, and Vincent H. Resh, eds. Encyclopedia of Insects. San Diego, CA: Academic Press, 2003.
Chu, H. F., and L.K. Cutkomp. How to Know the Immature Insects. 2nd ed. Burr Ridge, IL: McGraw-Hill Higher Education, 1992.
Fox, H. Amphibian Morphogenesis. Totowa, NJ: Humana Press, 1984.
Quiri, P.R. Metamorphosis. Danbury, CT: Franklin Watts, Inc., 1991.
OTHER
Cornell University: New York State Agricultural Experiment Station. “Insect Biology and Ecology: A Primer” <http://www.nysaes.cornell.edu/ent/biocontrol/info/primer.html> (accessed December 2, 2006).
North Dakota State University. “Insect Growth and Metamorphosis” <http://www.ndsu.nodak.edu/entomology/topics/growth.htm> (accessed December 4, 2006).
Peter A. Ensminger
Metamorphosis
Metamorphosis
Metamorphosis is the transition in overall body pattern that occurs during the life history of some animals following birth or hatching. Two well-known examples are the development of caterpillars into butterflies and tadpoles into frogs .
Metamorphosis is considered an indirect form of development, in that a metamorphic animal passes through morphologically distinct stages before reaching the adult form. In contrast, humans and many other animals undergo direct development, in that the young and old resemble one another, except in size and sexual maturity. Metamorphosis occurs in at least 17 phyla of the animal kingdom, including Porifera (sponges ), Cnidaria (jellyfish and others), Platyhelminthes (flat worms), Mollusca (mollusks ), Annelida (segmented worms ), Arthropoda (insects and others), Echinodermata (sea urchins and others), and Chordata (vertebrates and others). Although the term "metamorphosis" is generally not applied to plants, many plants have a developmental life cycle, called the alternation of generations, which is also characterized by a dramatic change in overall body pattern.
General features
In general, cells in the different parts of a multicellular organism all have the same genes, although only some of these genes are expressed (translated into proteins ) in any given cell. At the molecular level, highly regulated temporal and spatial changes in gene expression causes metamorphosis in all animals. Thus, in the case of a butterfly, a very simple model of metamorphosis is that one family of genes is expressed in the larva (caterpillar), a second family of genes in the pupa, and a third family of genes in the imago (adult). Such a model provides a framework for studies of metamorphosis, although there is clearly much more to metamorphosis than implied by this simple model.
Metamorphosis is associated with adaptive changes in the way an organism interacts with its environment, and this may be why it evolved independently in so many different phyla of the animal kingdom. For example, adult amphibians (Chordata phylum) often eat very different foods than their larvae. Thus, adults and larvae do not compete for food, a limiting resource in many environments. A second example of the adaptive significance of metamorphosis is in barnacles (Arthropoda phylum). Adult barnacles are sessile, but the larvae are free-swimming. Thus, the dispersal of larvae gives adults the opportunity to colonize new habitats, where the local environment might be more favorable.
Environmental cues often trigger hormonal changes in an animal that lead to metamorphosis. For example, many insects enter a dormant stage of development during the winter and often will not metamorphose unless exposed to low temperatures. Light is another important environmental cue which triggers metamorphosis. In one well known case, the length of the light period in a light-dark cycle controls metamorphosis of fruit fly pupae into adults.
Insects
Some of the best known cases of metamorphosis occur in insects, a class of the Arthropoda phylum. There are about a half million known species of insects, and great diversity in the way different insects develop. According to one classification scheme based on metamorphosis, insects are classified as Ametabola, Hemimetabola, or Holometabola.
The Ametabola do not undergo metamorphosis. This is an evolutionary primitive condition and is exemplified by insects such as the bristletails and springtails . During development, these insects increase in size, but do not undergo distinct changes in form. In general, the Ametabola do not have wings.
The Hemimetabola undergo gradual metamorphosis. This is exemplified by insects such as the dragonflies , termites , roaches, and grasshoppers . In the Hemimetabola, a form called the nymph hatches from the egg. Nymphs lack wings, but have compound eyes and otherwise resemble the adult form, except they are smaller. The wings of the Hemimetabola grow gradually during a series of molts, developmental periods in which the cuticular exoskeleton is shed, allowing for growth.
The Holometabola undergo complete metamorphosis. This is exemplified by insects such as moths , butterflies, wasps , and flies . In the Holometabola, a worm-like larva with short legs, no wings, and simple eyes, hatches from the egg. As in the Hemimetabola, the larva increases in size through a series of molts. Eventually, the larva develops into a pupa inside a cocoon. The pupa is often considered a resting stage and can often survive in unfavorable environments. Eventually, the pupa metamorphoses into an adult. In this process, the pupa resorbs larval organs and uses them as nutrients while special groups of cells, called imaginal discs, form and reshape the insect. The adult typically has wings, compound eyes, legs, antennae, and sexual organs.
Hormones
Hormones have an important role in insect metamorphosis. In many species, two classes of hormones, molting hormones (made by the prothoracic glands ) and juvenile hormones (made by the corpora allata) act together to control metamorphosis. Each regulates the expression of different genes, so that a change in their relative concentrations causes metamorphosis, the development of different body patterns.
The two well known molting hormones are ecdysone and 20-hydroxyecdysone. These control the molting of larvae prior to metamorphosis and their level remains relatively constant during development. There are three known juvenile hormones. These gradually decrease in concentration during the many stages of larval molting. When their concentration falls below a critical level, the larva transforms into a pupa. Then, production of juvenile hormones ceases and the pupa metamorphoses into an adult.
Interestingly, the leaves of some trees, such as hemlock, make a chemical similar to the juvenile hormones of insects. When larvae (caterpillars) feed on these leaves, they cannot metamorphose into the adult form. Apparently, this is a chemical defense mechanism some trees use to prevent leaf-feeding insects from reaching sexual maturity.
Amphibians
Metamorphosis has also been extensively studied in amphibians, a class of vertebrates which includes frogs, toads , and salamanders . "Amphibian" means dual (amphi-) life form (-bian) and refers to the typical life history of these animals, in which an aquatic larva metamorphoses into a terrestrial adult. The reptiles , such as turtles , lizards, and snakes , is another class of vertebrates whose species superficially resemble adult amphibians, but do not undergo metamorphosis.
Metamorphosis differs in the many different amphibian species. In frog development, the eggs hatch and give rise to tadpoles, small aquatic larvae that have external gills and are mainly vegetarian. As the tadpole grows, internal gills and limbs form. Several significant changes occur during metamorphosis into the adult, including growth of a large mouth and tongue, loss of gills, formation of lungs, growth of the front legs, and resorption of the tail. Numerous biochemical changes accompany these morphological changes, such as synthesis of a new visual pigment in the eyes and a new oxygen-binding hemoglobin protein in the blood . The adult is mainly insectivorous and partly terrestrial.
Interestingly, the sexually mature adults of some amphibians, such as the axolotl, have a larval morphology. The retention of larval or juvenile characteristics in adulthood is defined as neoteny. Neoteny is apparently caused by a genetic mechanism which uncouples development of body cells and the development of the sexual organs. Although neoteny is most apparent in amphibians, because they are normally metamorphic, changes in developmental timing may underlie the evolution of many species, including humans.
Hormones
As in insects, a complex interaction of hormones in the amphibian larva precipitates metamorphosis. Ultimately, two major classes of hormones act together to control amphibian metamorphosis: the thyroid hormones (made by the thyroid gland) and prolactin (made by the pituitary gland). Thyroid hormones function somewhat like the molting hormones of insects, in that an increase of their concentration relative to prolactin leads to metamorphosis of the larva into the adult. Prolactin functions somewhat like the juvenile hormones of insects, in that it tempers the action of the thyroid hormones. In most species, thyroid hormones increase dramatically in concentration during metamorphosis and this stimulates resorption of certain larval organs and differentiation of new adult organs.
Developmental biologists often investigate amphibian metamorphosis by experimentally manipulating hormone levels. For example, injection of thyroxine into a young larva can induce metamorphosis, although the injection must be at an appropriate stage of larval development and injection of high levels can lead to developmental abnormalities. If the thyroid gland is removed from a larva, it will not metamorphose into the adult form; moreover, a larva without a thyroid will metamorphose into an adult if thyroid tissue is implanted.
The relative ease with which these and other experimental manipulations of hormone levels can alter metamorphosis indicates that hormones have a profound effect on development. It also indicates that the endocrine system is relatively malleable. These two features suggest that natural selection may dramatically affect the course of animal evolution by altering the endocrine system.
Resources
books
Carde, Ring, and Vincent H. Resh, eds. Encyclopedia of Insects. San Diego, CA: Academic Press, 2003.
Chu, H.F., and L.K. Cutkomp. How to Know the Immature Insects. 2nd ed. Burr Ridge, IL: McGraw-Hill Higher Education, 1992.
Fox, H. Amphibian Morphogenesis. Totowa, NJ: Humana Press, 1984.
Quiri, P.R. Metamorphosis. Danbury, CT: Franklin Watts, Inc., 1991.
Peter A. Ensminger
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Alternation of generations
—General feature of the life cycle of many plants, characterized by the occurrence of multiple reproductive forms which often have very different overall body patterns.
- Endocrine system
—A system of glands and other structures that secrete hormones to regulate certain body functions such as growth and development of sex characteristics.
- Evolution
—In biology, inheritable changes occurring over a time span greater than one generation.
- Gene
—A discrete unit of inheritance, represented by a portion of DNA located on a chromosome. The gene is a code for the production of a specific kind of protein or RNA molecule, and therefore for a specific inherited characteristic.
- Gene expression
—Molecular process in which a gene is transcribed into a specific RNA (ribonucleic acid), which is then translated into a specific protein.
- Hormone
—Chemical regulator of physiology, growth, or development which is typically synthesized in one region of the body and active in another and is typically active in low concentrations.
- Imago
—Adult form of an insect which develops from a larva and often has wings.
- Larva
—Immature form (wormlike in insects; fishlike in amphibians) of a metamorphic animal which develops from the embryo and differs radically from the adult.
- Molting
—Shedding of the outer layer of an animal, such as the cuticle during growth of insect larvae.
- Natural selection
—The differential survival and reproduction of organisms, producing evolutionary change in populations.
- Neoteny
—Retention of larval or juvenile characteristics in a sexually mature adult.
metamorphosis
Stories of such transformations were very popular in the Hellensitic period; several authors are known to have complied collections of them, although all such collections are now lost, with the exception of some excerpts which have survived under the name of a writer of the early Roman empire, Antoninus Liberalis. However, the Roman poet Ovid collected about 250 transformation stories, which survive as the Metamorphoses. In Apuleius' book of the same title, more commonly known as the Golden Ass, the hero Lucius tries to uncover the secrets of witchcraft but is transformed into an ass. After a series of adventures, he is restored to human form by the goddess lsis. In all these stories the theme of metamorphosis is used to question the established boundaries between human and beast, god and mortal, animate and inanimate, thus becoming a way of exploring the limits of what humanity can do.
The following accounts of metamorphosis are best known from Ovid, and are also common in Renaissance as well as classical art. Daphne was a river nymph who was chased by the god Apollo; she asked her mother, the earth goddess, to change her form, and became a laurel tree, thus escaping the god's advances. Callisto was a princess or a nymph, who was seduced by Zeus. In some versions of her myth, she was transformed into a bear so that this could take place, but in others her metamorphosis is a punishment for her pregnancy, and she gives birth to a human son while she is a still a bear. Her son later kills her by mistake when hunting. In a particularly violent myth of transformation, Tereus leaves his wife Procne and rapes her sister, Philomela, telling Philomela that her sister is dead. To ensure Philomela's silence, he cuts out her tongue. Philomela discovers the truth and sends her sister a message woven into a tapestry; Procne then kills her son by Tereus and serves his flesh to his father. When Tereus pursues the sisters threatening to murder them, they ask the gods for help; all three are transformed into birds, Tereus becoming a hoopoe, Philomela a nightingale, and Pronce a swallow. The order of the last two is reversed by some ancient writers, because an alternative version has Procne as the victim of Tereus' silencing, and the bird forms are significant; the hoopoe, being a crested bird, is ‘royal’, while whichever sister loses her tongue must be the one who becomes the bird with the power of song who, in the words of T. S. Eliot, ‘Filled all the desert with inviolable voice’ (The Waste Land).
In all these stories, violent attempts to transgress the boundaries of the female body by rape act as the catalyst for the dissolution of the boundaries between humans and the rest of the natural world. It is, however, not only women who are the subjects of metamorphosis in a sexual context. A myth concerned with the preservation of the boundary between gods and mortals tells how, when the hunter Actaeon accidentally came upon the goddess Artemis bathing, she turned him into a stag and he was then torn apart by his own hunting dogs. In some artistic representations of his metamorphosis, seen as a particularly cruel one because he did not intend to disturb the virgin goddess, the still-human Actaeon is shown in the process of sprouting horns. Some versions of the myth take his violation of Artemis' chosen isolation further by suggesting that he wanted to marry her. In a further story of metamorphosis in the context of sexual transgression, the mortals Atalanta and Melanion make love in the sanctuary of a god, and are then punished by being turned into lions.
As well as the human/animal boundary, that between male and female is also vulnerable; in classical myth, Caenis was a girl who was loved by the god Poseidon, who changed her into the boy Caenus at her request as a gift. Myths that describe transgression do not, however, serve to sanction it in real life. The Oedipus myth, with its central character unwittingly killing his father, then marrying his mother, only to blind himself when the truth comes out, acts as a warning of what can happen if social boundaries are crossed. The fact that Oedipus puts out his own eyes when he ‘sees’ the truth is only one example of the connection in ancient myth between physical blindness and moral vision; other expressions of this are the tradition that the poet Homer was blind, and the myths of the blind seer Teiresias.
In Greek mythology, several gods had the power to change shape. For example, Dionysos was able to do this, while when the deities Poseidon and Demeter slept together they took the respective forms of a stallion and a mare. But most frequently it was the supreme god, Zeus, who used metamorphosis in the course of finding ways of disguising himself when seducing mortal women; he took Leda in the form of a swan, Europa in the form of a bull, and Danae in the form of a shower of gold. As well as confusing the objects of his sexual interest, Zeus' power of metamorphosis was supposed to protect them. One of Zeus' mortal lovers, Semele, was tricked by Zeus' jealous wife Hera into testing him by demanding that he should appear to her in his true shape; this immediately killed her, since Zeus appeared as a thunderbolt.
Some mortals have also been thought to possess the power to change shape by witchcraft or by other methods, sometimes deliberately, sometimes against their will.
Helen King
See also Greeks; mythology and the body; werewolves; witchcraft.
Metamorphosis
Metamorphosis
Metamorphosis, or a change in form, in biology means the transition from a larval stage to an adult stage. In most animals, embryonic development leads to the formation of a larval stage with very different characteristics than the adult stage. Larval morphology, or form, may be specialized for some function such as growth (which requires feeding and associated structures) or dispersal. Some larval forms, called exotrophic, feed, while others, called endotrophic , are nonfeeding. Juvenile and adult forms often live in very different environments.
Cnidarians have varying types of metamorphosis. Some species have three distinct life history stages: the planula, medusa, and polyp. The planula stage is the free-living larval stage. The medusa stage involves a single individual or a colony of individuals that act as a single free-swimming organism (examples include jellyfish and man-o-war). The polyp stage is sessile (adhered to the substrate) and may involve a single individual or a colony of individuals (examples include sea anemones and corals). Some species lack the free-swimming medusa stage. In others, the medusa is the dominant life history stage and the polyp stage is lacking completely.
Molting and metamorphosis in arthropods is controlled by environment and hormones . Insects experience no size increase in the egg, pupal (the thirdstage in the life of an insect that undergoes complete metamorphosis), or adult stages. All growth occurs during the intermediate larval or nymphal stages. Anametabolous (without change) metamorphosis occurs in the primitive insect groups Colembola, Diplura, Protura, and Thysanura. Juveniles change little except in size and proportion from egg to adult. After reaching adulthood, defined as sexual maturity, they continue to molt, adding antennal segments.
Many insects (including dragonflies, grasshoppers, and cockroaches) and crustaceans (crawfish and crabs) develop through hemimetabolous (incomplete or gradual) metamorphosis. In hemimetabolous metamorphosis, the insect egg hatches into a nymph. The nymph is similar to the adult in general morphology, only smaller. The nymph is an actively feeding stage, and as it grows it must shed its exoskeleton and produce a new, larger one. This process is called molting. In insects with hemimetabolous metamorphosis, the gonads do not mature until after the final molt.
Some insects (including flies, butterflies, wasps, and bees) have holometabolous metamorphosis (they undergo a complete metamorphosis, having distinct larval and pupal stages). There are four distinct stages in the life cycle of holometabolous insects: egg, larva, pupa, and imago (adult). The larva is segmented and wormlike. The larval stage is a feeding stage and consists of several subdivisions called instars . Each instar ends in molting, which allows the larva to grow.
The final instar ends with pupation. Prior to pupation, the animal stops feeding and the cuticle hardens and darkens to form the puparium (pupal chamber), where metamorphosis will take place. The pupa begins to darken just prior to the emergence of the imago. In the larvae of these organisms, imaginal disks (clusters of cells carried with a larva that will develop into different adult body parts) are formed. These disks will produce adult organs, but they remain quiescent, or inactive, in the embryo until the appropriate time.
Most, but not all, amphibians have a biphasic (two-phase) life history with an aquatic larval stage that metamorphoses to become an adult, a process known as indirect development. Many frogs have a free-living, aquatic larval stage as a tadpole. Near the end of the larval stage, many larval structures are reabsorbed or remodeled and adult structures begin to form.
During metamorphosis bones begin to ossify, the tail is reabsorbed, limbs form, and larval respiratory and feeding structures (including gills and a beak with keratinized—covered with a tough protein like our fingernails— mouth parts) are replaced by adult structures (including lungs and movable jaws). The digestive system is remodeled to accommodate a transition from a largely herbivorous diet to one that is strictly carnivorous .
In salamanders, the larval stage is more similar to the adult stage than is the case with frogs. Metamorphosis usually involves the replacement of larval gills with lungs; ossification of the skull, vertebral column, and limbs; and the remodeling of the tail and feeding apparatus to conform to the requirements of life on land.
Although this familiar mode of development is common among amphibians, some salamanders, caecilians, and frogs lack a free-living larval stage. In many species with a monphasic life history, a miniature version of the adult is hatched directly from the egg (direct development) in what is called ovoviparity , or birthed by the female, in what is known as viviparity .
The loss of a free-swimming larval stage has been hypothesized to release a major limit on morphological diversification in some groups of direct-developing frogs, because the pre-pattern established by larval structures is no longer present. Evidence of this morphological release can be seen in the great diversity of species and morphologies attained by some amphibian groups that have lost the free-living tadpole stage.
Andrew G. Gluesenkamp
Bibliography
Brusca, Richard G., and Gary J. Brusca. Invertebrates. Sunderland, MA: Sinauer Associates, 1990.
Gilbert, Scott F. Developmental Biology, 5th ed. Sunderland, MA: Sinauer Associates, 1997.
Gould, Stephen Jay. Ontogeny and Phylogeny. Cambridge, MA: Harvard University Press, Belknap Press, 1977.
Kalthoff, Klaus. Analysis of Biological Development, 2nd ed. Boston: McGraw-Hill, Inc., 2001.
Metamorphosis
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 ]
Metamorphosis
Metamorphosis
Metamorphosis is a series of changes through which an organism goes in developing from an early immature stage to an adult. Most people are familiar with the process, for example, by which a butterfly or moth emerges from a chrysalis (cocoon) in its adult form or a frog or toad passes through its tadpole stage.
Metamorphosis is perhaps best known among insects and amphibians (organisms such as frogs, toads, and salamanders that can live either on land or in the water). However, the process of metamorphosis has been observed in at least 17 phyla (a primary division of the animal kingdom), including Porifera (sponges), Cnidaria (jellyfish and others), Platyhelminthes (flat worms), Mollusca (mollusks), Annelida (segmented worms), Arthropoda (insects and others), Echinodermata (sea urchins and others), and Chordata (vertebrates and others).
In addition, although the term metamorphosis is generally not applied to plants, many plants do have a developmental life cycle—called the alternation of generations—which is also characterized by a dramatic change in overall body pattern.
Forms of metamorphosis
Metamorphosis in an organism is generally classified as complete or incomplete. Complete metamorphosis involves four stages: egg, larva, pupa, and adult. Consider the sequence of these stages in an insect. After a fertilized egg is laid, a wormlike larva is hatched. The larva may look like the maggot stage of a housefly or the caterpillar stage of a butterfly or moth. It is able to live on its own and secures its own food from the surrounding environment.
After a period of time, the larva builds itself some kind of protective shell such as a cocoon. The insect within the shell, now known as a pupa, is in a resting stage. It slowly undergoes a fairly dramatic change in its body structure and appearance. The energy needed for these changes comes from food eaten and stored during the larval stage.
When the process of body reorganization has been completed, the pupa breaks out of its shell and emerges in its mature adult form, also called the imago.
Words to Know
Alternation of generations: A general feature of the life cycle of many plants, characterized by the occurrence of different reproductive forms that often have very different overall body patterns.
Imago: Adult form of an insect that develops from a larva and often has wings.
Larva: Immature form (wormlike in insects; fishlike in amphibians) of a metamorphic animal that develops from the embryo and is very different from the adult.
Molting: Shedding of the outer layer of an animal, as occurs during growth of insect larvae.
Pupa: A stage in the metamorphosis of an insect during which its tissues are completely reorganized to take on their adult shape.
Incomplete metamorphosis involves only three stages, known as egg, nymph, and adult. When the fertilized egg of an insect hatches, for example, an organism appears that looks something like the adult but is smaller in size. In many cases, winged insects have not yet developed their wings, and they are still sexually immature. In this form, the insect is known as a nymph.
Eventually, the nymph reaches a stage of maturity at which it loses its outer skin (it molts) and takes on the appearance of an adult. These stages can be seen in a grasshopper, for example, which hatches from its egg as a nymph and then passes through a series of moltings before becoming a mature adult.
[See also Amphibians; Insects ]
metamorphosis
met·a·mor·pho·sis / ˌmetəˈmôrfəsəs/ • n. (pl. -pho·ses / -fəˌsēz/ ) Zool. (in an insect or amphibian) the process of transformation from an immature form to an adult form in two or more distinct stages. ∎ a change of the form or nature of a thing or person into a completely different one, by natural or supernatural means: his metamorphosis from presidential candidate to talk-show host.