Metamorphosis is a biological process A biological process is a process of a living organism. Biological processes are made up of any number of chemical reactions or other events that results in a transformation by which an animal Animals are a major group of mostly multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also physically develops Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis", which is the process that gives rise to tissues, organs and anatomy after birth Birth is the act or process of bearing or bringing forth offspring. The offspring is brought forth from the mother. Different forms of birth are oviparity, vivipary and ovovivipary or hatching, involving a conspicuous and relatively abrupt change in the animal's body With regard to living things, a body is the physical body of an individual. "Body" often is used in connection with appearance, health issues and death. The study of the workings of the body is physiology structure through cell growth The term cell growth is used in the contexts of cell development and cell division When used in the context of cell division, it refers to growth of cell populations, where one cell (the "mother cell") grows and divides to produce two "daughter cells" and differentiation In developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a single zygote to a complex system of tissues and cell types. Differentiation is a common. Some insects Insects are a class within the arthropods that have a chitinous exoskeleton, a three-part body (head, thorax, and abdomen), three pairs of jointed legs, compound eyes, and two antennae. They are among the most diverse group of animals on the planet and include more than a million described species and represent more than half of all known living, amphibians Amphibians , such as frogs, toads, salamanders, newts, and caecilians, are ectothermic (or cold-blooded) animals that metamorphose from a juvenile water-breathing form, either to an adult air-breathing form, or to a paedomorph that retains some juvenile characteristics. Proteidae (mudpuppies and waterdogs) are good examples of paedomorphic species, mollusks The Mollusca, common name molluscs or mollusks,[note 1] is a large phylum of invertebrate animals. There are around 85,000 recognized extant species of molluscs. This is the largest marine phylum, comprising about 23% of all the named marine organisms. Numerous molluscs also live in freshwater and terrestrial habitats. Molluscs are highly diverse,, crustaceans Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at 0.1 mm (0.004 in), to the Japanese spider crab with a leg span of up to 14 ft (4.3 m) and a mass, Cnidarians Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are, echinoderms Echinoderms are a phylum of marine animals. Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone. Aside from the problematic Arkarua, the first definitive members of the phylum appeared near the start of the Cambrian period and tunicates Tunicates, also known as urochordates, are members of the subphylum Tunicata or Urochordata, a group of underwater saclike filter feeders with incurrent and excurrent siphons that is classified within the phylum Chordata. While most tunicates live on the ocean floor and are commonly known as sea squirts and sea pork, others – such as salps, undergo metamorphosis, which is usually accompanied by a change of habitat A habitat is an ecological or environmental area that is inhabited by a particular species of animal, plant or other type of organism. It is the natural environment in which an organism lives, or the physical environment that surrounds (influences and is utilized by) a species population.[citation needed] or behavior Behavior, or behaviour , refers to the actions of an organism or system, usually in relation to its environment, which includes the other organisms or systems around as well as the physical environment. It is the response of the organism or system to various stimuli or inputs, whether internal or external, conscious or subconscious, overt or.
Scientific usage of the term is exclusive, and is not applied to general aspects of cell growth The term cell growth is used in the contexts of cell development and cell division When used in the context of cell division, it refers to growth of cell populations, where one cell (the "mother cell") grows and divides to produce two "daughter cells", including rapid growth spurts In humans puberty is the process of physical changes by which a child's body becomes an adult body capable of reproduction. Puberty is initiated by hormone signals from the brain to the gonads . In response, the gonads produce a variety of hormones that stimulate the growth, function, or transformation of brain, bones, muscle, skin, breasts, and. References to "metamorphosis" in mammals Mammals are a class of vertebrate, air-breathing animals whose females are characterized by the possession of mammary glands while both males and females are characterized by hair and/or fur, three middle ear bones used in hearing, and a neocortex region in the brain. Some mammals have sweat glands, but most do not are imprecise and only colloquial, but historically idealist ideas of transformation and monadology The Monadology is one of Gottfried Leibniz’s best known works representing his later philosophy. It is a short text which sketches in some 90 paragraphs a metaphysics of simple substances, or monads, as in Goethe's Johann Wolfgang von Goethe (German pronunciation: [ˈjoːhan ˈvɔlfɡaŋ fɔn ˈɡøːtə] , 28 August 1749 – 22 March 1832) was a German writer and polymath. Goethe's works span the fields of poetry, drama, literature, theology, philosophy, pantheism, and science. His magnum opus, lauded as one of the peaks of world literature, is the two-part Metamorphosis of Plants, influenced the development of ideas of evolution Evolution is the change in the inherited traits of a population of organisms through successive generations. After a population splits into smaller groups, these groups evolve independently and may eventually diversify into new species. Ultimately, life is descended from a common ancestory through a long series of these speciation events,.
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Etymology
The word "metamorphosis" derives from Greek Greek , an independent branch of the Indo-European family of languages, is the language of the Greeks. Native to the southern Balkans, it has the longest documented history of any Indo-European language, spanning 34 centuries of written records. In its ancient form, it is the language of classical ancient Greek literature and the New Testament of μεταμόρφωσις, "transformation, transforming"[1], from μετα- (meta- Meta- , is a prefix used in English (and other Greek-owing languages) to indicate a concept which is an abstraction from another concept, used to complete or add to the latter), "change" + μορφή (morphe) "form"[2].
Insect metamorphosis
Metamorphosis usually proceeds in distinct stages, starting with larva A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle or nymph In biology, a nymph is the immature form of some invertebrates, particularly insects, which undergoes gradual metamorphosis before reaching its adult stage. Unlike a typical larva, a nymph's overall form already resembles that of the adult. In addition, while a nymph moults it never enters a pupal stage. Instead, the final moult results in an, optionally passing through pupa A pupa is the life stage of some insects undergoing transformation. The pupal stage is found only in holometabolous insects, those that undergo a complete metamorphosis, going through four life stages; embryo, larva, pupa and imago. (For a list of such insects see Holometabolism), and ending as adult An adult is a human being or living organism that is of relatively mature age, typically associated with sexual maturity and the attainment of reproductive age. In human context, the term has other subordinate meanings associated to social and legal concepts, for example a legal adult is a legal concept for a person who has attained the age of. There are two main types of metamorphosis in insects, hemimetabolism Hemimetabolism or hemimetaboly, also called incomplete metamorphosis, is a term used to describe the mode of development of certain insects that includes three distinct stages: the egg, nymph, and the adult stage, or imago. These groups go through gradual changes; there is no pupal stage. The nymph often somewhat resemble the adult stage but and holometabolism Holometabolism, also called complete metamorphism, is a term applied to insect groups to describe the specific kind of insect development which includes four life stages - as an embryo, a larva, a pupa and an imago or adult. Holometabolism is a monophyletic trait that all insects in the Endopterygota superorder have. This type of development gives.
Incomplete metamorphosis in the grasshopper with different instar nymphsThe immature stages of a species that metamorphosises are usually called larvae A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle, and in these stages may grow quite quickly. But in the complex metamorphosis of many insect species, only the first stage is called a larva and sometimes even that bears a different name; the distinction depends on the nature of the metamorphosis.
In hemimetabolism, the development of larva often proceeds in repeated stages of growth and ecdysis Ecdysis is the moulting of the cuticula in arthropods and related groups . Since the cuticula of these animals is also the skeletal support (the exoskeleton) of the body and is inelastic, it is shed during growth and a new, larger covering is formed. The old, empty exoskeleton is called an exuvia (or "exuvium") (moulting); these stages are called instars An instar is a developmental stage of arthropods, such as insects, between each moult , until sexual maturity is reached. Arthropods must shed the exoskeleton in order to grow or assume a new form. Differences between instars can often be seen in altered body proportions or changes in the number of body segments. Some arthropods can continue to. The juvenile forms closely resemble adults, but are smaller and, if the adult has wings, lack wings. This process is also known as "simple", "gradual" or "incomplete" metamorphosis. The differences between juveniles in different instars are small, often just differences in body proportions and the number of segments.
In holometabolism, the larvae differ markedly from the adults. Insects which undergo holometabolism pass through a larval stage, then enter an inactive state called pupa A pupa is the life stage of some insects undergoing transformation. The pupal stage is found only in holometabolous insects, those that undergo a complete metamorphosis, going through four life stages; embryo, larva, pupa and imago. (For a list of such insects see Holometabolism), or chrysalis, and finally emerge as adults. Holometabolism is also known as "complete" and "complex" metamorphosis. Whilst inside the pupa, the insect will excrete digestive juices, to destroy much of the larva's body, leaving a few cells intact. The remaining cells will begin the growth of the adult, using the nutrients from the broken down larva. This process of cell death is called histolysis, and cell regrowth histogenesis Histogenesis is the formation of different tissues from undifferentiated cells. These cells are constituents of three primary germ layers, the endoderm, mesoderm, and ectoderm. The science of the microscopic structures of the tissues formed within histogenesis is termed histology.
Whether the insect spends more time in its adult stage or in its juvenile form depends on the species. Notable examples are the mayfly Mayflies are insects which belong to the Order Ephemeroptera . They have been placed into an ancient group of insects termed the Palaeoptera, which also contains dragonflies and damselflies. They are aquatic insects whose immature stage (called naiad or, colloquially, nymph) usually lasts one year in freshwater. The adults are short-lived, from a, whose non-eating, adult stage lives for one day, and the cicada A cicada is an insect of the order Hemiptera, suborder Auchenorrhyncha, in the superfamily Cicadoidea, with large eyes wide apart on the head and usually transparent, well-veined wings. There are about 2,500 species of cicada around the world, and many remain unclassified. Cicadas live in temperate to tropical climates where they are among the, whose juvenile stage live underground for 13 or 17 years. These species have incomplete metamorphosis. Typically, though not exclusively, species in which the adult form outlives the juvenile form undergo complex metamorphosis.
Many observations have indicated that programmed cell death Programmed cell-death is death of a cell in any form, mediated by an intracellular program. In contrast to necrosis, which is a form of cell-death that results from acute tissue injury and provokes an inflammatory response, PCD is carried out in a regulated process which generally confers advantage during an organism's life-cycle. PCD serves plays a considerable role during physiological processes of multicellular organisms, particularly during embryogenesis Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus. It starts with the fertilization of the ovum by sperm. The fertilized ovum is referred to as a zygote. The zygote undergoes rapid mitotic divisions with no significant growth (a process known as cleavage) and cellular differentiation, leading and metamorphosis.
Hormonal control
Insect Insects are a class within the arthropods that have a chitinous exoskeleton, a three-part body (head, thorax, and abdomen), three pairs of jointed legs, compound eyes, and two antennae. They are among the most diverse group of animals on the planet and include more than a million described species and represent more than half of all known living growth and metamorphosis are controlled by hormones A hormone is a chemical released by a cell in one part of the body, that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. It is essentially a chemical messenger that transports a signal from one cell to another. All multicellular organisms produce hormones; synthesized by endocrine glands Endocrine glands are glands of the endocrine system that secrete their products, hormones, directly into the blood rather than through a duct. The main endocrine glands include the pituitary gland, pancreas, ovaries, testes, thyroid gland, and adrenal glands. The hypothalamus is a neuroendocrine organ. Other organs which are not so well known for near the front of the body.
neurosecretory cells of an insect's brain The brain is the center of the nervous system in all vertebrate, and most invertebrate, animals. Some primitive animals such as jellyfish and starfish have a decentralized nervous system without a brain, while sponges lack any nervous system at all. In vertebrates, the brain is located in the head, protected by the skull and close to the primary secrete a hormone, the prothoracicotropic hormone that activates prothoracic glands, which secrete a second hormone, usually Ecdysone (a steroid A steroid is a type of organic compound that contains a specific arrangement of four rings that are joined to each other. Examples of steroids include cholesterol, the sex hormones estradiol and testosterone, and the anti-inflammatory drug dexamethasone), that induces metamorphosis.
Moreover, the corpora allata, a retrocerebral organ produces the juvenile hormone Juvenile hormones are a group of acyclic sesquiterpenoids that regulate many aspects of insect physiology. JHs regulate development, reproduction, diapause, and polyphenisms, whose effect is to prevent the development of adult characteristics while allowing ecdysis Ecdysis is the moulting of the cuticula in arthropods and related groups . Since the cuticula of these animals is also the skeletal support (the exoskeleton) of the body and is inelastic, it is shed during growth and a new, larger covering is formed. The old, empty exoskeleton is called an exuvia (or "exuvium"). Therefore, the insect is subject to a series of molting, controlled by Ecdysone, until the production of juvenile hormone ceases and metamorphosis occurs.
Amphibian metamorphosis
File:Greenfrog-male-tadpoles.jpg A male northern green frog, [[Rana Clamitans Melanota]] with the year before's tadpoles Just before metamorphosis, only 24 hours are needed to reach the stage in the next picture Almost functional common frog with some remains of the gill sac and a not fully developed jawIn typical amphibian development the eggs are laid in water and the larvae are adapted to an aquatic lifestyle. Both frogs, toads, and newts hatch from the egg as larvae with external gills. After that the newt larvae are starting a predatory lifestyle, the tadpoles are mostly scraping off food from surfaces with their horny tooth ridges. Metamorphosis in amphibians is regulated by thyroxin concentration in the blood, that stimulates metamorphosis and prolactin Prolactin or Luteotropic hormone (LTH) is a peptide hormone discovered by Dr. Henry Friesen, primarily associated with lactation. In breastfeeding, the act of an infant sucking the nipple stimulates the production of prolactin, which fills the breast with milk via a process called lactogenesis, in preparation for the next feed. Oxytocin, another that counteracts its effect. The specific events are dependent of the threshold values for different tissues. Because most of the embryonic development is outside the parental body the development is subject to a lot of adaptations due to specific ecological circumstances. For this reason tadpoles can have horny ridges for teeth, whiskers and fins. They also make use of the lateral line organ. After metamorphosis these organs become redundant and will be resorbed by controlled cell death called apoptosis. The amount of adaptations to specific ecological circumstances is amazing and there are still discoveries being made.
Frogs and toads
With frogs and toads the external gills of the newly hatched tadpole are covered in a few days with a gill sac and lungs are quickly formed. Under the gill sac the front legs are formed and the hindlegs are also visible a few days later. Then there usually is a longer stage where the tadpole is growing on a vegetarian diet. The tadpoles use a spiral shaped relatively long gut to digest that diet.
After that, very quick changes in the body can be observed when the lifestyle of the frog changes completely. The trunk shaped mouth with horny tooth ridges is resorbed together with the spiral gut. The animal develops a big jaw, the gills disappear as well as the gill sac. The eyes grow at a very fast rate as well as the legs, a tongue is formed and all this is completed with the associated changes in the neural networks (development of stereoscopic vision, loss of the lateral line system etc.) All of this can happen in about a day, so it is truly a metamorphosis. A few days later also the tail is reabsorbed, due to the higher thyroxin concentrations required for tail resorption.
Newts
The large external gills of the crested newtIn newts there is not really such a thing as metamorphosis, because the larvae start feeding as predators always and continue to do so in the adult stage. The gills of newts are never covered by a gill sac and will be resorbed only just before the animal leaves the water. Just like in tadpoles the lungs are functional very soon also, but they don't make as much use of them as tadpoles do. Newts often have an aquatic phase in spring and summer and a land phase in winter. For adaptation to a water phase prolactin is the required hormone and for adaptations to the land phase thyroxin. The external gills do not return in subsequent aquatic phases because they are completely absorbed upon leaving the water for the first time.
Metamorphosis in fish and invertebrate aquatic animals
Little known is that also fish, i.e. bony fish, undergo metamorphosis. Fish metamorphosis is typically under strong control by thyroid hormone. Examples include the agnatha, salmon, and lamprey, which must change from a freshwater to saltwater lifestyle (diadromous). Additionally, the flatfish begins its life bilaterally symmetrical, and one eye must move to join the other side of the fish in its adult form.
See also
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References
- Davies, R.G., "Outlines of Entomology", Chapman and Hall: chapter 3
- Williamson D I (2003). "The Origins of Larvae", xviii + 261 pp, ISBN 1-4020-1514-3. Kluwer. Dordrecht.
External links
- Description of metamorphosis and its different forms
- Info on butterflies and butterfly metamorphosis
- Info on insect hormones
Categories: Developmental biology | Animal anatomy | Greek loanwords
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