Metamorphosis is a biological process by which an animal physically develops including birth transformation or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation.  Some insects, fish, amphibians, mollusks, crustaceans, cnidarians, echinoderms, and tunicates undergo metamorphosis, which is often accompanied by a change of nutrition source or behavior.  Animals can be divided into species that undergo complete metamorphosis ("holometaboly"), incomplete metamorphosis ("hemimetaboly"), or no metamorphosis ("ametaboly"). 
Scientific usage of the term is technically precise, and it is not applied to general aspects of cell growth, including rapid growth spurts. Generally organisms with a larva stage undergo metamorphosis, and during metamorphosis the organism loses larval characteristics.  References to "metamorphosis" in mammals are imprecise and only colloquial, but historically idealist ideas of transformation and morphology, as in Goethe's Metamorphosis of Plants , have influenced the development of ideas of evolution.
The word metamorphosis derives from Ancient Greek μεταμόρφωσις, "transformation, transforming",  from μετα- ( meta- ), "after" and μορφή (morphe), "form". 
In insects, growth and metamorphosis are controlled by hormones synthesized by endocrine glands near the front of the body (anterior). Neurosecretory cells in an insect's brain secrete a hormone, the prothoracicotropic hormone (PTTH) that activates prothoracic glands, which secrete a second hormone, usually ecdysone (an ecdysteroid), that induces ecdysis.  PTTH also stimulates the corpora allata, a retrocerebral organ, to produce juvenile hormone, which prevents the development of adult characteristics during ecdysis. In holometabolous insects, molts between larval instars have a high level of juvenile hormone, the moult to the pupal stage has a low level of juvenile hormone, and the final, or imaginal, molt has no juvenile hormone present at all.  Experiments on firebugs have shown how juvenile hormone can affect the number of nymph instar stages in hemimetabolous insects.  
Metamorphosis is iodothyronine-induced and an ancestral feature of all chordates. 
All three categories of metamorphosis can be found in the diversity of insects, including no metamorphosis ("ametaboly"), incomplete or partial metamorphosis ("hemimetaboly"), and complete metamorphosis ("holometaboly"). While ametabolous insects show very little difference between larval and adult forms (also known as "direct development"), both hemimetabolous and holometabolous insects have significant morphological and behavioral differences between larval and adult forms, the most significant being the inclusion, in holometabolus organisms, of a pupal or resting stage between the larval and adult forms.
In hemimetabolous insects, immature stages are called nymphs. Development proceeds in repeated stages of growth and ecdysis (moulting); these stages are called instars. The juvenile forms closely resemble adults, but are smaller and lack adult features such as wings and genitalia. The size and morphological differences between nymphs in different instars are small, often just differences in body proportions and the number of segments; in later instars, external wing buds form. The period from one molt to the next is called a stadium. 
In holometabolous insects, immature stages are called larvae and differ markedly from adults. Insects which undergo holometabolism pass through a larval stage, then enter an inactive state called pupa (called a "chrysalis" in butterfly species), and finally emerge as adults. 
The earliest insect forms showed direct development (ametabolism), and the evolution of metamorphosis in insects is thought to have fuelled their dramatic radiation (1,2). Some early ametabolous "true insects" are still present today, such as bristletails and silverfish. Hemimetabolous insects include cockroaches, grasshoppers, dragonflies, and true bugs. Phylogenetically, all insects in the Pterygota undergo a marked change in form, texture and physical appearance from immature stage to adult. These insects either have hemimetabolous development, and undergo an incomplete or partial metamorphosis, or holometabolous development, which undergo a complete metamorphosis, including a pupal or resting stage between the larval and adult forms. 
A number of hypotheses have been proposed to explain the evolution of holometaboly from hemimetaboly, mostly centering on whether or not the intermediate stages of hemimetabolous forms are homologous in origin to the pupal stage of holometabolous forms.
According to a 2009 study, temperature plays an important role in insect development as each individual species are found to have specific thermal windows that allow them to progress through their developmental stages. These windows are not significantly affected by ecological traits, rather, the windows are phylogenetically adapted to the ecological circumstances insects are living in. 
According to research from 2008, adult Manduca sexta is able to retain behavior learned as a caterpillar.  Another caterpillar, the ornate moth caterpillar, is able to carry toxins that it acquires from its diet through metamorphosis and into adulthood, where the toxins still serve for protection against predators. 
Many observations published in 2002, and supported in 2013 indicate that programmed cell death plays a considerable role during physiological processes of multicellular organisms, particularly during embryogenesis, and metamorphosis.   Additional research in 2019 found that both autophagy and apoptosis, the two ways programmed cell death occur, are processes undergone during insect metamorphosis. 
Below is the sequence of steps in the metamorphosis of the butterfly (illustrated):
1 – The larva of a butterfly
2 – The pupa is now spewing the thread to form chrysalis
3 – The chrysalis is fully formed
4 – Adult butterfly coming out of the chrysalis
In cephalochordata, metamorphosis is iodothyronine-induced and it could be an ancestral feature of all chordates. 
Some fish, both bony fish (Osteichthyes) and jawless fish (Agnatha), undergo metamorphosis. Fish metamorphosis is typically under strong control by the thyroid hormone. 
Examples among the non-bony fish include the lamprey. Among the bony fish, mechanisms are varied.
The salmon is diadromous, meaning that it changes from a freshwater to a saltwater lifestyle.
Many species of flatfish begin their life bilaterally symmetrical, with an eye on either side of the body; but one eye moves to join the other side of the fish – which becomes the upper side – in the adult form.
The European eel has a number of metamorphoses, from the larval stage to the leptocephalus stage, then a quick metamorphosis to glass eel at the edge of the continental shelf (eight days for the Japanese eel), two months at the border of fresh and salt water where the glass eel undergoes a quick metamorphosis into elver, then a long stage of growth followed by a more gradual metamorphosis to the migrating phase. In the pre-adult freshwater stage, the eel also has phenotypic plasticity because fish-eating eels develop very wide mandibles, making the head look blunt. Leptocephali are common, occurring in all Elopomorpha (tarpon- and eel-like fish).
Most other bony fish undergo metamorphosis initially from egg to immotile larvae known as sac fry (fry with a yolk sac), then to motile larvae (often known as fingerlings due to them roughly reaching the length of a human finger) that have to forage for themselves after the yolk sac resorbs, and then to the juvenile stage where the fish progressively start to resemble adult morphology and behaviors until finally reaching sexual maturity.  
In typical amphibian development, eggs are laid in water and larvae are adapted to an aquatic lifestyle. Frogs, toads, and newts all hatch from the eggs as larvae with external gills but it will take some time for the amphibians to interact outside with pulmonary respiration. Afterwards, newt larvae start a predatory lifestyle, while tadpoles mostly scrape food off surfaces with their horny tooth ridges.
Metamorphosis in amphibians is regulated by thyroxin concentration in the blood, which stimulates metamorphosis, and prolactin, which counteracts its effect. Specific events are dependent on threshold values for different tissues. Because most embryonic development is outside the parental body, development is subject to many 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 adaptation to specific ecological circumstances is remarkable, with many discoveries still being made.
With frogs and toads, the external gills of the newly hatched tadpole are covered with a gill sac after a few days, and lungs are quickly formed. Front legs are formed under the gill sac, and hindlegs are visible a few days later. Following that there is usually a longer stage during which the tadpole lives off a vegetarian diet. Tadpoles use a relatively long, spiral‐shaped gut to digest that diet. Recent studies suggest tadpoles don't have a balanced homeostatic feedback control system until the beginning stages of metamorphosis. At this point, their long gut shortens and begins favoring the diet of insects. 
Rapid changes in the body can then be observed as the lifestyle of the frog changes completely. The spiral‐shaped mouth with horny tooth ridges is resorbed together with the spiral gut. The animal develops a big jaw, and its gills disappear along with its gill sac. Eyes and legs grow quickly, a tongue is formed, and all this is accompanied by associated changes in the neural networks (development of stereoscopic vision, loss of the lateral line system, etc.) All this can happen in about a day, so it is truly a metamorphosis. It is not until a few days later that the tail is reabsorbed, due to the higher thyroxin concentrations required for tail resorption.
Salamander development is highly diverse; some species go through a dramatic reorganization when transitioning from aquatic larvae to terrestrial adults, while others, such as the axolotl, display pedomorphosis and never develop into terrestrial adults. Within the genus Ambystoma , species have evolved to be pedomorphic several times, and pedomorphosis and complete development can both occur in some species. 
In newts, metamorphosis occurs due to the change in habitat, not a change in diet, because newt larvae already feed as predators and continue doing so as adults. Newts' gills are never covered by a gill sac and will be resorbed only just before the animal leaves the water. Adults can move faster on land than in water.  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 adaptation to the land phase, thyroxin. External gills do not return in subsequent aquatic phases because these are completely absorbed upon leaving the water for the first time.
Basal caecilians such as Ichthyophis go through a metamorphosis in which aquatic larva transition into fossorial adults, which involves a loss of the lateral line.  More recently diverged caecilians (the Teresomata) do not undergo an ontogenetic niche shift of this sort and are in general fossorial throughout their lives. Thus, most caecilians do not undergo an anuran-like metamorphosis. 
Amphibians are four-limbed and ectothermic vertebrates of the class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terrestrial, fossorial, arboreal or freshwater aquatic ecosystems. Thus amphibians typically start out as larvae living in water, but some species have developed behavioural adaptations to bypass this.
Salamanders are a group of amphibians typically characterized by their lizard-like appearance, with slender bodies, blunt snouts, short limbs projecting at right angles to the body, and the presence of a tail in both larvae and adults. All ten extant salamander families are grouped together under the order Urodela from the group Caudata. Salamander diversity is highest in eastern North America, especially in the Appalachian Mountains; most species are found in the Holarctic realm, with some species present in the Neotropical realm.
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.
In biology, a nymph is the immature form of some invertebrates, particularly insects, which undergoes gradual metamorphosis (hemimetabolism) before reaching its adult stage. Unlike a typical larva, a nymph's overall form already resembles that of the adult, except for a lack of wings. In addition, while a nymph moults, it never enters a pupal stage. Instead, the final moult results in an adult insect. Nymphs undergo multiple stages of development called instars.
A tadpole is the larval stage in the biological life cycle of an amphibian. Most tadpoles are fully aquatic, though some species of amphibians have tadpoles that are terrestrial. Tadpoles have some fish-like features that may not be found in adult amphibians such as a lateral line, gills and swimming tails. As they undergo metamorphosis, they start to develop functional lungs for breathing air, and the diet of tadpoles changes drastically.
A pupa is the life stage of some insects undergoing transformation between immature and mature stages. Insects that go through a pupal stage are holometabolous: they go through four distinct stages in their life cycle, the stages thereof being egg, larva, pupa, and imago. The processes of entering and completing the pupal stage are controlled by the insect's hormones, especially juvenile hormone, prothoracicotropic hormone, and ecdysone. The act of becoming a pupa is called pupation, and the act of emerging from the pupal case is called eclosion or emergence.
Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is a term used as a "bridging" form of reproduction between egg-laying oviparous and live-bearing viviparous reproduction. Ovoviviparous animals possess embryos that develop inside eggs that remain in the mother's body until they are ready to hatch.
An instar is a developmental stage of arthropods, such as insects, between each moult (ecdysis), 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, colors, patterns, changes in the number of body segments or head width. After shedding their exoskeleton (moulting), the juvenile arthropods continue in their life cycle until they either pupate or moult again. The instar period of growth is fixed; however, in some insects, like the salvinia stem-borer moth, the number of instars depends on early larval nutrition. Some arthropods can continue to moult after sexual maturity, but the stages between these subsequent moults are generally not called instars.
Juvenile hormones (JHs) are a group of acyclic sesquiterpenoids that regulate many aspects of insect physiology. The first discovery of a JH was by Vincent Wigglesworth. JHs regulate development, reproduction, diapause, and polyphenisms.The chemical formula for juvenile hormone is .
In animal dormancy, diapause is the delay in development in response to regular and recurring periods of adverse environmental conditions. It is a physiological state with very specific initiating and inhibiting conditions. The mechanism is a means of surviving predictable, unfavorable environmental conditions, such as temperature extremes, drought, or reduced food availability. Diapause is observed in all the life stages of arthropods, especially insects.
Holometabolism, also called complete metamorphosis, is a form of insect development which includes four life stages: egg, larva, pupa, and imago. Holometabolism is a synapomorphic trait of all insects in the superorder Endopterygota. Immature stages of holometabolous insects are very different from the mature stage. In some species the holometabolous life cycle prevents larvae from competing with adults because they inhabit different ecological niches. The morphology and behavior of each stage are adapted for different activities. For example, larval traits maximize feeding, growth, and development, while adult traits enable dispersal, mating, and egg laying. Some species of holometabolous insects protect and feed their offspring. Other insect developmental strategies include ametabolism and hemimetabolism.
A juvenile is an individual organism that has not yet reached its adult form, sexual maturity or size. Juveniles can look very different from the adult form, particularly in colour, and may not fill the same niche as the adult form. In many organisms the juvenile has a different name from the adult.
An imaginal disc is one of the parts of a holometabolous insect larva that will become a portion of the outside of the adult insect during the pupal transformation. Contained within the body of the larva, there are pairs of discs that will form, for instance, the wings or legs or antennae or other structures in the adult. The role of the imaginal disc in insect development was first elucidated by Jan Swammerdam.
Cotesia congregata is a parasitoid wasp of the genus Cotesia. The genus is particularly noted for its use of polydnaviruses. Parasitoids are distinct from true parasites in that a parasitoid will ultimately kill its host or otherwise sterilize it.
The enzyme juvenile hormone esterase (EC 22.214.171.124, systematic name methyl-(2E,6E,10R)-10,11-epoxy-3,7,11-trimethyltrideca-2,6-dienoate acylhydrolase, JH esterase) catalyzes the hydrolysis of juvenile hormone:
A newt is a salamander in the subfamily Pleurodelinae. The terrestrial juvenile phase is called an eft. Unlike other members of the family Salamandridae, newts are semiaquatic, alternating between aquatic and terrestrial habitats. Not all aquatic salamanders are considered newts, however. More than 100 known species of newts are found in North America, Europe, North Africa and Asia. Newts metamorphose through three distinct developmental life stages: aquatic larva, terrestrial juvenile (eft), and adult. Adult newts have lizard-like bodies and return to the water every year to breed, otherwise living in humid, cover-rich land habitats.
Neurosecretion is the storage, synthesis and release of hormones from neurons. These neurohormones, produced by neurosecretory cells, are normally secreted from nerve cells in the brain that then circulate into the blood. These neurohormones are similar to nonneural endocrine cells and glands in that they also regulate both endocrine and nonendocrine cells. Neurosecretion cells synthesize and package their product in vesicles and exocytose them at axon endings just as normal neurons do, but release their product farther from their target than normal neurons, typically releasing their neurohormones into the circulatory system to reach their distant targets.
Ichthyoplankton are the eggs and larvae of fish. They are mostly found in the sunlit zone of the water column, less than 200 metres deep, which is sometimes called the epipelagic or photic zone. Ichthyoplankton are planktonic, meaning they cannot swim effectively under their own power, but must drift with the ocean currents. Fish eggs cannot swim at all, and are unambiguously planktonic. Early stage larvae swim poorly, but later stage larvae swim better and cease to be planktonic as they grow into juveniles. Fish larvae are part of the zooplankton that eat smaller plankton, while fish eggs carry their own food supply. Both eggs and larvae are themselves eaten by larger animals.
Fish go through various life stages between fertilization and adulthood. The life of a fish start as spawned eggs which hatch into immotile larvae. These larval hatchlings are not yet capable of feeding themselves and carry a yolk sac which provides stored nutrition. Before the yolk sac completely disappears, the young fish must mature enough to be able to forage independently. When they have developed to the point where they are capable of feeding by themselves, the fish are called fry. When, in addition, they have developed scales and working fins, the transition to a juvenile fish is complete and it is called a fingerling, so called as they are typically about the size of human fingers. The juvenile stage lasts until the fish is fully grown, sexually mature and interacting with other adult fish.
Direct development is a concept in biology. It refers to forms of growth to adulthood that do not involve metamorphosis. An animal undergoes direct development if the immature organism resembles a small adult rather than having a distinct larval form. A frog that hatches out of its egg as a small frog undergoes direct development. A frog that hatches out of its egg as a tadpole does not.
The dictionary definition of metamorphosis at Wiktionary