Metamorphosis

Metamorphosis is a biological process by which an animal physically develops after birth or hatching, involving a conspicuous and relatively abrupt change in the animal's body structure through cell growth and differentiation. Metamorphosis is iodothyronine-induced and an ancestral feature of all chordates.[1] Some insects, fishes, amphibians, mollusks, crustaceans, cnidarians, echinoderms, and tunicates undergo metamorphosis, which is often accompanied by a change of nutrition source or behavior. Animals that go through metamorphosis are called metamorphoses. 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. References to "metamorphosis" in mammals are imprecise and only colloquial, but historically idealist ideas of transformation and monadology, as in Goethe's Metamorphosis of Plants, have influenced the development of ideas of evolution.

Dragonfly metamorphosis
A dragonfly in its final moult, undergoing metamorphosis from its nymph form to an adult

Etymology

The word metamorphosis derives from Greek μεταμόρφωσις, "transformation, transforming",[2] from μετα- (meta-), "after" and μορφή (morphe), "form".[3]

Hormonal control

Metamorphosis is iodothyronine-induced and an ancestral feature of all chordates.[1]

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.[4] 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.[5] Experiments on firebugs have shown how juvenile hormone can affect the number of nymph instar stages in hemimetabolous insects.[6][7]

Insects

Grasshoppermetasnodgrass
Incomplete metamorphosis in the grasshopper with different instar nymphs

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.

Development and terminology

Holometabolous vs. Hemimetabolous
Two types of metamorphosis are shown. In a complete (holometabolous) metamorphosis the insect passes through four distinct phases, which produce an adult that does not resemble the larva. In an incomplete (hemimetabolous) metamorphosis an insect does not go through a full transformation, but instead transitions from a nymph to an adult by molting its exoskeleton as it grows.

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.

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.[8]

Evolution

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.[9]

A number of hypotheses have been proposed to explain the evolution of holometaboly from hemimetaboly, mostly centering on whether or not the intermediate hemimetabolous forms are homologous to pupal form of holometabolous forms.

More recently, scientific attention has turned to characterizing the mechanistic basis of metamorphosis in terms of its hormonal control, by characterizing spatial and temporal patterns of hormone expression relative to metamorphosis in a wide range of insects.

Recent research

According to research from 2008, adult Manduca sexta is able to retain behavior learned as a caterpillar.[10] 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.[11]

Many observations have indicated that programmed cell death plays a considerable role during physiological processes of multicellular organisms, particularly during embryogenesis and metamorphosis.

ChristianBauer Pieris rapae cocoon

pupa ready to hatch

Amphibians

RanaTemporariaLarva2
Just before metamorphosis, only 24 hours are needed to reach the stage in the next picture.
Rana Temporaria - Larva Final Stage
Almost functional common frog with some remains of the gill sac and a not fully developed jaw

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.

Frogs and toads

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.

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.

Salamanders

The 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 paedomorphosis and never develop into terrestrial adults. Within the genus Ambystoma, species have evolved to be paedomorphic several times, and paedomorphosis and complete development can both occur in some species.[12]

Newts

LarveKamsalamander
The large external gills of the crested newt

In newts, there is no true metamorphosis 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. Just as in tadpoles, their lungs are functional early, but newts use them less frequently than tadpoles. 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.

Caecilians

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.[13] 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.[14]

Fish

Some fish, both bony fish (Osteichthyes) and jawless fish (Agnatha), undergo metamorphosis. Fish metamorphosis is typically under strong control by the thyroid hormone.[12]

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 from embryo to larva (fry) and then to the juvenile stage during absorption of the yolk sac, because after that phase the individual needs to be able to feed for itself.[15][16]

See also

References

  1. ^ a b Robert J. Denser Chordate Metamorphosis: Ancient Control by Iodothyronines Current Biology, 2008, Vol 18 No 13, R567-9. DOI: 10.1016/j.cub.2008.05.024
  2. ^ "Metamorphosis, Henry George Liddell, Robert Scott, ''A, at Perseus". Perseus.tufts.edu. Retrieved 2012-08-26.
  3. ^ "Online Etymology Dictionary". Etymonline.com. Retrieved 2012-08-26.
  4. ^ Davies, 1998. Chapter 3.
  5. ^ Gullan, P.J. & Cranston, P.S. 6.3 Process and Control of Moulting in The Insects: An Outline of Entomology. Blackwell Publishing, 2005. pp. 153-156.
  6. ^ Slama; Williams (1965). "Juvenile hormone activity for the bug Pyrrhocoris apterus" (PDF). Proceedings of the National Academy of Sciences. 54 (2): 411–414. doi:10.1073/pnas.54.2.411. PMC 219680.
  7. ^ Singh, Amit; Konopova, Barbora; Smykal, Vlastimil; Jindra, Marek (2011). "Common and Distinct Roles of Juvenile Hormone Signaling Genes in Metamorphosis of Holometabolous and Hemimetabolous Insects". PLoS ONE. 6 (12): e28728. doi:10.1371/journal.pone.0028728. ISSN 1932-6203. PMC 3234286. PMID 22174880. open access publication – free to read
  8. ^ Lowe, Tristan; Garwood, Russell P.; Simonsen, Thomas; Bradley, Robert S.; Withers, Philip J. (July 6, 2013). "Metamorphosis revealed: three dimensional imaging inside a living chrysalis". Metamorphosis revealed: three dimensional imaging inside a living chrysalis. 10 (84). 20130304. doi:10.1098/rsif.2013.0304. PMC 3673169. Retrieved June 11, 2015.
  9. ^ Gullan, P.J. & Cranston, P.S. 6.2 Life History Patterns and Phases in The Insects: An Outline of Entomology. pp. 143–153. 2005 by Blackwell Publishing
  10. ^ Douglas J. Blackiston, Elena Silva Casey & Martha R. Weiss (2008). "Retention of memory through metamorphosis: can a moth remember what it learned as a caterpillar?". PLoS ONE. 3 (3): e1736. doi:10.1371/journal.pone.0001736. PMC 2248710. PMID 18320055. open access publication – free to read
  11. ^ Conner, W.E. (2009). Tiger Moths and Woolly Bears—behaviour, ecology, and evolution of the Arctiidae. New York: Oxford University Press. pp. 1–10.
  12. ^ a b Laudet, Vincent (September 27, 2011). "The Origins and Evolution of Vertebrate Metamorphosis". Current Biology. 21: R726–R737. doi:10.1016/j.cub.2011.07.030.
  13. ^ Dunker, Nicole; Wake, Marvalee H.; Olson, Wendy M. (January 2000). "Embryonic and Larval Development in the Caecilian Ichthyophis kohtaoensis (Amphibia, Gymnophiona): A Staging Table". Journal of Morphology. 243: 3–34. doi:10.1002/(sici)1097-4687(200001)243:1<3::aid-jmor2>3.3.co;2-4.
  14. ^ San Mauro, D.; Gower, D. J.; Oommen, O. V.; Wilkinson, M.; Zardoya, R. (November 2004). "Phylogeny of caecilian amphibians (Gymnophiona) based on complete mitochondrial genomes and nuclear RAG1". Molecular Phylogenetics and Evolution. 33: 413–427. doi:10.1016/j.ympev.2004.05.014. PMID 15336675.
  15. ^ Mader, Sylvia, Biology 9th ed. Ch. 31
  16. ^ Peter B. Moyle and Joseph J. Cech Jr, Fishes: an introduction to ichthyology 5th ed. 9.3: "Development" pp 148ff

Bibliography

  • Davies, R.G. (1998). Outlines of Entomology. Chapman and Hall. Second Edition. Chapter 3.
  • Williamson D.I. (2003). The Origins of Larvae. Kluwer.

External links

Developmental biology

Developmental biology is the study of the process by which animals and plants grow and develop. Developmental biology also encompasses the biology of regeneration, asexual reproduction, metamorphosis, and the growth and differentiation of stem cells in the adult organism.

In the late 20th century, the discipline largely transformed into evolutionary developmental biology.

Endopterygota

Endopterygota, also known as Holometabola, is a superorder of insects within the infraclass Neoptera that go through distinctive larval, pupal, and adult stages. They undergo a radical metamorphosis, with the larval and adult stages differing considerably in their structure and behaviour. This is called holometabolism, or complete metamorphism.

The Endopterygota are among the most diverse insect superorders, with about 850,000 living species divided between 11 orders, containing insects such as butterflies, flies, fleas, bees, ants, and beetles.They are distinguished from the Exopterygota (or Hemipterodea) by the way in which their wings develop. Endopterygota (meaning literally "internal winged forms") develop wings inside the body and undergo an elaborate metamorphosis involving a pupal stage. Exopterygota ("external winged forms") develop wings on the outside their bodies and do not go through a pupal stage. The latter trait is plesiomorphic, however, and not exclusively found in the exopterygotes, but also in groups such as Odonata (dragonflies and damselflies), which are not Neoptera, but more basal among insects.

The earliest endopterygote fossils date from the Carboniferous.

Heaven's Gate (religious group)

Heaven's Gate was an American UFO religious millenarian cult based in San Diego, California, founded in 1974 and led by Marshall Applewhite (1931–1997) and Bonnie Nettles (1927–1985). On March 26, 1997, police discovered the bodies of 39 members of the group, who had participated in a mass suicide in order to reach what they believed was an extraterrestrial spacecraft following Comet Hale–Bopp.Just before the suicide, the group's website was updated with the message: "Hale-Bopp brings closure to Heaven's Gate ... Our 22 years of classroom here on planet Earth is finally coming to conclusion – 'graduation' from the Human Evolutionary Level. We are happily prepared to leave 'this world' and go with Ti's crew."

Hemimetabolism

Hemimetabolism or hemimetaboly, also called incomplete metamorphosis and paurometabolism, is 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 resembles the adult stage but lacks wings and functional reproductive organs.

Holometabolism

Holometabolism, also called complete metamorphosis, is a form of insect development which includes four life stages: egg, larva, pupa, and imago or adult. 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.

Imago

In biology, the imago is the last stage an insect attains during its metamorphosis, its process of growth and development; it also is called the imaginal stage, the stage in which the insect attains maturity. It follows the final ecdysis of the immature instars.In a member of the Ametabola or Hemimetabola, in which metamorphosis is "incomplete", the final ecdysis follows the last immature or nymphal stage.

In members of the Holometabola, in which there is a pupal stage, the final ecdysis follows emergence from the pupa, after which the metamorphosis is complete, although there is a prolonged period of maturation in some species.The imago is the only stage during which the insect is sexually mature and, if it is a winged species, has functional wings. The imago often is referred to as the adult stage.Members of the order Ephemeroptera (mayflies) do not have a pupal stage, but they briefly pass through an extra winged stage called the subimago. Insects at this stage have functional wings but are not yet sexually mature.The Latin plural of imago is imagines, and this is the term generally used by entomologists –

however, imagoes is also acceptable.

Insect

Insects or Insecta (from Latin insectum) are hexapod invertebrates and the largest group within the arthropod phylum. Definitions and circumscriptions vary; usually, insects comprise a class within the Arthropoda. As used here, the term Insecta is synonymous with Ectognatha. Insects have a chitinous exoskeleton, a three-part body (head, thorax and abdomen), three pairs of jointed legs, compound eyes and one pair of antennae. Insects are the most diverse group of animals; they include more than a million described species and represent more than half of all known living organisms. The total number of extant species is estimated at between six and ten million; potentially over 90% of the animal life forms on Earth are insects. Insects may be found in nearly all environments, although only a small number of species reside in the oceans, which are dominated by another arthropod group, crustaceans.

Nearly all insects hatch from eggs. Insect growth is constrained by the inelastic exoskeleton and development involves a series of molts. The immature stages often differ from the adults in structure, habit and habitat, and can include a passive pupal stage in those groups that undergo four-stage metamorphosis. Insects that undergo three-stage metamorphosis lack a pupal stage and adults develop through a series of nymphal stages. The higher level relationship of the insects is unclear. Fossilized insects of enormous size have been found from the Paleozoic Era, including giant dragonflies with wingspans of 55 to 70 cm (22 to 28 in). The most diverse insect groups appear to have coevolved with flowering plants.

Adult insects typically move about by walking, flying, or sometimes swimming. As it allows for rapid yet stable movement, many insects adopt a tripedal gait in which they walk with their legs touching the ground in alternating triangles, composed of the front & rear on one side with the middle on the other side. Insects are the only invertebrates to have evolved flight, and all flying insects derive from one common ancestor. Many insects spend at least part of their lives under water, with larval adaptations that include gills, and some adult insects are aquatic and have adaptations for swimming. Some species, such as water striders, are capable of walking on the surface of water. Insects are mostly solitary, but some, such as certain bees, ants and termites, are social and live in large, well-organized colonies. Some insects, such as earwigs, show maternal care, guarding their eggs and young. Insects can communicate with each other in a variety of ways. Male moths can sense the pheromones of female moths over great distances. Other species communicate with sounds: crickets stridulate, or rub their wings together, to attract a mate and repel other males. Lampyrid beetles communicate with light.

Humans regard certain insects as pests, and attempt to control them using insecticides, and a host of other techniques. Some insects damage crops by feeding on sap, leaves, fruits, or wood. Some species are parasitic, and may vector diseases. Some insects perform complex ecological roles; blow-flies, for example, help consume carrion but also spread diseases. Insect pollinators are essential to the life cycle of many flowering plant species on which most organisms, including humans, are at least partly dependent; without them, the terrestrial portion of the biosphere would be devastated. Many insects are considered ecologically beneficial as predators and a few provide direct economic benefit. Silkworms produce silk and honey bees produce honey and both have been domesticated by humans. Insects are consumed as food in 80% of the world's nations, by people in roughly 3000 ethnic groups.

Maria Sibylla Merian

Maria Sibylla Merian (2 April 1647 – 13 January 1717) was a German-born naturalist and scientific illustrator, a descendant of the Frankfurt branch of the Swiss Merian family. Merian was one of the first naturalists to observe insects directly.

Merian received her artistic training from her stepfather, Jacob Marrel, a student of the still life painter Georg Flegel. Merian published her first book of natural illustrations in 1675. She had started to collect insects as an adolescent and at age thirteen she raised silk worms. In 1679 Merian published the first volume of a two-volume series on caterpillars, the second volume followed in 1683. Each volume contained 50 plates engraved and etched by Merian. Merian documented evidence on the process of metamorphosis and the plant hosts of 186 European insect species. Along with the illustrations Merian included a descriptions of their life cycles.

In 1699 Merian travelled to Dutch Surinam to study and record the tropical insects. In 1705 she published Metamorphosis insectorum Surinamensium. Few colour images of the New World were printed before 1700 and thus Merian's Metamorphosis has been credited with influencing a range of naturalist illustrators. Because of her careful observations and documentation of the metamorphosis of the butterfly, she is considered by David Attenborough to be among the most significant contributors to the field of entomology. She was a leading entomologist of her time and she discovered many new facts about insect life through her studies.

Metamorphism

Metamorphism is the change of minerals or geologic texture (distinct arrangement of minerals) in pre-existing rocks (protoliths), without the protolith melting into liquid magma (a solid-state change). The change occurs primarily due to heat, pressure, and the introduction of chemically active fluids. The chemical components and crystal structures of the minerals making up the rock may change even though the rock remains a solid. Changes at or just beneath Earth's surface due to weathering or diagenesis are not classified as metamorphism. Metamorphism typically occurs between diagenesis (max. 200°C), and melting (~850°C).Three types of metamorphism exist: contact, dynamic, and regional. Metamorphism produced with increasing pressure and temperature conditions is known as prograde metamorphism. Conversely, decreasing temperatures and pressure characterize retrograde metamorphism.

Metamorphoses

The Metamorphoses (Latin: Metamorphōseōn librī: "Books of Transformations") is a Latin narrative poem by the Roman poet Ovid, considered his magnum opus. Comprising 11,995 lines, 15 books and over 250 myths, the poem chronicles the history of the world from its creation to the deification of Julius Caesar within a loose mythico-historical framework.

Although meeting the criteria for an epic, the poem defies simple genre classification by its use of varying themes and tones. Ovid took inspiration from the genre of metamorphosis poetry, and some of the Metamorphoses derives from earlier treatment of the same myths; however, he diverged significantly from all of his models.

One of the most influential works in Western culture, the Metamorphoses has inspired such authors as Dante Alighieri, Giovanni Boccaccio, Geoffrey Chaucer, and William Shakespeare. Numerous episodes from the poem have been depicted in acclaimed works of sculpture, painting, and music. Although interest in Ovid faded after the Renaissance, there was a resurgence of attention to his work towards the end of the 20th century. Today the Metamorphoses continues to inspire and be retold through various media. The work has been the subject of numerous translations into English, the first by William Caxton in 1480.

Metamorphosis (The Rolling Stones album)

Metamorphosis is the third compilation album of The Rolling Stones music released by former manager Allen Klein's ABKCO Records (who usurped control of the band's Decca/London material in 1970) after the band's departure from Decca and Klein. Released in 1975, Metamorphosis centres on outtakes and alternate versions of well-known songs recorded from 1964 to 1970.

Metamorphosis (World Saxophone Quartet album)

Metamorphosis is an album by the jazz group the World Saxophone Quartet released in 1990 on the Elektra/Nonesuch label and features performances by Hamiet Bluiett, Arthur Blythe, Oliver Lake and David Murray with Chief Bey, Melvin Gibbs and Mor Thiam. It was the first album recorded by the group after the departure of foundation member Julius Hemphill and their first to feature additional musicians.

Ovoviviparity

Ovoviviparity, ovovivipary, ovivipary, or aplacental viviparity is a mode of reproduction in animals in which embryos that develop inside eggs remain in the mother's body until they are ready to hatch. This method of reproduction is similar to viviparity, but the embryos have no placental connection with the mother and receive their nourishment from a yolk sac. In some species, this is supplemented by uterine secretions or other maternal provisioning.

The young of ovoviviparous amphibians are sometimes born as larvae, and undergo metamorphosis outside the body of the mother, and in some insect species, such as the tachinid flies, the embryos develop to the first larval instar stage before they are laid and the eggs hatch almost immediately.

Pupa

A pupa (Latin: pūpa, "doll"; plural: pūpae) is the life stage of some insects undergoing transformation between immature and mature stages. The pupal stage is found only in holometabolous insects, those that undergo a complete metamorphosis, with four life stages: 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 pupae of different groups of insects have different names such as chrysalis for the pupae of butterflies and tumbler for those of the mosquito family. Pupae may further be enclosed in other structures such as cocoons, nests, or shells.

Reflexology

Reflexology, also known as zone therapy, is an alternative medicine involving application of pressure to the feet and hands with specific thumb, finger, and hand techniques without the use of oil or lotion. It is based on a pseudoscientific system of zones and reflex areas that purportedly reflect an image of the body on the feet and hands, with the premise that such work effects a physical change to the body.There is no convincing evidence that reflexology is effective for any medical condition.

Smallville (season 1)

Season one of Smallville, an American television series developed by Alfred Gough and Miles Millar, began airing on October 16, 2001, on The WB television network. The series recounts the early adventures of Kryptonian Clark Kent as he adjusts to his developing superpowers in the fictional town of Smallville, Kansas, during the years before he becomes Superman. The first season comprises 21 episodes and concluded its initial airing on May 21, 2002. Regular cast members during season one include Tom Welling, Kristin Kreuk, Michael Rosenbaum, Eric Johnson, Sam Jones III, Allison Mack, Annette O'Toole, and John Schneider.

The season's stories focus on Martha and Jonathan Kent's (O'Toole and Schneider) attempts to help their adopted son Clark (Welling) cope with his alien origin and control his developing superhuman abilities. Clark must deal with the meteor-infected individuals that begin appearing in Smallville, his love for Lana Lang (Kreuk), and not being able to tell his two best friends, Pete Ross (Jones III) and Chloe Sullivan (Mack), about his abilities or his origins. Clark also befriends Lex Luthor (Rosenbaum) after saving Lex's life. The season also follows Lex, as he tries to assert his independence from his father, Lionel Luthor (John Glover).

The episodes were filmed primarily in Vancouver and post-production work took place in Los Angeles. Gough and Millar assisted the writing staff with week-to-week story development. "Villain of the week" storylines were predominant during the first season; physical effects, make-up effects, and computer generated imagery became important components as well. Limited filming schedules sometimes forced guest actors to perform physical stunts, and the series regulars were more than willing to do stunt work. Episode budgets ultimately became strictly regulated, as the show frequently ran over budget during the first half of the season. The pilot broke The WB's viewership record for a debut series, and was nominated for various awards. Although the villain of the week storylines became a concern for producers, critical reception was generally favorable, and the series was noted as having a promising start. The first season was released on DVD on September 23, 2003, and included various special features that focused on individual episodes and the series as a whole. It has also been released on home media in regions 2 and 4 in the international markets.

Tadpole

A tadpole (also called a pollywog) is the larval stage in the life cycle of an amphibian, particularly that of a frog or toad. They are usually wholly aquatic, though some species have tadpoles that are terrestrial. When first hatched from the egg they have a more or less globular body, a laterally compressed tail and internal or external gills. As they grow they undergo metamorphosis, during which process they grow limbs, develop lungs and reabsorb the tail. Most tadpoles are herbivorous and during metamorphosis the mouth and internal organs are rearranged to prepare for an adult carnivorous lifestyle.

Having no hard parts, it might be expected that fossil tadpoles would not exist. However, traces of biofilms have been preserved and fossil tadpoles have been found dating back to the Miocene. Tadpoles are eaten in some parts of the world and are mentioned in folk tales and used as a symbol in ancient Egyptian numerals.

The Metamorphosis

The Metamorphosis (German: Die Verwandlung) is a novella written by Franz Kafka which was first published in 1915. One of Kafka's best-known works, The Metamorphosis tells the story of salesman Gregor Samsa who wakes one morning to find himself inexplicably transformed into a huge insect and subsequently struggling to adjust to this new condition. The novella has been widely discussed among literary critics, with differing interpretations being offered.

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