Metamorphosis Download

Many ancient myths end in a metamorphosis. As Apollo is chasing the nymph Daphne, she calls on her river-god father for help and he turns her into a laurel tree to save her. Out of anger and jealousy, the goddess Athena turns the marvelous weaver Arachne into a spider that will spin only beautiful webs. But natural substances may also metamorphose, or undergo metamorphosis. Heat and pressure over thousands of years may eventually turn tiny organisms into petroleum, and coal into diamonds. And the most beloved of natural metamorphoses (notice how this plural is formed) is probably the transformation of caterpillars into butterflies.

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.[1] Some insects, fish, amphibians, mollusks, crustaceans, cnidarians, echinoderms, and tunicates undergo metamorphosis, which is often accompanied by a change of nutrition source or behavior.[2] Animals can be divided into species that undergo complete metamorphosis ("holometaboly"), incomplete metamorphosis ("hemimetaboly"), or no metamorphosis ("ametaboly").[3]

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The word metamorphosis derives from Ancient Greek , "transformation, transforming",[5] from - (meta-), "after" and (morphe), "form".[6]

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 (shedding of the exoskeleton).[7]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.[8] Experiments on firebugs have shown how juvenile hormone can affect the number of nymph instar stages in hemimetabolous insects.[9][10]

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.

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.[14]

According to research from 2008, adult Manduca sexta is able to retain behavior learned as a caterpillar.[16] 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.[17]

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.[18][19] Additional research in 2019 found that both autophagy and apoptosis, the two ways programmed cell death occur, are processes undergone during insect metamorphosis. [20]

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.[22][23]

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.

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.[25] 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.[26] 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.[27]

Gregor Samsa wakes up one morning to find himself transformed into a "monstrous vermin". He initially considers the transformation to be temporary and slowly ponders the consequences of this metamorphosis. Stuck on his back and unable to get up and leave the bed, Gregor reflects on his job as a traveling salesman and cloth merchant, which he characterizes as being full of "temporary and constantly changing human relationships, which never come from the heart". He sees his employer as a despot and would quickly quit his job if he were not his family's sole breadwinner and working off his bankrupt father's debts. While trying to move, Gregor finds that his office manager, the chief clerk, has shown up to check on him, indignant about Gregor's unexcused absence. Gregor attempts to communicate with both the manager and his family, but all they can hear from behind the door is incomprehensible vocalizations. Gregor laboriously drags himself across the floor and opens the door. The clerk, upon seeing the transformed Gregor, flees the apartment. Gregor's family is horrified, and his father drives him back into his room, injuring his side by shoving him when he gets stuck in the doorway.

Gregor is the main character of the story. He works as a traveling salesman in order to provide money for his sister and parents. He wakes up one morning finding himself transformed into an insect. After the metamorphosis, Gregor becomes unable to work and is confined to his room for most of the remainder of the story. This prompts his family to begin working once again. Gregor is depicted as isolated from society and often both misunderstands the true intentions of others and is misunderstood.

Grete is Gregor's younger sister, and she becomes his caretaker after his metamorphosis. They initially have a close relationship, but this quickly fades. At first, she volunteers to feed him and clean his room, but she grows increasingly impatient with the burden and begins to leave his room in disarray out of spite. Her initial decision to take care of Gregor may have come from a desire to contribute and be useful to the family, since she becomes angry and upset when the mother cleans his room. It is made clear that Grete is disgusted by Gregor, as she always opens the window upon entering his room to keep from feeling nauseous and leaves without doing anything if Gregor is in plain sight. She plays the violin and dreams of going to the conservatory to study, a dream Gregor had intended to make happen; he had planned on making the announcement on Christmas Day. To help provide an income for the family after Gregor's transformation, she starts working as a salesgirl. Grete is also the first to suggest getting rid of Gregor, which causes Gregor to plan his own death. At the end of the story, Grete's parents realize that she has become beautiful and full-figured and decide to consider finding her a husband.[4] ff782bc1db