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

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. Some insects, amphibians, mollusks, crustaceans, Cnidarians, echinoderms and tunicates undergo metamorphosis, which is usually (but not always) accompanied by a change of habitat or behavior.

Scientific usage of the term is exclusive, and 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, influenced the development of ideas of evolution.

Contents

Etymology

The word "metamorphosis" derives from Greek μεταμόρφωσις, "transformation, transforming"[1], from μετα- (meta-), "change" + μορφή (morphe) "form"[2].

Insect metamorphosis

Metamorphosis usually proceeds in distinct stages, starting with larva or nymph, optionally passing through pupa, and ending as adult. There are two main types of metamorphosis in insects, hemimetabolism and holometabolism.

Incomplete metamorphosis in the grasshopper with different instar nymphs

The immature stages of a species that metamorphosises are usually called larvae, 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. An example of metamorphosis where the name is changed is that of a tadpole. When a tadpole metamorphosises, it becomes amphibious, whereas a tadpole itself may not be considered amphibious.

In hemimetabolism, the development of larva often proceeds in repeated stages of growth and ecdysis (moulting), these stages are called instars. 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, 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.

Whether the insect spends more time in its adult stage or in its juvenile form depends on the species. Notable examples are the mayfly, whose non-eating, adult stage lives for one day, and the cicada, 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 plays a considerable role during physiological processes of multicellular organisms, particularly during embryogenesis and metamorphosis.

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Hormonal control

Insect growth and metamorphosis are controlled by hormones synthesized by endocrine glands near the front of the body.

neurosecretory cells of an insect's brain secrete a hormone, the prothoracicotropic hormone that activates prothoracic glands, which secrete a second hormone, usually Ecdysone (a steroid), that induces metamorphosis.

Moreover, the corpora allata, a retrocerebral organ produces the juvenile hormone, whose effect is to prevent the development of adult characteristics while allowing ecdysis. 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

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 jaw

In 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 that counteracts its effect. The specific events are dependent of the treshold 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 we can observe very quick changes in the body 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 newt

In 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 are not returning in the water phase because they are completely resorbed when 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.

See also

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


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