Difference between revisions of "Metamorphosis" - New World Encyclopedia

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A cicada in the process of shedding.

In zoology, metamorphosis is a process of pronounced and relatively abrupt developmental change in the internal and external morphology of an animal after the embryonic stage, accompanied by changes in physiology (body functions)and usually (but not always) striking changes in behavior or habitat. Some insects, amphibians, molluscs, crustaceans, echinoderms and tunicates undergo metamorphosis.

In metamorhposis, the immature form can resemble the adult form (incomplete metamorphosis) or can look very dissimilar to the adult form (complete metamorphosis). The transformation from an egg to an adult butterfly involves first a segemented, worm-like larva (caterpillar), then a immobile pupa stage, until final the winged butterfly emerges. Likewise, a tadpole, with gills, a fish-like tail and no legs, metamorphosizes into an adult, airbreathing, legged and tailess frog. While some crustaceans simply increase in size with each molt, in other crustaceans, such as the Slipper Lobster, the larval stage looks very different from the adult. On the other hand, after a grasshopper egg hatches, the emerging nymph looks similar to the adult grasshopper, but lacks wings and has underdeveloped reproductive organs.

Metamorphosis is an adaptation that contributes to the success of insects and the other organisms. It allows exploitation of different niches, aids in dispersal, eliminates competition between larvae and adults for food and space and helps in surviving harsh weather (Towle 1989). For example, caterpillars fead on plants while butterflies derive energy from nectar. Also, with the caterpillar specialized for growth and the adult for dispersal and reproduction, conflict is eliminated between two activities that require great energy expenditure (Towle 1989).

Scientific usage of the term metamorphosis is exclusive, and is not applied to general aspects of cell growth, including rapid growth spurts. References to "metamorphosis" in mammals is imprecise and only colloquial.

Insect metamorphosis

**Comparative Lengths of Metamorphosis**
Species	Egg	Larva/Nymph	Pupa	Adult
Housefly	1 day	2 weeks	1 week	2 weeks
Ladybird	4 days	2 weeks	2 weeks	3-9 months
Monarch Butterfly	4 days	2 weeks	10 days	2-6 weeks
Periodical Cicada	1 month	13/17 years	no such stage	2 months
Mayfly	1 month	3 years		1 day
Cockroach	1 month	3 months		9 months

There are two main types of metamorphosis in insects, complete metamorphosis (holometabolism) and incomplete metamorphosis (hemimetabolism).

Holometabolism, also called complete metamorphosis and complex metamorphosis, is a term applied to metamorphosis where the larvae differ markedly from the adults. Insects that undergo holometabolism pass through a larval stage, then enter an inactive state called pupa, or chrysalis, and finally emerge as adults (imago).

For example, in the life cycle of a butterfly, the embryo grows within the egg, hatching into the larval stage (caterpillar), before entering the pupal stage within its chrysalis and finally emerging as an adult butterfly imago. The caterpillar looks more like a worm than an adult butterfly, but has the typical insect three pairs of jointed legs as well as several pairs of fleshy legs (Towle 1989) In the immobile pupal stage, the animal is encased in a protective coating called a chrysalis (a cocoon in moths). In this stage, the insect will excrete digestive juices, to destroy much of the larva's body, leaving a few cells intact, while groups of cells, called imaginal disks, develop into the tissues of the adult, using the nutrients from the broken down larva. This process of cell death is called histolysis, and cell regrowth histogenesis.

Examples of holometabolic insects include beetles, flies, bees, wasps, ants, butterflies, moths, fleas, and caddisflies.

Hemimetabolism, also called incomplete metamorphosis, simple metamorphosis, gradual metamorphosis, or hemimetaboly, is a term applied to metamorphosis where the larvae resembles the adult somewhat, as they have compound eyes, developed legs and wing stubs visible on the outside. The juvenile forms closely resemble adults, but are smaller and, if the adult has wings, lack wings. In this mode of development there are three distinct stages: the egg, nymph, and the adult stage, or imago. These groups go through gradual changes; there is no pupal stage. In hemimetabolism, the development of larva often proceeds in repeated stages of growth and ecdysis (moulting), these stages are called instars. The differences between juveniles in different instars are small, often just differences in body proportions and the number of segments.

Examples of hemimetabolous insects include grasshoppers, crickets, praying mantids, cockroaches, earwigs, dragonflies, damselflfies, aphids, scale insects, true bugs, and cicadas.

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 complete metamorphosis.

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

Hormonal control

Insect growth and metamorphosis are controlled by hormones synthesized by endocrine glands near the front of the body. Three key hormones are brain hormone, molting hormone (called ecdysone), and juvenile hormone (Towle 1989).

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

Moreover, the corpora allata produce 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 molts, controlled by ecdysone, until the production of juvenile hormone ceases and metamorphosis occurs.

Pieris rapae larva
Pieris rapae pupa
Pieris rapae pupa, ready to hatch.
A Pieris rapae adult

Amphibian metamorphosis

Amphibian metamorphosis undergoes a single change from larvae, called a tadpole, to an adult. In the typical amphibian lifecycle, eggs are laid in water. The tadpole then emerges from the egg, and swims freely within the water. The tadpole has gills, a tail, a small circular mouth, and lacks legs. The tadpole will grow, until it begins metamorphosis. Metamorphosis begins with the development of the hind legs, then the front legs. The lungs develop, and the tapole begins to swim to the surface of the water to breathe. The intestine shortens to accommodate a carnivorous diet, and the eyes migrate rostrally and dorsally. In frogs the tail is absorbed by the body, for the last stage of metamorphosis.

There are many derivations from the typical amphibian lifecycle. Some species of salamander do not need to metamorphose to be sexually mature, and will only metamorphose under certain environmental stresses. Many species of frog from the tropics lay their eggs on land, where the tadpoles undergo metamorphosis within the egg. Once they hatch, they are already adults, sometimes possessing a tail which is re-absorbed in a couple of days.

Crustacean metamorphosis

Growth in arthropods, given the hardened exoskeleton, must proceed through series of molts (ecdysis), where the old exoskeleton is shed and a new cuticle is secreted. Molting actually takes after secretion of the new and larger cuticle but before it is hardened (Russell-Hunter 1969).

References
ISBN links support NWE through referral fees

Davies, R.G., Outlines of Entomology, Chapman and Hall: chapter 3

External links

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@@ Line 30: / Line 30: @@
 | [[Cockroach]] || 1 month || 3 months || 9 months
 |}
-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]].
+There are two main types of metamorphosis in insects, complete metamorphosis (holometabolism) and incomplete metamorphosis (hemimetabolism).
-The immature stages of a species that metamorphoses are usually called [[larva|larvae]]. 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.
+Holometabolism, also called complete metamorphosis and complex metamorphosis, is a term applied to metamorphosis where the larvae differ markedly from the adults. Insects that undergo holometabolism pass through a larval stage, then enter an inactive state called pupa, or chrysalis, and finally emerge as adults (imago).
-In hemimetabolism, the development of larva often proceeds in repeated stages of growth and [[ecdysis]] (moulting), these stages are called [[instar]]s. 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.
+For example, in the life cycle of a butterfly, the embryo grows within the egg, hatching into the larval stage (caterpillar), before entering the pupal stage within its chrysalis and finally emerging as an adult butterfly imago. The caterpillar looks more like a worm than an adult butterfly, but has the typical insect three pairs of jointed legs as well as several pairs of fleshy legs (Towle 1989) In the immobile pupal stage, the animal is encased in a protective coating called a chrysalis (a cocoon in moths). In this stage, the insect will excrete digestive juices, to destroy much of the larva's body, leaving a few cells intact, while groups of cells, called imaginal disks, develop into the tissues of the adult, using the nutrients from the broken down larva. This process of cell death is called [[histolysis]], and cell regrowth [[histogenesis]].
-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 larvas 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]].
+Examples of holometabolic insects include beetles, flies, bees, wasps, ants, butterflies, moths, fleas, and caddisflies.
-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.
+Hemimetabolism, also called incomplete metamorphosis, simple metamorphosis, gradual metamorphosis, or hemimetaboly, is a term applied to metamorphosis where the larvae resembles the adult somewhat, as they have compound eyes, developed legs and wing stubs visible on the outside. The juvenile forms closely resemble adults, but are smaller and, if the adult has wings, lack wings. In this mode of development there are three distinct stages: the egg, nymph, and the adult stage, or imago. These groups go through gradual changes; there is no pupal stage. In hemimetabolism, the development of larva often proceeds in repeated stages of growth and [[ecdysis]] (moulting), these stages are called [[instar]]s.  The differences between juveniles in different instars are small, often just differences in body proportions and the number of segments.
+Examples of hemimetabolous insects include grasshoppers, crickets, praying mantids, cockroaches, earwigs, dragonflies, damselflfies, aphids, scale insects, true bugs, and cicadas.
+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 complete metamorphosis.
 Many observations have indicated that [[cell death]] plays a considerable role during physiological processes of multicellular organisms, particularly during [[embryogenesis]] and metamorphosis.
-<gallery>
-Image:ChristianBauer Pieris rapae caterpiller.jpg|''[[Small White|Pieris rapae]]'' [[larva]]
-Image:ChristianBauer Pieris rapae caterpiller2.jpg|''Pieris rapae'' [[pupa]]
-Image:ChristianBauer Pieris rapae cocoon.jpg|''Pieris rapae'' pupa, ready to hatch.
-Image:ChristianBauer Pieris rapae youngadult.jpg|A ''Pieris rapae'' [[adult]]
-</gallery>
 ===Hormonal control===
-[[Insect]] growth and metamorphosis are controlled by [[hormone]]s synthesized by [[endocrine]] [[gland]]s near the front of the body.
+[[Insect]] growth and metamorphosis are controlled by [[hormone]]s synthesized by [[endocrine]] [[gland]]s near the front of the body. Three key hormones are brain hormone, molting hormone (called ecdysone), and juvenile hormone (Towle 1989).
 Some [[cell (biology)|cell]]s of an insect's [[brain]] secrete a hormone that activates [[Thorax|thoracic]] glands, which secrete a second hormone, usually [[Ecdysone]] (a [[steroid]]), that induces metamorphosis.
-Moreover, the [[corpora allata]] produce 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 moult, controlled by [[Ecdysone]], until the production of juvenile hormone ceases and metamorphosis occurs.
+Moreover, the [[corpora allata]] produce 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 molts, controlled by [[ecdysone]], until the production of juvenile hormone ceases and metamorphosis occurs.
+<gallery>
+Image:ChristianBauer Pieris rapae caterpiller.jpg|''Small White|Pieris rapae'' larva
+Image:ChristianBauer Pieris rapae caterpiller2.jpg|''Pieris rapae'' pupa
+Image:ChristianBauer Pieris rapae cocoon.jpg|''Pieris rapae'' pupa, ready to hatch.
+Image:ChristianBauer Pieris rapae youngadult.jpg|A ''Pieris rapae'' adult
+</gallery>
 ==Amphibian metamorphosis==
-[[Amphibian]] metamorphosis undergoes a single change from larvae, called a [[tadpole]], to an adult. In the typical amphibian lifecycle, eggs are laid in water. The tadpole then emerges from the egg, and swims freely within the water. The tadpole has gills, a tail and a small circular mouth. The tadpole will grow, until it begins metamorphosis. Metamorphosis begins with the development of the hind legs, then the front legs. The lungs develop, and the tapole begins to swim to the surface of the water to breathe. The intestine shortens to accommodate a carnivorous diet, and the eyes migrate rostrally and dorsally. In [[frog]]s the tail is absorbed by the body, for the last stage of metamorphosis.
+[[Amphibian]] metamorphosis undergoes a single change from larvae, called a [[tadpole]], to an adult. In the typical amphibian lifecycle, eggs are laid in water. The tadpole then emerges from the egg, and swims freely within the water. The tadpole has gills, a tail, a small circular mouth, and lacks legs. The tadpole will grow, until it begins metamorphosis. Metamorphosis begins with the development of the hind legs, then the front legs. The lungs develop, and the tapole begins to swim to the surface of the water to breathe. The intestine shortens to accommodate a carnivorous diet, and the eyes migrate rostrally and dorsally. In [[frog]]s the tail is absorbed by the body, for the last stage of metamorphosis.
 There are many derivations from the typical amphibian lifecycle. Some species of [[salamander]] do not need to metamorphose to be sexually mature, and will only metamorphose under certain environmental stresses. Many species of frog from the [[tropics]] lay their eggs on land, where the tadpoles undergo metamorphosis within the egg. Once they hatch, they are already adults, sometimes possessing a tail which is re-absorbed in a couple of days.
-==See also==
+==Crustacean metamorphosis==
-* [[Ametabolism]]
+Growth in [[arthropod]]s, given the hardened exoskeleton, must proceed through series of molts (ecdysis), where the old exoskeleton is shed and a new cuticle is secreted. Molting actually takes after secretion of the new and larger cuticle but before it is hardened (Russell-Hunter 1969).
-* [[Hypermetamorphosis]]
-* [[Morphogenesis]]
 ==References==