How did the metamorphosis develop?
The metamorphosis (Greek: μεταμόρφωση = transformation, transformation, transformation) or Metabolism is a biological process in which an animal, after hatching from the egg, changes its physique or structure clearly and relatively quickly due to cell growth and / or changes. Insects (Insecta), Amphibians (Amphibia), some mollusks (Mollusca), Crustaceans (Crustacea), Echinoderms (Echinodermata) and tunicates (Tunicata) undergo metamorphosis, usually, but not necessarily, associated with a change in habitat or behavior of the animal.
The metamorphosis includes three or two stages, depending on whether there is a pupal stage or not. The animal changes from a larva to an immobile pupa and hatches from it as an adult animal (complete metamorphosis). If there is no pupal stage, the larva transforms directly into an adult animal (incomplete metamorphosis). Adult insects will too Imago called, the larval stages of certain insect orders are also called nymph.
In insects, growth and metamorphosis are controlled by hormones from endocrine glands located in the head. A hormone is released from the nervous system that activates the thoracic glands, which in turn release the steroid ecdysone. This hormone initiates metamorphosis. Another larval hormone produced in the corpora allata prevents adult traits from developing during the larval moulting stages. According to this, the larva will molt until juvenile hormone production ceases and metamorphosis begins.
In incomplete metamorphosis (hemimetabolism), the larva goes through several stages of molting. If the skin or exoskeleton becomes too tight due to cell growth, they tear and the larva strips them off. A new one has already formed under the skin or under the exoskeleton, which is now still flexible and must first harden in order to protect the larva from external influences. These larvae are very similar to the adults, but are smaller and either have no wings or only wing stubs if the adults are winged. The differences between the different stages of moulting are small, mostly only the body proportions and / or the number of body segments change.
In the complete metamorphosis (holometabolism) the larvae differ considerably from the adult animal. It occurs only in certain insect orders. As with incomplete metamorphosis, the larva goes through several molting stages. During the pupal stage, the insect secretes digestive juices that break down much of the larva, but some cells remain intact. From these cells the imago begins to grow within the pupa. It gets the nutrients it needs from the pre-digested remains of the larva. The process of cell breakdown is called “histolysis”, the new formation of cells is called “histogenesis”.
The following images illustrate the metamorphosis of the monarch butterfly (Danaus plexippus) shown:
Metamorphosis in amphibians
The larvae of the amphibians are called tadpoles in the frogs and larvae in the tail amphibians. They hatch, usually eggs laid in water, and spend their larval stage swimming there. Tadpoles have gills and a rounded mouth opening.
The metamorphosis to the young begins with the development of the hind legs, then the front legs of the tadpoles follow the frogs; the forelegs grow first in the larvae of the tail amphibians. When the lungs develop, the tadpoles swim to the surface of the water to breathe. The intestinal tract shortens to accommodate a predatory diet. The eyes gradually migrate rostrally and dorsally (towards the mouth and upwards). In the last stage of metamorphosis, the tail of frogs is completely absorbed by the body. This metamorphosis is controlled with thyroid hormones.
There are also deviations from normal metamorphosis in some amphibians. Some species of salamander, for example alpine salamander (Salamandra atra), and frogs, for example Great Honeycomb Toad (Pipa pipa), do not need to undergo any metamorphosis in the water and still develop into reproductive amphibians. Lots of tropical frog species, for example Eleutherodactylus limbatus, lay their eggs on land and the tadpoles undergo their metamorphosis in the egg. In the almost or fully developed stage, the frogs hatch, sometimes with a tail that is absorbed in a few days. Axolotl (Ambystoma mexicanum), on the other hand, will never metamorphose naturally and keep their gills throughout their lives. They lack the thyroid gland necessary for the final transformation. This is only very small and available in remnants. Axolotl reproduce in the adult larval stage.
Duration of the metamorphosis
How much time the individual stages take depends on the individual species. They can be very different. Often, but not always, one can observe that hemimetabolic insects as imago only have a comparatively shorter lifespan than the holometabolic imagines. The opposite is the case with the duration of the larval stages. The following table compares the individual stages of the metamorphosis of different insects over time:
- Battalion, Eugène: Recherches anatomiques et expérimentales sur la métamorphose des amphibens anoures. Masson, Paris 1891.
- Chia, Fu-Shiang & Mary E. Rice: Settlement and metamorphosis of marine invertebrate larvae. Elsevier, New York 1978. ISBN 0-444-00277-4
- Hüsing, Johannes O .: The metamorphosis of the insects. Geest, Portig & Ziemsen, Leipzig, Wittenberg 1952-63.
- Jenkin, Penelope M .: Animal hormones. Pergamon, Oxford 1962-70. ISBN 0-080-15648-7
- Evans Snodgrass, Robert: Insect metamorphosis. Washington 1954.
- Todd, Kim: Chrysalis. Harcourt, Orlando 2007. ISBN 0-151-01108-7
- Sir Wigglesworth, Vincent B .: The physiology of insect metamorphosis. Cambridge 1954.
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