The metamorphosis ( Greek μεταμόρφωσις = transformation, transformation, transformation), also metabolism (Greek μεταβολή = change), is in zoology the transformation of the larval form to the adult stage , the sexually mature, adult animal (shape change). The term refers specifically to animals whose youth stages differ in shape and lifestyle from the adult state. During metamorphosis, the larval organs are resorbed or rejected and the existing systems of the adult organs are developed to function. Hormones control the various processes (in vertebrates through the thyroid gland ).
Forms of metamorphosis in animals
- The continuous metamorphosis takes place during the entire after embryonic development - degradation processes then play a minor role (e.g. anamerism in cancers ).
- The catastrophic metamorphosis represents the transition from the last larval to the adult stage. Large parts of the larval body are shed or receded (example: Pluteus larva of the sea urchin ). The profound remodeling often takes place in an immobile pupal stage that is unable to take in food .
Classic examples of animal groups with metamorphosis are the frogs (conversion of the aquatic tadpole to the terrestrial form) and the insects (e.g. the conversion of a caterpillar via an insect pupa to a butterfly ). It should be noted that the term metamorphosis does not stand for the entire ontogeny, including the embryonic phase, but generally only refers to the transformation of a larval stage into an adult stage.
Gradual metamorphosis ( hemimetabolism ) is also observed in many insects , with the types epimetabolism, prometabolism , heterometabolism and neometabolism. They do not have a pupal stage, but instead, for example, larvae-specific features of the wing and genital systems are gradually re-formed in the direction of the imaginal final stage, or the external systems develop late, so that wingless larvae and wing-bearing pronymph and nymph stages can be distinguished. The hemimetabolic insects do not go through a caterpillar stage, but the juvenile nymphs grow up and transform themselves into adult animals, whereby they moult several times .
In the case of complete metamorphosis ( holometabolism ), the types homometabolism, remetabolism, parametabolism and allometabolism are differentiated. This form also includes the example of the butterfly with its intermediate pupal stage and the complete reconstruction of the organs that takes place. In the case of holometabolic insects, only the caterpillars that shed their skin several times grow . The adults are fixed to their size by their chitin shell and cannot grow. Regarding the process of complete metamorphosis, it can be said that the creature emerging from the larva is usually a completely new creature. The original larva is initially almost completely dissolved by its own digestive juices and thus dies. This partial self- digestion is called histolysis . Only a few collections of special cells called histioblasts , which did not fulfill any function during the larval stage, are spared from this process and form the basis for the newly emerging organs. How much original tissue is preserved differs from species to species, but in butterflies , for example, it is only a few percent.
Large monarch butterfly caterpillar
Also from parasitic worms , e.g. B. in liver fluke , complicated transformation processes with several intermediate stages are known. The extensive research on the fruit fly Drosophila melanogaster revealed interesting details about the metamorphosis of the insects.
The concept of metamorphosis in natural philosophy
The idea of metamorphoses was widespread in the sciences of the 18th and 19th centuries, originally this idea came from Johann Wolfgang von Goethe , who was also a natural scientist and, among other things, examined the metamorphosis of plants in search of the original plant . The term was also used by Carl von Linné (1707–1778).
- Adolf Portmann : Metamorphosis of the animals. In: Adolf Portmann: Biology and Spirit. Göttingen 2000, ISBN 3-89762-002-2
- Rudolf Steiner: Goethe's natural scientific writings. Novalis Verlag 1949.
- Johann Wolfgang von Goethe: The metamorphosis of plants. De Gruyter Verlag 1995. ISBN 978-3050039169 .