Metamorphosis is the transition during the life history of some animals from birth or hatching to the adult stage. Metamorphosis is considered an indirect form of development, in that a metamorphic animal passes through morphologically distinct stages before reaching the adult form. In contrast, higher animals undergo direct development in which young and old resemble one another except in size and sexual maturity.
Metamorphosis occurs in at least 17 phyla of the animal kingdom, including Porifera (sponges), Cnidaria (jellyfish), Platyhelminthes (flat worms), Mollusca (snails), Annelida (worms), Arthropoda (insects), Echinodermata (sea urchins), and Chordata (vertebrates).
Metamorphosis is associated with adaptive changes in the way an organism interacts with its environment. For example, adult amphibians often eat very different foods than their larvae. Thus, adults and larvae do not compete for food. A second example of the adaptive significance of metamorphosis is in barnacles in which adults are sessile but the larvae are free-swimming. Thus, the dispersal of larvae gives adults the opportunity to colonize new habitats where the local environment might be more favorable.
Cnidarians have varying types of metamorphosis. Some species have three distinct life history stages: the planula larva, medusa, and polyp. The former two stages are free swimming and the polyp stage is sessile and may involve a single individual or a colony of individuals.
Metamorphosis differs in many amphibian species. In frog development the eggs hatch into tadpoles that have external gills and are vegetarian. Several significant changes occur during metamorphosis into the adult, including growth of a large mouth and tongue, loss of gills, formation of lungs, growth of the front legs and loss of tail.
Numerous biochemical changes accompany these morphological changes, such as synthesis of a new visual pigment in the eyes and a new oxygen-binding hemoglobin protein in the blood. The adult is mainly insectivorous and partly terrestrial. During metamorphosis bones begin to ossify, the tail is reabsorbed, limbs form, and larval respiratory and feeding structures are replaced by adult structures. The digestive system is remodeled to accommodate strictly carnivorous diet.
Some of the best known cases of metamorphosis occur in insects. The transformation from larva to adult butterfly undergoes during pupation and is an example of metamorphosis. The Ametabola such as silverfish do not undergo metamorphosis. During development, these insects increase in size, but do not undergo distinct changes. This is called direct metamorphosis or anamorphosis.
The Hemimetabola undergo gradual metamorphosis. This is exemplified by insects such as the dragonflies, termites, roaches, and grasshoppers. In these, the nymph hatches from the egg and resembles the adult, except that they are smaller.
The wings of the Hemimetabola grow gradually during a series of molts in which the cuticular exoskeleton is shed, allowing for growth. 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.
The Holometabola, which includes moths, butterflies, beetles etc. undergo complete metamorphosis. A worm-like larva with short legs, no wings, and simple eyes, hatches from the egg and increases in size through a series of molts, eventually developing into a pupa inside a cocoon.
The pupa is often considered a resting stage and can often survive in unfavorable environments. Eventually, the pupa metamorphoses into an adult. In this process, the pupa resorbs larval organs and an imaginal discs is formed and reshape the insect. The adult typically has wings, compound eyes, legs, antennae, and sexual organs.
Holometabolism is also known as complete and complex or indirectmetamorphosis. 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. Histolysis is the decay and dissolution of organic tissues or of blood and cell regrowth histogenesis. Histogenesis is the formation of different tissues from undifferentiated cells.
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