Metamorphosis
In animals, metamorphosis is the change in structure from larva to adult. Examples of animals that undergo metamorphosis include insects, tunicates, and anurans (frogs and toads). Tunicates are animals with a notochord in the larva and no internal skeleton in the adult. Amphibians are the only vertebrates that have metamorphosis as part of their life cycle.
In Anurans, tadpoles (larvae) live in water, while adults live on land. Tadpoles are herbivores with internal gills, a lateral line system that detects water movements, and a long finned tail. They have no legs, swim with undulating movements and resemble fish. During metamorphosis, they develop legs and lose their gills and lateral line system. A double-loop circulatory system with a three-chambered heart replaces the single-loop two-chambered heart. The young amphibians develop air breathing lungs, and a pair of external eardrums. They become carnivores as teeth and jaws appear, and they can digest animal protein. The hormone, thyroxine, controls metamorphosis in amphibians. Different anurans have different rates of metamorphosis. Desert toads require eight days, while bullfrogs take two to three years. Scientists believe that in some species the rate of metamorphosis is related to ponds that have wet and dry phases. They are of the opinion that some amphibians survive by changing into adult form before the pond dries up.
Many tunicates (sea squirts) are marine animals that, as adults, cling to rocks, piers, and boats. Their larvae resemble tadpoles externally. Internally, however, tunicate larvae have the characteristics of chordates, the phylum in which they are classified. They possess a dorsal hollow nerve tube, and a notochord, which is a firm but flexible tube below the nerve tube. They have gill slits that function in filter-feeding and a tail. The larvae swim freely, eventually attach to a surface by their heads, and undergo metamorphosis. As adults, tunicates have no nerve tube, no notochord, and no tail. Their only remaining chordate characteristic is their gill slits.
There are three possibilities regarding insect metamorphosis. In some insects, such as silverfish, there is no metamorphosis. The juveniles look like the adults only smaller. Other insects, such as grasshoppers, undergo incomplete metamorphosis during which adults lay eggs that develop into nymphs. Nymphs are smaller than adults and lack wings and reproductive organs. Still other insects, such as butterflies, flies, and beetles, undergo complete metamorphosis. In this process, the adults lay eggs that develop into larvae (caterpillars, grubs, or maggots). The larvae eat voraciously and grow. As they increase in size, they molt or shed their skins several times. They replace their old skins with new exoskeletons. When the larvae reach a certain size, they settle down, form a protective case, and become inactive. Now they are called pupae. During the pupal stage, their tissues break down, and adult tissues develop. They emerge from their pupa cases when development is complete. Molting and metamorphosis in insects are controlled by the interaction of three hormones: ecdysone, brain hormone, and juvenile hormone.
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