Metamorphosis means transforming, or change in nature. In biology, metamorphosis is the process by which an animal develops continually, after birth or hatching. This involves the change in body structure; both internal and external features, regulated by hormones. Metamorphosis is occasionally accompanied by changes in the animal’s behaviour, physiology and biochemistry.
Animals that undergo metamorphosis
Insects exhibit metamorphosis in different ways, depending on the species. Metamorphosis in insects is regulated by hormones such as ecdysone, prothoracicotropic hormone (PTTH), and juvenile hormone. Each hormone plays a different role in the transformation. In insects, there are three types of metamorphosis: complete (holometabolous), ametabolous, and incomplete (hemimetabolous) metamorphosis.
In holometabolous development of insects (e.g. flies, mosquitoes, butterflies, beetles and tsetse flies), it comprises four different stages: egg, larva, chrysalis (pupa) and imago (adult). Holometabolous formation starts from the larval stage after hatching. The larva moults about 3-4 times before proceeding to the pupal stage; this process lasts for days. The pupal stage is more advanced in the development of some body features, though the features are not developed fully. After some days or weeks, the pupa finally emerges as an adult, which is the final stage. All the features are developed fully in the adult stage.
Insects that exhibit hemimetabolous or incomplete metamorphosis (e.g. grasshoppers, aphids, termites and true bugs) only undergo three stages: egg, nymph and imago. In this type of transformation, an egg hatches into a nymph which resembles the imago but is very tiny and lacks some external features such as wings. The nymph only moves by hopping, feeding on tender grasses. By moulting, or ecdysis, several times, the nymph finally emerges as an imago. As an adult, its wings are formed perfectly and used for flight. The internal features such as the digestive and reproductive systems are also well-developed.
Ametabolous metamorphosis occurs in insects, such as springtails and silverfish. These insects undergo no true metamorphosis. They hatch from eggs as nymphs. The nymphs have all the features found in adults but smaller in size. The nymphs only grow in size until a reasonable adult size.
In amphibians (e.g. frogs and toads), their embryos develop into fish-like larvae called tadpoles. These tadpoles have some features similar to bony fishes. Features like fins, gills and lateral line are all present in both creatures. Tadpoles develop in stages before becoming adults:
First stage. After hatching from the eggs, which takes some days, pairs of external gills appear in both sides of the head. They feed actively on aquatic plants and microscopic organisms, using their pair of horny toothed jaws to chew. At this stage, tadpoles are totally vegetarians and live in water.
Second stage. At this stage, the tadpoles increase in size. External gills disappear, leading to the formation of internal gills. The mouth, nostrils and eyes are well formed. The ears develop into tympanic membranes. Also, at this stage, the tadpoles resemble fishes and breathe like them because of the development of internal gills.
Third stage. After some weeks of active feeding, the tadpoles begin to develop hind-limbs. The hind-limbs are very tiny; hanging between the head and the tail. Tadpoles do not use them for movement because they are not well-developed.
Fourth stage. The development of forelimbs begins. The tail also begins to depreciate in length. Also, they start to feed on small dead organisms. In addition, the lungs start to develop, and the gills gradually disappear.
Fifth stage. This is the final stage. The tadpoles are now adults. They now feed on either insects or worms. They migrate from water to land and use well-developed lungs to breathe. The tail disappears, thereby forming a trunk. They use both the forelimbs and hind-limbs for locomotion. Both the internal and external features are well-developed at this stage.
Bony fishes (e.g. tilapia, carp etc) also undergo metamorphosis. The eggs start to develop embryos which hatch as small young fishes (larvae). Each larva bears a yolk sac, which is attached to its body. This yolk sac provides food to the individual. The provision of food by the yolk sac continues until the larva migrates into the adult stage. While in this stage, the yolk sac disappears because the now-adult fish is able to fend for itself.