The following points highlight the three main types of metamorphosis in insects. The types are: 1. Ametabolic Metamorphosis 2. Heterometabolic Metamorphosis 3. Holometabolic Metamorphosis.
Type # 1. Ametabolic Metamorphosis:
In lower insects (Collembola, Thysanura) the young which hatches from an egg is a miniature of the adult and is called a nymph, it differs from the adult in having immature reproductive organs; by several moultings and growth it becomes an adult.
These insects are primitively wingless, they are also called Apterygota, e.g., Lepisma, the change from young to adult is negligible, such insects are ametabolic because there is no metamorphosis.
Type # 2. Heterometabolic Metamorphosis:
In winged insects the adult differs in several respects from the young, such insects are said to undergo metamorphosis in becoming adults. The nymph which hatches from the egg has a general resemblance to the adult in body form, type of mouth parts and possession of compound eyes, though these nymphs may have adaptations associated with their particular habits of being aquatic, swimming or burrowing.
In these the change from nymphs to adults is a gradual process in which appendages, mouth parts, antennae and legs of the nymph grow directly into those of the adult.
Wings develop gradually as external outgrowths of thorax and are visible externally in the nymphal instars, because of their external wing development they are also called exopterygota. The reproductive organs mature gradually. Insects showing this slight change from nymph to adult are known as heterometabolic (gradual), they include Dictyoptera, Orthoptera, Isoptera, Hemiptera and Anoplura.
Though nymphs of dragon flies, may flies, etc., are quite different from the adult in having special nymphal adaptations because their nymphs are aquatic, while the adults are aerial, the nymphal adaptations are shed in changing into adults, such forms with slightly greater changes are called hemimetabolic (incomplete), they include Odonata, Plecoptera and Ephemeroptera.
Type # 3. Holometabolic Metamorphosis:
In Lepidoptera, Coleoptera, Hymenoptera, Diptera, Siphonoptea, etc., the young which hatches from the egg is called a larva, the larva is very different from the adult in structure, body form, mouth parts, legs and in its mode of life, the larva has lateral ocelli in place of compound eyes, it feeds voraciously, grows, moves about and undergoes ecdyses.
The larva is so different from the adult that it first changes into a resting, quiscent instar called a pupa which is often enclosed in a cocoon secreted by the labial glands of the larva. Great transformation occurs in this instar, wings develop internally from pockets of the hypodermis, and they are not visible from outside.
Because wings develop from internal imaginal discs these insects are also called endopterygota. Appendages are formed, muscles, tracheae and parts of the alimentary canal are replaced by corresponding organs of the imago. Such vast changes are called holometabolic metamorphosis.
In holometabolic insects there is an internal reconstruction during late larval and pupal instars. Larval organs, with the exception of central nervous system and developing reproductive organs, are disrupted, their breaking down is called histolysis, this is brought about by phagocytes which feed on the organs, and products of their digestion are then used for building new structures.
The building of new structures is brought about by growth centres called imaginal buds or discs. Imaginal discs are groups of formative cells which are set aside in the larva, they are the rudiments of future organs of the imago, they form legs, mouth parts, internal organs and wings.
This process of formation of organs of an imago from imaginal discs inside the pupa is known as histogenesis and it results in the formation of the imago.
Thus, two postembryonic processes occur in all insects, the first is growth in the young and the second is metamorphosis, in both of which moulting takes place; both processes are controlled by hormones of endocrine glands. Insects have two such endocrine glands, they are corpora allata and prothoracic glands.
The juvenile hormone of corpora allata controls growth and moulting up to the end of the larval period. So long as the juvenile hormone of corpora allata is produced the final moulting into a pupa or into an adult cannot take place.
The prothoracic glands are a pair of small glands in the first thoracic segment, they produce a hormone called ecdyson which brings about moulting and development of imaginal discs and reproductive organs.
When both hormones are secreted, then moulting of the larva only will take place. The result of the two hromones is supression of adult characters from appearing during larval and pupal instars. When only ecdyson is secreted, and the juvenile hormone is not produced, then the larva will moult into a pupa, and the pupa into an imago.
Thus, it is seen that ecdyson is essential for each moulting, but its action is modified as long as the juvenile hormone is present. Removal of the old cuticle in ecdysis is brought about by an enzyme secreted by the hypodermis, the enzyme erodes the lower surface of the cuticle, then the hypodermis secretes a new cuticle below the old one.