Types of Metamorphosis
1) Incomplete Metamorphosis or Direct or Simple Metamorphosis:
The group of the insects undergoing this type of metamorphosis is also called Hemimetabola. Insects under this type complete their postembryonic development without assuming much striking morphological changes. There are three stages in the life of these insects’ viz. egg, nymph and adult. The young ones on hatching from eggs are called nymphs. These nymphs resemble their parents very much in their structure of body (mouth parts, simple and compound eyes, antennae, legs etc). Similarly they have the same mode of life, feeding habits, food and habitat. The difference between the nymphs and adults is that the nymphs do not process wings and reproductive organs until they turn into full-grown adults. Besides, nymphs are smaller in size and shape.
The wings develop gradually from a small wing pads in the nymphs to a fully developed functional wings in matured adults. This type of metamorphosis is therefore, also called gradual metamorphosis. This wings development takes place externally and hence this group if insect is called Exopterygota. The nymphs grow in size and shape by the process of moulting and the successive nymphs grow in size and shape by the process of moulting and the successive instars look more alike to adults. There is no resting or transitional phase for transformation into adults e.g. dragonflies, damselflies, grasshoppers, cockroach, crickets, aphids, jassids, bugs etc. The degree of metamorphosis is not the same in all Exopterygota. A few insects like white flies and thrips on their developmental period, pass through a stage called incipient or false pupal stage (comparable to pupa of Holometabola) before emerging as an adults. Such deviation from Hemimetabola is called aberrant Hemimetabola. In few Exopterygota insects, the wings may be lost or may not develop at all, for example, head louse, bed etc. But there are rudiments of wings in the form of wing pad/ buds in these insects and hence these insects are called secondarily wingless insects. In dragonflies the nymphs are aquatic while the adults are aerial and therefore this group is called Hemimetabola.
2) Complete Metamorphosis or Complex or Indirect Metamorphosis:
The group of insects undergoing this type of metamorphosis is also called Holometabola. The insects in this type complete their postembryonic development by assuming much striking morphological changes. In order to attain maturity, this insect pass through four different stages viz. egg, larva, pupa and adult. Since there are many changes in form it is called complex metamorphosis. Young one after hatching from the egg is called larva. The larva differs from its parents in the structure, food, feeding habits, mode of life and habitat. The larvae may have biting type of mouthparts, while adults may have different mouthparts such as siphoning type. Similarly they do not have compound eyes but possesses simple ocelli. Legs are also subjected to compound modifications. Some of the larvae have only three pairs of thoracic legs (beetles & weevils) while in other there may be one or more pairs of abdominal legs in addition to thoracic legs (butterflies & month). In some larvae legs are altogether absent (flies). There are several instars during larval stage. There is altogether absent (flies). There are several instars during larval stage. There are no external signs of presence of wing pads or buds on the larvae. However, these pads are present inside the body cavity in thoracic region. These wing pads develop internally hence; this group of insects is called Endopterygota. Further, for transformation into adults the larva has to pass through a resisting phase or transitional phase called pupa. Feeding and movement ceases and metabolic activities are lowered down during the pupal stage but conspicuous changes in morphological forms in the development of wings and reproductive organs occur in the pupal stage. The adult come out of the pupal covering with development of compound eyes, antennae, thoracic legs, wings reproductive organs and changes in mouth parts. Since, a pupal stage is necessary for the transformation of larva into adult, this type of metamorphosis is called indirect or complete metamorphosis. As Endopterygota insects are included under this type of metamorphosis e.g. Butterflies, moths, beetle, weevils, flies, honeybees, wasps, mosquitoes, etc.
This is specialized type of metamorphosis found in higher orders of Endopterygota insects. It is a type of complete metamorphosis in which different larval instars represent two or more markedly different types of larvae. The first instars larva is active and usually campodaeiform and the subsequent larval instars are vermiform or scarabaeiform e.g. blister beetles.