Metamorphosis in Insect : Post Embryonic development


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Metamorphosis in insects 🐝

One of the characteristics features of the insect development is that they almost always hatch in a condition morphologically different from that assumed in the imago. In order to reach the later instar they pass through changes in the form and structures and sometimes in habit which is collectively termed Metamorphosis. However, the degree of metamorphosis is varies in different groups. Broadly three types of metamorphosis have been recognised :

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1. Slight or No metamorphosis i.e, ametabolous development:

In primitive wingless insects ( Apterygota ) like silver fish, spining tails etc. And in secondary apterous exopterygotes the young one that emerges from the eggs is essentially similar to the imago, different from it only in size, immature reproductive organs and external genitalia. It moults several times leading to gradual increase in the size accompanied by maturation of sex organ and appearance of external genitalia. The moulting and growth continues even after reproductive maturity. These insects are therefore regarded to have no metamorphosis. But , since the changes are marked, it is now described that these have slight metamorphosis and these insects are called Ametabolous.

Egg => young=> Imago

2. Hemimetabolous or incomplete metamorphosis:

Insects of exopterygota exhibits simple metamorphosis. The young one that hatches out of the eggs resemble the adults or imago in general form and metamorphosis in them includes gradual growth and equitation of wings, genitals and gonads. Therefore, a pupil stage is one thing and the young one are known as nymphs. The degree of metamorphosis varies in hemimetabolous form but two major types are very distinct.

a) Gradual Metamorphosis:

 In insects like cockroaches, grasshopper and cricket etc. the nymphs is similar to adult both in structures as well as in habits, but lacks wings, gonads and external genitalia and Also the wingpad in the second and third thoracic segments. In the later level period the transformation of wingpads into functional wings occur at the last moult. This last nymphal stages with wingpads and genitalia is called imago stage which moults finally into a winged adult, after which the insect doesn't moult anymore. These is no quiescent stage and no disappearance and remodeling of larvel Parts. 

             Egg => Nymph => Imago

b) Partial Metamorphosis:

 In insects like Dragonflies, Mayflies, Damselflies the nymphs differ from adults both in external structures as well as in habits. The nymphs are aquatic and herbivores, therefore they passess tracheal gills for respiration, mendi bulate mouth parts for cutting vegetables and some organs for locomotion. These young once are known as naids. These undergo extensive remodelling to assume adult organization by shielding gills, modifying mouth parts and developing wings. There may be one or more quiescent or semi quiescent stages ( recalling pupa ) and disappearance of a few larvel Parts ( gills. etc. ) and remodelling of some other parts ( mouth parts ) . Therefore, this type of metamorphosis is known as partial or incomplete metamorphosis.

Egg => Naide => Imago.

3. Complete metamorphosis or Holometaboly

In all endopterygota insects like butterflies, moths , bees, wasps, beetles and fly etc. the egg hatches in a worm like larva,very much different from imago on adult in structure, appearance and habits. The larva has a worm like body , bitting and chewing type of mouth parts, simple eyes and poorly developed walking legs. It has quote a different ways of life. For example, the larva of butterfly crolls on the ground, eats leaves while adult butterfly flies in the air and drinks nectar from flowers. The mosquito larva lives in water and eats algae and protozoa while the adults sucks blood or flower and fruit juice.

    Larva whether crolls or swims, eat voraciously , grow in size and mouth several times and finally changes into a quiescent stages called pupa. The pupa neither moves nor feeds and is enclosed in a pupal cage on puparium. Inside the puparium two type of changes occur at a fast speed. There are histolysis and histogenesis.

i) Histolytic Changes: 

In histolysis , all the larvel organs except the central nervous system are broken down by special ambebocyte cells called phagocytes. The tissue fluid from by their distruction is utilised as raw material in the construction of adult organs.

ii) Histogenesis: 

This includes the development of adults organs from group cells of imaginal buds or imaginal discs. The imaginal buds are actually the rudiment of future organs of the adults. Such as wings, walking legs, antenna, mouth parts and internal organs etc. These developed directly from eggs and remain quiescent or non functional all through the larvel stage. During pupal stages these discs or buds grow in size and differentiate into adults structure which remain collaps and folded. When recognisation is completed, it undergoes another moult and the young insect or imago immerges out. At each immergent the blood is pumped into these collaps structure, which unfold and inflate and chitin is deposited to make them hard.

Egg => Larva => Pupa => Imago.

Content reference Asst. Prof. S. Bhattacharjee  BAHONA COLLEGE Dept. Zoology. {alertInfo}

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