Metamoephosis in insects

In insects the development is usually indirect the egg hatches into a dissimilar larva instead of a miniature adult.The transformation of an immature larval individual into a  sexually mature reproducing adult of very different form, structure and habit, is called metamorphosis. Insects show four types of metamorphosis:

1. No-metamorphosis or ametabolous development : 

In ametabolous development, metamorphosis does  not occur. These insects are most primitive and wingless. E.g., silver fish, spring tails.

2 Incomplete metamorphosis or hemimetabolous development :

In this type of metamorphosis, the  life history includes egg, naid(young) and imago (adult). Young is aquatic and respires by tracheal gills, whereas, the adult is  aerial and respires by tracheae. E.g.,  mayflies, dragon flies, and stone flies.

3.Gradual metamorphosis or paurometabolous development : 

In this type of metamorphosis, the  life history includes egg, nymph (young) and imago  (adult). The nymph resembles an adult in general body  form, but lacks wings and external genital appendages. E.g., cockroaches, grasshoppers, locusts, termites, stick insect, praying mantis, bed bug and lice etc.

4. Complete metamorphosis or holometabolous development : 

In this type of metamorphosis, the life history includes fourdevelopmental stages – egg, larva, pupa and imago (adult). Larva after hatching, moults several times to become a fully grown adult. It later becomes a pupa within a secreted case, called  puparium. Pupa differentiates into the young adult that breaks the puparium open and emerges out. It grows to a mature form, e.g., butterflies, moths, beetles , houseflies,  mosquitoes, fleas, honey bees, ants, wasps, etc.

Hormonal control of insect metamorphosis:

Metamorphosis or post embryonic growth of insects, is under hormonal control. The various hormones secreted are as follows :

1.Brain hormone (BH) : 

It is secreted by the neurosecretory cells of the brain. It serves to activate  corpora cardiaca.

2.Prothoracicotropic hormone (PTTH) : 

This hormone is secreted by the corpora cardiaca, which in turn stimulates the prothoracic glands.

3.Prothoracic gland hormone (PGH) or ecdyzone: 

This hormone is secreted by the paired, bilateral sheet of cells in thorax, constituting the prothoracic glands. This hormone is known to trigger moulting as it acts on the tissues to promote all of the changes characterising a moult.

4.Juvenile hormone (JH) :

This hormone is secreted by corpora allata. Ecdysone enhances larva-to-larva moults as long as there is sufficient JH. Ecdysone increases pupation when JH levels are reduced. The adult is formed when JH is completely absent.