14.4.      Insect Metamorphosis

Insect metamorphosis involves the degeneration of larval tissues and their complete replacement by different cell population. Usually, insects grow by moulting, shedding their cuticle and developing new cuticle as their size increases. The three major patterns of insect development.

Ametabolous development: 

In this type of development, the insects hatch the egg as a pronymph that resembles the adult characters and grow directly as an adult without an intermediate stage. e. g.  springtails and mayflies.

Hemimetabolous development: 

The development of insect includes the hatching from pronymph as the immature adult, nymph. The nymph develops gradually with each moult and finally become an adult. e.g. bugs and grasshopper.

Holometabolous development: 

The insect hatches in a juvenile form, the larva that grows in size with each moult. Then the larvas shows metamorphose is to pupa, the stage of transformation. Then finally adult shows a resemblance from the pupa. e.g. beetles, moths and butterflies.

The larva composed of two distinct populations of cells: the larval cells- for juvenile insect, and the cluster of imaginal cells awaiting the signal to differentiate.

Hormonal control of insect metamorphosis

The moulting process is initiated by the release of a peptide family, prothoracicotropic hormone (PTTH) in response to neural, hormonal, or environmental factors, to stimulate the production of ecdysone from the prothoracic gland. Ecdysone is a prohormone converted into 20-hydroxyecdysone, an active form by a heme-containing oxidase in the mitochondria and microsomes of peripheral tissues. Each moulting organized by the surge of 20-hydroxyecdysone that commits and stimulates the epidermal cells to synthesize enzymes for digestion and recycling of the cuticle components.

Environmental conditions may also control moulting for example in silkworm moth the PTTH secretion ceases after the pupa formation and the development suspended throughout the winter season at a state, called diapause. If the pupa not exposed to cold weather the diapause lasts for a very long time but if exposed to cold for even two weeks, then pupa can moult when returned to a warmer temperature.

The juvenile hormone (JH) is the second major hormone in insect development secreted from corpora allata. It prevents metamorphosis during larval moults. In the presence of juvenile hormone, the hydroxyecdysone-stimulated moults lead to a new larval instar. As juvenile hormone levels drop below a critical threshold value it triggers the secretion of prothoracicotropic hormone from the brain that stimulates the prothoracic glands to secrete a small amount of ecdysone.

Then the hydroxyecdysone formed in the absence of juvenile hormone commits the cells to pupal development through which new mRNAs are synthesized and translated to inhibit the transcription of the larval mRNAs. The second ecdysone surge activates the expression of new pupal genes to transform the larva to pupa.

As shown in the curves of the above Figure, the different cells show responses to the JH at different times. The onset and duration of the JH-sensitive time period is an autonomous state of the cell that is not controlled by hormones. The critical weight during the last larval stage acts as a checkpoint to initiates metamorphosis through an endocrine cascade. When the insect's body has stored all the food it needs to undergo these major changes and no needs to feed. If the diet is poor then the larval period is extended to ensure the critical weight but if corpora allata is removed from larvae in their last instar then they don’t show any adjustment to starvation. Thus, the continuing presence of the juvenile hormone in the last instar can delay the initiation of metamorphosis and the removal of only juvenile hormone (JH) can’t allow the larva to enter metamorphosis. Means there is not only an optimum weight but also a minimum time that is required to initiating metamorphosis.

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