Metamorphosis in Insects - Hormonal Control 

               Or

What role do hormones play in insect development?

                    Or

What is metamorphosis? Describe the hormonal regulation of metamorphosis in insects.

Answer:

The process of transformation of an immature larva into a sexually mature reproducing adult, passing through different stages is called metamorphosis.

( meta - " change" and morphe - " form" )

  For your information: metamorphosis is mostly seen in insects and amphibians, but some fish and marine invertebrates also show metamorphosis

In insects, a system of endocrine glands produces hormones that regulate moulting and metamorphosis.

Metamorphosis in insects - hormonal control: Diagram

Hormones are secreted by certain modified brain cells. These nerve cells are called neurosecretory cells in the brain( pars intercerebralis) secreted by corpora cardiaca which secrete the prothoracico-tropic hormone PTTH ( ecdysiotropin). This stimulates prothoracic glands in the thorax to secrete ecdysone (a steroid hormone: 20- Hydroxyecdysone) which triggers moulting by acting on the tissues promoting it. The initiation of the moulting cycles in insects is not a random or periodic event.

In moulting, 

• the retraction of the epidermis from the old cuticle, 
• secretion of the new cuticle, 
• and finally the shedding of the partially digested old cuticle are controlled by hormones. 

This shedding of old exoskeleton at the end of each moult in insects is called ecdysis and is typically followed by the expansion and tanning of a new one. 

Metamorphosis - Molting, shedding of old exoskeleton (butterfly larva): Ecdysis

The corpora allata, which is another component of the retro-cerebral complex, secretes another hormone called juvenile hormone (JH).

In the presence of JH, ecdysone can bring about moulting only up to the larval stage, but when the secretion of JH diminishes to a certain level, the larva moults to a pupa and with the entire ceasing of JH, the pupa moults to adult form.

Adult butterfly from pupa(chrysalis)

In the final stages of moulting into adulthood, the postecdysial processes include expansion and hardening of wings.

Studies have shown that premature treatment of midge larvae with ecdysone shows the prompt appearance of localised puffs on their chromosomes inferring a visible indication of this hormone being involved in the activation of specific genes.

SUMMARY: Hormonal control of metamorphosis in Butterfly. 

Diagram: metamorphosis in butterfly, different stages of larva, ecdysis in larva, pupa (chrysalis), adult butterfly

Certain brain cells secrete a prothoracicotropic hormone( PTTH) which prompts the prothoracic gland to release ecdysone, a steroid hormone that triggers both moulting and metamorphosis. However, in the presence of the juvenile hormone (JH) from corpora allata, the metamorphosis is suppressed and so the larva only moults to another larval stage. At this phase, the juvenile hormone ceases to be produced and now ecdysone initiates metamorphosis forming pupa and then emerging from the chrysalis ( pupa) and within a few hours grows full length to become a beautiful adult butterfly.

Hormones secreted

Brain Hormone (BH): 

They are secreted by neurosecretory cells of the brain. It is a lipid. It activates the corpora cardiaca in the retro-cerebral complex of the stomatogastric nervous system.

PTTH ( Prothoracicotropic Hormone): 

Secreted by the corpora cardiaca, which stimulates the prothoracic glands.

PGH (Prothoracic gland hormone):

This hormone is secreted by the cells of the thorax which are present as bilateral sheets and constitute the pro-thoracic glands. They are in pairs. This hormone is ecdysone which triggers moulting as it acts on tissues to bring about the changes required to moult.

JH ( Juvenile Hormone):

JH is secreted by corpora allata, another component of the intracerebral complex.

Chemically it is an unsaponifiable, non-sterolic lipid. This hormone governs and promotes the morphogenesis of larva to adult.

Related QnA:

Question:How does insect development proceed?

Answer:

Insects lay land-adapted (cleidoic) eggs. The chitinous shell (chorion) is several layers thick, protecting the developing embryos from dehydration. It is secreted by ovarian follicle cells in the ovarioles and is penetrated by one or more pores,(micropyles) through which sperm from the spermathecae enter as the eggs are laid. 

Except for the eggs of wingless collembolans, which cleave holoblastically. Cleavage is superficial on the surface of the central yolk mass. 

At hatching, the young of primitive, wingless insects resemble the adults and undergo no metamorphosis, simply increasing in size with each moult and finally attaining sexual maturity. All other insects undergo some type of metamorphosis.

Question: Give a brief account of types of metamorphosis in insects.

Answer:

The transformation of an immature larval individual into a sexually mature, reproducing adult of different forms, structure and habits is called metamorphosis.

Types of Metamorphosis in Insects:

Insects display four types of metamorphosis.

1. No metamorphosis or  ametabolous development:

Here the maturation occurs without metamorphosis. Example silverfish. 

In this case, the newly hatched creature looks like an adult except in size and armature of spines and Setae.

2. Incomplete metamorphosis or hemimetabolous development:

This is seen in some terrestrial insects that have aquatic larvae, e.g. Mayflies, dragonflies, and stoneflies.

Incomplete metamorphosis involves an abrupt moult from the wingless, often gilled ( having gills) naiad to the winged adult which is terrestrial or aerial and respire by trachea. This moult takes place in the air after the naiad has crawled above the water line.

3. Gradual Metamorphosis or Pauro-metabolous development:

Gradual metamorphosis involves the progressive transformation through several molts, of the wingless nymph to the winged adult.

Here the newly hatched creature resembles the adult in general body form but has legs wings and external genital appendages.

The young ones or the nymphs, undergo several stages of successive moulting to become an adult,e.g. grasshoppers, aphids, stink bugs etc.

4. Complete metamorphosis or holometabolous development:

The life cycle showing complete metamorphosis includes 4 developmental stages.  Egg, larva, pupa and adult.

 The pupa is the quiescent stage that intervenes between larva and adult. The larva often called maggot, grub or caterpillar, grows through several moults. Then moults to the pupal state. The pupa is a secreted case called puparium or chrysalis. This in turn moults to the adult and breaks the puparium open and emerges outside, sometimes after a prolonged period of winter or dry season dormancy. 

Pupal dormancy is often caused by adverse external conditions and this type of programmed dormancy is known as diapause which is initiated or terminated by photoperiod. e.g. Butterfly, housefly, mosquito.

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