Arthropoda: Metamorphosis in insects and its hormonal regulation | Zoology Optional Notes for UPSC PDF Download

Overview of Metamorphosis in Insects

Definition of Metamorphosis

• Metamorphosis refers to the transformation of an immature larval organism into a sexually mature, reproducing adult.
• The adult resulting from this process is markedly distinct from the larval stage in terms of form, structure, and behavior.

Insect Growth and Maturation

• Insects undergo stages of growth and maturation, which are categorized into specific phases known as metamorphosis.
• These phases segregate their lives into discrete periods for resting, growing, and reproduction.

Comparison with Human Growth

• Human growth is gradual, starting from infancy and progressing in small increments until adulthood.
• In contrast, insects experience metamorphosis, involving distinct stages that can be significantly different from one another.

Types of Metamorphosis in Insects

• There are four primary types of metamorphosis in insects, each characterized by specific developmental patterns:
  1. No-Metamorphosis
  2. Complete Metamorphosis
  3. Gradual Metamorphosis
  4. Incomplete Metamorphosis

Variability in Egg Development

• In most cases, insects begin life as eggs, which typically hatch a few days after being laid.
• Some insect species, however, remain in the egg stage for an entire season before hatching.
• Longer egg incubation periods allow these insects to grow and gain strength within the protective egg environment.

Detailed Description of Metamorphosis Types

• In ametabolous development, newly hatched insects closely resemble adult forms, except for differences in size and armature, such as spines and setae.

Incomplete Metamorphosis (Hemimetabolous Development)

• Hemimetabolous development involves immature stages referred to as nymphs or naiads.
• Nymphs are aquatic and respire using tracheal gills, while adults are terrestrial and respire through tracheae.

Gradual Metamorphosis (Paurometabolous Development)

• Paurometabolous development, or gradual metamorphosis, features newly hatched young insects that generally resemble adults in body form.
• These young nymphs lack wings and external genital appendages and undergo multiple nymphal stages through successive molts to reach adulthood.

Complete Metamorphosis (Holometabolous Development)

• Holometabolous development, known as complete metamorphosis, includes four distinct metamorphic stages: egg, larva, pupa, and adult.
• Larvae undergo multiple molts, growing into fully developed larvae, and then pupate within a secreted case called a puparium.
• Inside the puparium, pupation transforms them into adults.

This structured information provides a comprehensive understanding of the concept of metamorphosis in insects, including its types and the developmental changes that insects undergo in each type. It also highlights the significance of egg incubation periods in insect growth and maturation.

Incomplete Metamorphosis (Hemimetabolous Development)

Life Stages

• Immature stages in incomplete metamorphosis are known as nymphs or naiads. These nymphs bear resemblance to the adult form in many aspects.
• The distinctive feature of this type of metamorphosis is that the nymphs and adults coexist in similar habitats during certain periods of the insect's life cycle.

Respiration

• Nymphs, being the immature stages, are primarily aquatic and respire using specialized tracheal gills that facilitate respiration in aquatic environments.
• In contrast, the adults of these insects are terrestrial and employ tracheae for respiration, adapting to life on land.

Examples
Insects exhibiting incomplete metamorphosis belong to various orders, and some notable examples include:

• Grasshoppers (Order Orthoptera)
• Termites (Order Isoptera)
• Booklice (Order Corrodentia)
• Thrips (Order Thysanoptera)
• True bugs (Order Hemiptera)
• Aphids (Order Homoptera)
• Earwigs (Order Dermaptera)

Incomplete metamorphosis, characterized by the coexistence of nymphs and adults and the transition between aquatic and terrestrial habitats, is observed in a diverse range of insect orders. This type of metamorphosis facilitates adaptation to varying environmental conditions during different life stages.

Gradual Metamorphosis (Paurometabolous Development)

Life Stages

• Newly hatched young insects exhibit a striking resemblance to the adult in terms of general body structure. However, they lack wings and external genital appendages at this stage.
• Over time, these young nymphs undergo several nymphal stages through successive molting processes. With each molt, they gradually acquire additional adult characteristics.

Examples
Insects displaying gradual metamorphosis belong to various orders. Some prominent examples include:

• Mayflies (Order Ephemeroptera)
• Dragonflies (Order Odonata)
• Stoneflies (Order Plecoptera)

Gradual metamorphosis enables insects to gradually develop into adults, with each nymphal stage representing a step toward full maturation. This type of metamorphosis is common among specific insect orders, allowing them to adapt to their environments as they mature.

Complete Metamorphosis (Holometabolous Development)

Metamorphic Stages

1. Egg: The insect begins its life as an egg, where it undergoes initial development.
2. Larva: After hatching from the egg, the insect takes the form of a larva, which is typically a feeding and growth stage. Larvae often have different habits and body structures from the adult form.
3. Pupa: Following the larval stage, the insect transforms into a pupa. This stage involves a period of rest and metamorphosis within a protective casing.
4. Adult: The final stage is the emergence of the adult insect from the pupal casing.

Examples
Insects exhibiting complete metamorphosis belong to various orders. Notable examples include:

• Lacewings (Order Neuroptera)
• Beetles (Order Coleoptera)
• Scorpionflies (Order Mecoptera)
• Caddisflies (Order Trichoptera)
• Moths and Butterflies (Order Lepidoptera)
• Flies (Order Diptera)
• Fleas (Order Siphonoptera)
• Wasps and Bees (Order Hymenoptera)

Complete metamorphosis allows insects to undergo a thorough transformation from the egg stage to the adult stage, with each stage serving specific functions in the insect's development. This type of metamorphosis is observed in diverse insect orders, enabling them to adapt to various ecological niches.

Hormonal Regulation of Metamorphosis in Insects

• Hormonal control of metamorphosis in insects was first elucidated by V. B. Wigglesworth in the 1930s, revealing the critical role of hormones in this process.

Hormonal Pathway

1. Sensory Input and Brain Hormone (BH) Release:

   • When an immature insect reaches a certain size, it requires a larger exoskeleton to accommodate its growth.
   • Sensory input from the body activates specific neurosecretory cells in the insect's brain.
   • These neurons secrete brain hormone (BH), initiating the metamorphic process.

2. Prothoracicotropic Hormone (PTTH) Activation:

   • The release of BH triggers the corpora cardiaca to release prothoracicotropic hormone (PTTH) into the circulatory system.

3. Prothoracic Gland Hormone (PGH) Release:

   • PTTH stimulates the prothoracic glands to secrete molting hormone, also known as Prothoracic gland hormone (PGH).
   • PGHs belong to the class of ecdysteroids and play a pivotal role in initiating the molting process.

4. Apolysis and New Exoskeleton Synthesis:

   • PGH affects various cells throughout the insect's body, primarily triggering a series of physiological events called apolysis.
   • During apolysis, a new exoskeleton begins to form as a soft, wrinkled layer underneath the existing hard exoskeleton.

5. Variable Duration of Apolysis:

   • The duration of apolysis varies depending on the insect species and its characteristic growth rate, ranging from days to weeks.
   • Once the new exoskeleton is sufficiently developed, the insect is ready to shed the old exoskeleton, resulting in a stage where the insect's body is covered by two layers of exoskeleton, known as "pharate."

6. Ecdysis Triggered by Eclosion Hormone:

   • Towards the end of apolysis, ecdysteroid concentration decreases.
   • Neurosecretory cells in the ventral ganglia then begin to secrete eclosion hormone, initiating the physical process of ecdysis (shedding the old exoskeleton).
   • Rising eclosion hormone concentration stimulates other neurosecretory cells in the ventral ganglia to secrete bursicon, a hormone responsible for the hardening and darkening of the integument through quinone cross-linkages in the exocuticle (sclerotization).

7. Role of Juvenile Hormone (JH):

   • In immature insects, the corpora allata secrete juvenile hormone (JH) before each molt.
   • JH acts by inhibiting genes that promote the development of adult characteristics, keeping the insect in an immature state.
   • During the last larval stage, the corpora allata atrophies and ceases JH production, lifting the inhibition on adult development, and enabling the insect to molt into an adult.

8. Reactivation of Juvenile Hormone (JH):

   • At the approach of sexual maturity in adult insects, brain neurosecretory cells release a brain hormone that reactivates the corpora allata.
   • Reactivated corpora allata stimulate the renewed production of JH.
   • In adult females, JH stimulates yolk production for egg development, while in adult males, it triggers accessory glands to produce seminal fluid and spermatophore casing.
   • Lack of normal JH production renders adults sexually sterile.

Hormones Involved in Metamorphosis

This complex hormonal control system orchestrates the metamorphosis of insects, ensuring the precise timing and execution of each developmental stage.