Growth and Metamorphosis In Insects

Introduction

Metamorphosis can be defined as "a process of profound postembryonic reorganization of tissues which usually prepares an animal for the rest of its life in a different habitat."

Metamorphosis is regulated by juvenile hormone secreted from the insect’s brain. Juvenile hormone secretion causes moulting of the insect and results in the emergence of the adult. The soft body of most insects remains covered by a thick envelope called exoskeleton for protection against external environment. An insect pass through egg, larva, pupa, and adult in its life, through growth and transformation. But its exoskeleton has limited capability of expansion. Therefore, insects shed their old skin (cuticle) many times, called ecdysis and form new cuticles through the process of moulting to transform into adults.

The moulting is stimulated by a steroid hormone-ecdysone (moulting hormone) secreted from the prothoracic gland of the insect. Intervals between moults are called 'stages' or 'stadia' and a particular 'stadium' formed by the insect is called 'instar'. An insect undergoes moulting, passes through conspicuous changes, the process collectively known as metamorphosis.

Types of Metamorphosis Found in Insects

Four types of metamorphosis are found in insects. They are –

Ametamorphosis (or Ametabola/ Paleometabola/ Anamorphosis/ No metamorphosis)

There is no metamorphosis. When these insects emerge from eggs, the immatures, called nymphs resemble miniature versions of the adults. They moult and grow only to achieve sexual maturity. This type of metamorphosis is usually observed in primitive wingless insects (Apterygota).

Examples are springtails, silverfish, firebrats, chewing lice, sucking lice, campodeids.

Gradual metamorphosis (or Paurometabola)

In this type of metamorphosis, immatures (nymphs) resemble their adults in general body and food preferences, but lack wings and external reproductive structures. There are three stages in their life cycle - egg, nymph, and adult (imago). No pupal stage is present. Wings are developed for second and third thoracic segments from an early stage, gradual increase in size, colour and markings occur with each moult and external genital organs are formed. The nymph lives an independent life, its span is species- specific and lives in the same habitat as the adults.

Examples are found in grasshoppers, cockroaches, crickets, true bugs, bed bugs, aphids, and termites.

Incomplete metamorphosis (or Hemimetabola/Heterometabola)

Immature or nymphs, called naiads, are aquatic with tracheal gills for respiration, lack wings and genital organs. They slightly resemble adults; transformation occurs by gradual metamorphosis and successive moults developing their appendages, mouth parts, antennae, and legs. Adults and nymphs usually live in different habitats.

Some immature characters are lost in the final moult, therefore incomplete (or 'hemi') metamorphosis is observed. On transformation, naiads come out of water and release their wings externally (called Exopterygota) and develop genital structures.

Examples are cockroaches, grasshoppers, dragonflies, earwigs, thrips, booklice, true bugs.

Complete metamorphosis (or Holometabola)

Most insects develop through complete metamorphosis. Four metamorphic stages-egg, larva, pupa, and adult are present, each stage is distinctly different from the other, therefore complete (or 'holo'-total) metamorphosis takes place. After hatching from eggs, larvae undergo several moults, its final instar secretes an enclosed case-called puparium to form a resting phase-called pupa, inside the puparium, the pupal body tissues differentiate to form adult structures and lastly emerge as adults by breaking the case.

The larva differs in eye structure-presence of lateral ocelli instead of compound eyes like in adults, eats voraciously, and transforms through ecdysis. Secretion from its labial glands forms the puparium, the pupa then develops wings internally from the pockets of hypodermis (called Endopterygota).

Examples can be found in most insects like butterflies, moths, bees, wasps, ants, true flies, and beetles.

Hormonal control of moulting and metamorphosis

Moulting and metamorphosis in insects are regulated by steroid-ecdysone (20- hydroxyecdysone) and lipid-juvenile hormone (JH). Ecdysone induces moulting by a change in gene expression during metamorphosis, while JH inhibits the effect of ecdysone and induces to produce another instar, instead of pupa or an adult.

The moulting process begins in the neurosecretory cells of the brain. They release prothoracicotropic hormone (PTTH) in response to neural, hormonal, or environmental stimuli. PTTH, a peptide hormone, stimulates the prothoracic gland to secrete ecdysone. In the peripheral tissues, this ecdysone is activated into 20-hydroxyecdysone and moulting is initiated. At first, a small amount of hydroxyecdysone is secreted in the larval blood (haemolymph) which signals a change in cellular function. Later, large amounts of hydroxyecdysone initiate cellular differentiation necessary for moulting and stimulate epidermal cells to secrete enzymes for digestion and recycling of the old cuticle.

Juvenile hormone is secreted from the corpora allata. Neurosecretory cells of corpora allata are active during larval moulting but inactive during metamorphic moulting.

Co-ordinated regulation and secretion of hormones during metamorphosis

In the presence of JH, hydroxyecdysone stimulates moults to form larval instars. In the last instar, the medial nerve from the brain to corpora allata inhibits JH secretion and signals the body to degrade the existing JH to drop its level below the threshold value.

The decrease in the threshold value of JH triggers the brain to secrete PTTH, which stimulates prothoracic glands to secrete ecdysone. The active 20-hydroxyecdysone, in the absence of JH signals cellular function for pupal development.

New mRNAs are synthesized; whose protein products inhibit the synthesis of larva- specific genes. Further secretions of hydroxyecdysone synthesize pupa-specific genes and moulting occurs to transform larva to pupa.

Lastly, at the imaginal moult, differentiation of cells (imaginal discs) occurs and the moult transforms the pupa into a fully-grown adult.

Conclusion

An insect has a complex life cycle and through continuous changes from egg, larva, and pupa attains adulthood, known as metamorphosis. This transformation occurs through skin shedding or 'moulting' initiated by ecdysone and JH. Both these hormones are released by neurosecretory cells of the brain during the process of moulting. Not all insects show similar metamorphosis. Some primitive wingless insects like springtails, silverfish, and firebrats have no metamorphosis. Their nymphs resemble miniature versions of their adults. Insects like grasshoppers, cockroaches, crickets, and termites resemble their adults but lack wings and reproductive organs. They undergo gradual metamorphosis into their prolific adult counterparts. Incomplete metamorphosis is also found in insects like- cockroaches, grasshoppers, dragonflies, true bugs, where nymphs are aquatic, and have tracheal gills, but lack wings and genital structures. Some of these features are lost in development and adults emerge, form their external wings and genital organs. Thus, insects survive and live for generations through this amazing boon of Nature through physical transformation and balance of hormones inside their organized bodies.

FAQs

Qns 1. Do all insects undergo metamorphosis?

Ans. Except for some primitive insects, all insects undergo metamorphosis.

Qns 2. Is complete metamorphosis found in all insects?

Ans. No, some insects show incomplete metamorphosis.

Qns 3. Is metamorphosis in insects regulated by hormones?

Ans. Yes, it is regulated by juvenile hormone secreted by the insect.

Qns 4. What are ecdysis and moulting?

Ans. Ecdysis is the shedding of the larval skin and the successive stages of change are known as moulting.