Metamorphosis: Kinds, Events and Role of Hormones

In this article we will discuss about Metamorphosis in Insects:- 1. Meaning of Metamorphosis 2. Types of Metamorphosis 3. Events 4. Role of Hormones.

Meaning of Metamorphosis:

Metamorphosis can be defined as “a rapid and complete transfor­mation from an immature larval life to a sexually adult form involving morphology, function and habitat changes”.

Ecdysis or moulting is the periodic shed- ding off the old exoskeleton. The duration of the period between two successive moults of a developing insect is called stadium. The form of the developing insect between two moults is called instar.

A larva is a motile, immature feeding stage in arthropods which is morphologi­cally different from the adult stage. The larvae of hemimetabolous insects are called nymphs. The adult stage of holometabolous insects is called imago.

Types of Metamorphosis:

On the basis of degree of changes there are 5 basic types of metamorphosis seen in insects.

They are:

(1) Ametabolous deve­lopment or Ametamorphic

(2) Gradual meta­morphosis or Paurometabolous develop­ment

(3) Incomplete metamorphosis or Hemimetabolous development

(4) Complete metamorphosis or Holometabolous devel­opment and

(5) Hypermetamorphosis or Hypermetabolous development.

(1) Ametabolous Development or Direct De­velopment:

Ametabolous type of development is called when the insects undergo little or no metamorphosis. Here the young’s emerge from the eggs resemble the adults in all respects except in size and sexual structures. It grows only in size by replacing its old skin through a process, called moulting.

The young which emerges from the egg resem­bles a miniature adult, called nymph. In nymph the reproductive organs are undevel­oped, and after several moults the nymph becomes an adult. This type of development is seen in apterygotan (wingless) insects (e.g., Lepisma, Fig. 18.134 and spring tails or Collembola, etc.).

(2) Gradual Metamorphosis or Paurometabolous Development:

This type of metamorphosis is seen in less primitive forms like cockroaches, grasshop­pers (Fig. 18.135), mantis and white ants, etc. Here the newly young which comes out of egg closely resembles the adult in general body form, habits and habitat but many adult features, i.e., wings and reproductive organs are undeveloped and their relative proportions of the body also differ. The young’s re called nymphs.

The wings de­velop as wing pads in the second and third thoracic segments at early age and gradually increase in size by mitosis in each moult. The external genitalia develop gradually at each moult. These nymphs lead an independent life and attain adult form through several moults.

This type of metamorphosis is called gradual metamorphosis or paurometabolous development because the young undergoes slow but steady change in each moult and attains the adult form.

Sometimes the gradual metamorphosis or paurometabolous devel­opment is included under hemimetabolous development. In each moult the proportion of the head gradually becomes smaller and the abdomen becomes longer.

In the life cycle of these insects there are three stages, e.g., egg → nymph → imago (adult). There is no pupal stage.

(3) Incomplete Metamorphosis or Hemimetabolous Development:

The insects which attain adult forms by gradual morphological change with succes­sive moults are called incomplete metamor­phosis or hemimetabolous development.

In many insects, e.g., dragonflies (Fig. 18.136), mayflies and damselflies, the different stages of the life cycle resemble to paurometabolous development except the nymphs are called naiads which are aquatic and respire by ex­ternal gills but the adults are terrestrial.

When these nymphs are ready to be adult they come out of water and adult winged forms are released. The wings and genitalia develop externally but are not fully formed until adult­hood. No further moulting takes place after the formation of wings, only exception in mayfly where winged form comes out of aquatic nymph and rests on a tree to undergo another moulting to become an adult.

In the naiads of hemimetabolous insects there are 3 pairs of thoracic legs, a head with compound eyes, antennae and small abdomen with posterior tracheal gills.

The naiads when attain adult stage after several moults, the head of the adult becomes pro­portionately smaller but the abdomen be­comes larger. The tracheal gills are lost and spiracles appear for aerial breathing. Many insects live for longer period as nymphs and the adult stage is short, the chief purpose of which is multiplication.

The best example is the may fly where adult stage lasts only for a day but nyphs take one year to grow.

From the egg → nymph → adult cycle of incomplete metamorphosis, it is evident that insects which are advanced from the primi­tive Lepisma like forms, have started to ex­plore two types of environments. But success in such attempts is achieved in forms with complete metamorphosis.

This climax of double life has been attained through a cycle of egg → larva → pupa adult; larva existing in an altogether different environ­ment than the adult. Nearly 87% of known insects develop through this cycle which involves two changes of form—one is from egg to caterpillar and the other from caterpillar to pupa and the adult.

(4) Complete Metamorphosis or Holometabolous Development:

Complete metamorphosis or holometabolous development is a kind of rapid mor­phological change during post embryonic transformation in some forms of insects where larva has no similarity with the adult and there is always a pupal stage. Complete metamorphosis takes place in beetles, caddis- flies, butterflies, moths, mosquitoes, flies, bees and wasps (Fig. 18.137).

In the house-fly (order Diptera) the larva is worm-like and devoid of appendages. It is called maggot. The mature larva is about 12 mm long. The head is indistinct and with a pair of oral lobes and hooks.

In case of beetle (Order Coleoptera) the larva is known as grub. The body of the grub is thick and with thoracic legs and well-developed head. They are usually sluggish in nature.

In the moths and butterflies (Order Lepidoptera), the larva is known as Caterpillar, which possesses a distinct head with power­ful mandibles and three pairs of jointed thoracic legs.

The abdomen possesses four or five pairs of un-jointed, short abdominal legs or also called pseudo-legs or prolegs. The caterpillars are often with protective colour or defensively shaped. These larvae eat voraciously and grow rapidly with sev­eral moultings. After sometime the larva is transformed into a stage, called pupa.

Again, these above mentioned larvae are also included under three categories, such as the maggot is called apodous larva for the absence of appendages on thorax and abdo­men and segmented body with a small head with sense organs. The larvae among beetles are also called campodeiform or oligopod larvae for the resemblance to apterous Campodea (Order Thysanura).

The larvae are characterised by without abdominal append­ages except cerci and the skin of the body is thick, provided with thoracic legs and sense organs. The caterpillar type larva is also called polypod or eruciform larvae (Fig. 18.138) which is characterised by a fleshy body with a thin skin and prolegs on the abdomen and six legs on the thorax.

The pupa is the third stage in the life of holometabolous insects and usually immo­bile and often remain within a protective covering from predators, called cocoon. In the mosquito the pupa is very active but does not eat anything. Though the pupal stage considers as a quiescent stage but it undergoes many internal changes. There are three types of pupa among the holometa­bolous insects.

They are:

(i) Exarate pupa:

This type of pupa is very common and found in almost all holometabolous insects (Fig. 18.139A) except Lepidoptera. The pupa is chara­cterized by the presence of free legs and appendages and the abdomen is capable of movement. This type of pupa is called free pupa.

(ii) Obtect pupa:

This type of pupa is seen among moths and butterflies and is characterized by the wings, and ap­pendages are not moved and fixed to the body by a moulting fluid. The pupa of butterflies is called chrysalis and it possesses a slender stalk at the top by which the pupa remains at­tached to the twigs.

(iii) Coarctate pupa:

This type of pupa is seen among dipterans (Fig. 18.139B). The pupa of house-fly is enclosed by a hard barrel-shaped chitinous case, called puparium. The puparium is segmented externally and the spiracles remain projected outwardly. The pupa of mosquitoes is comma-shaped and contains broad anterior cephalothorax.

The dorsal side of thorax bears a pair of small respiratory trumpets; the openings are guarded by numerous hairs. The abdomen is nine segmented and a pair of paddles on the eighth segment by which the pupa swims. After a period of pupal existence, the young insect emerges out by breaking or dissolving the pupal case. The holometabolous insects include 4 stages in their life cycle.

Egg → Larva → Pupa → Imago (adult):

Prepupa:

In the holometabolous insects, a stage is seen before the pupal stage, called prepupa. During this stage the feeding usu­ally stops and sometimes a cocoon is pro­duced. The prepupa resembles the larva but it is often shrunken and less pigmented.

(5) Hypermetamorphosis or Hypermetabolous Development:

It is a kind of metamorphosis in which there are two or three distinct types of larval instars with different habits and structures found in certain insects. This type of meta­morphosis is seen in blister beetles (Fig. 18.140).

Events of Metamorphosis:

In recent years considerable amount of work has been done to understand the events of metamorphosis. It has been found that at the time of early development, in the deve­loping egg the cells are segregated into two groups—one group for working at the larval life and the second group to take charge during pupal and adult life.

In a growing larva, the larval cells increase only in size but never undergo division. The second group of cells, called imaginal buds and discs, re­main inactive in the body of larva. When the larva is full-grown, second group of cells take over the charge. Within the apparently inactive pupa tremendous activities go on at cellular level. Imaginal buds grow by divi­sion.

During metamorphosis, most of the larval organs in the pupa except the central nervous system and developing reproduc­tive organs are broken down by enzymes and the process of disintegration of the lar­val organs is called histolysis and these larval disintegrated cells die and is used up by the imaginal cells.

In certain insects, within larva, pupal cells become fluid in consistency and imaginal cells continue to form the adult structures. The imaginal buds are the groups of formative cells but remain inactive in the larva but form the rudiments of future organs by mitosis.

These formative cells set aside in the pupa and reach func­tional organs by differentiation in the imago (adult). The process of formation of tissues and organs from the imaginal buds, called histogenesis. The wings, mouth parts, inter­nal organs, muscles and legs develop from the imaginal buds.

Role of Hormones during Metamor­phosis:

It has also been well established that the moulting and metamorphosis in insects are controlled by hormones (Fig. 18.141). The secretions of three organs are related to this process.

These organs are:

(i) The brain (protocerebrum)

(ii) The prothoracic gland and

(iii) The corpora allata.

(i) The Brain (protocerebrum):

In the brain there are four groups of neurosecre­tory cells. Of these two groups lie on the midline and another two group’s lie on the sides-one group on each side. The neurosecretory cells secrete a kind of protein hor­mone, called prothoracotropic (PTTH) or brain hormone that activates the prothoracic glands which in turn produce moulting hormone.

The protocerebrum sends the neuro­secretory axons to the corpora cardiaca, a pair of small glands which lie posterior to the brain. Prothoracotropic hormone passes to the corpora cardiaca along the axons where it is released to the blood.

(ii) Prothoracic Glands (PG):

There are a pair of glands located in the prothoracic region. These are also called moult gland or ecdysial gland. Each gland appears V-shaped and is a mass of glandular tissue of non- nervous origin.

It produces a hormone, known as ecdysone. It is steroid in nature. Butenandt and Karlson (1951) first isolated from the pupa of silkworm. Its empirical formula is C₂₇H₄₄O₆. The hormone stimu­lates growth and initiates the process of moulting and shedding the old cuticle of the larva and the new cuticle is formed beneath the old cuticle.

(iii) Corpora Allata:

They are paired non- nervous secretory cells and situated behind the brain posterior to corpora cardiaca. The corpora allata secrete another fat soluble hormone, called juvenile hormone (JH). Chemically it is related to ecdysone and also a steroid in nature. The juvenile hormone keeps the larval cells active and also controls the qualitative changes in the body during metamorphosis.

As long as the juvenile hor­mone is secreted from the corpora allata, the pupa and imago (adult) stages are not devel­oped. After a certain period the production of ecdysone instructs to stop the flow of juvenile hormone on one hand and on the other hand triggers the imaginal buds to be active.

The absence of juvenile hormone causes the death of larval cells and they are used as nutrients for the growing imaginal buds. If the amount of juvenile hormone (JH) becomes lower in the blood, the moulting from larva to pupa takes place and absence of juvenile hormone in the blood, there oc­curs from pupa to adult moult. So it has been determined that the process of moulting is under hormonal control.