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METAMORPHIC PATTERNS IN can be defined as “a rapid and complete transformation from an immature larval life to a sexually adult form involving morphology, function and changes”. or is the periodic shed- ding off the old exoskeleton. The duration of the period between two successive moults of a developing is called stadium. The form of the developing insect between two moults is called . Types of Metamorphosis: On the basis of degree of changes there are 5 basic types of metamorphosis seen in insects.

1. Ametabolous development or Ametamorphic 2. Metabolous development or Metamorphic 1) Ametabolous Development or Direct De- velopment: Ametabolous type of development occurs when the insects undergo little or no metamorphosis. Here the young ones called emerges from the eggs and resembles 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. 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. Eg. Lepisma and spring tails (Collembola).

2) Metabolous Development or Indirect Development: This is a group of insects where metamorphosis occurs. Metamorphosis may be simple or complex; based on this, metabola is divided into 4 types as follows, i. Gradual Metamorphosis or Paurometabolous Development: Here the newly young which comes out of egg closely resembles the adult in general body form, habits and habitat. The young’s are called nymphs. Nymph is different from adult in many features, i.e., wings and reproductive organs are undeveloped.

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The wings develop as wing pads in the second and third thoracic segments at early age and gradually increase in size 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 t because the young undergoes slow but steady change in each moult and attains the adult form. There is no pupal stage. Eg. , , mantis and white . Sometimes the gradual metamorphosis or paurometabolous development is included under hemimetabolous development. Egg → Nymph → (Adult) ii. Incomplete Metamorphosis or Hemimetabolous Development: In this group, the metamorphosis is partial. Different stages of the life cycle resemble to paurometabolous development except the nymphs are called naiads which are aquatic and respire by external 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 Eg: , and . Egg → → Adult iii. Complete Metamorphosis or Holometabolous Development:

In this group metamorphosis is complete. Egg hatches into a larva. The larva transformed into . 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.The pupa is converted in to adult.Thus there are four stages in the life cycle. Eg:

2 , caddis- , , moths, mosquitoes, flies, bees and wasps Egg → Larva → Pupa → Imago (adult) iv. or Hypermetabolous Development

Here the metamorphosis is complete. It is a kind of metamorphosis in which there are two or three distinct types of larval with different habits and structures found in certain insects. This type of metamorphosis is seen in blister beetles

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NEURO ENDOCRINE REGULATION OF METAMORPHOSIS

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

(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 , 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 .

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It is steroid in nature. Butenandt and Karlson (1951) first isolated from the pupa of silkworm.

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 hormone (JH). Chemically it is a steroid in nature. The 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.

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CONTROL OF INSECT PESTS 1. Chemical Control

The most common method of pest control is the use of pesticides. Pesticides are chemicals synthetic chemicals that either kill the pest or inhibit their development. Pesticides are classified according to the pests the used to control

 Fungicides- Used to control fungal infections  Insecticides- Used to control insect pests  Herbicides- Used to control weeds  Rodenticide- Used to control rodents (rat, mice, squirrels)

Insecticides: Are chemicals used to kill insect pests. Insecticides are of three types

1. Stomach poisons: Kill the insects when the chemicals are swallowed. They are applied on the insect’s food. Eg: Lead arsenate, Calcium arsenate.

2. Contact Poisons: They penetrate the insect’s body through the cuticle or spiracle and cause death. They are sprayed on the infected plant part. Eg: DDT, BHC.

3. Fumigants: Are poisonous gases. They enter the body through spiracles and kill insect. They are used in closed rooms. Eg: Aluminium phosphide, Naphthalene

Advantages of Chemical control 1. They are fast acting and effective 2. Most of them are broad-spectrum chemicals that can kill different species of insects 3. They can kill insect pests of all life stages (egg, larva and adult) 4. It is readily available in the market, easy to use and have more shelf life compared to other pest control methods like botanical and microbial pest control methods.

Disadvantages of Chemical control 1. They causes bioaccumulation (accumulate in side biological tissue ) and biomagnifications 2. Affects human beings health 3. Insects develop insecticide resistance to several chemical insecticides 4. Kills useful insects like pollinators and natural enemies. 2. Biological control Definition: The action of biocontrol agents (natural enemies) against the target species to keep the pest below economic threshold level (ETL). Biological control includes reduction of pest population by natural enemies along with active human role. It is the control of pests and parasites by the use of other organisms, In other words, it is a practice in which an organism is used against another organism.e.g., of mosquitoes by which feed on their larvae.

Under this practice, there are four types of pest control: (i) Classical biological control or importation, in which a natural enemy from another geographical area, often the area in which the pest originated from, is introduced to contain the pest below the economic injury level, EIL.

(ii) Inoculation, in which the periodic release of a control agent is required so that it is available throughout the year.

(iii) Augmentation, which involves the release of an indigenous natural enemy in order to supplement an existing population, and is therefore carried out repeatedly usually to coincide with a period of rapid growth of pest population

iv) Inundation, which s the release of large numbers of natural enemy, with the aim of killing those pests present at the time.

The natural enemies of insects pests include 1. Predators: They are free living species which consume a large number of prey during their life time. Example, a small predaceous ladybird , Rodolia cardinalis, commonly called vedalia, has been used to control the cottony- cushion scale insect (Icerya purchasi), a pest of citrus trees. 2. : are species whose immature stages develop on or within the body of the host insect-Egg parasitoids, larval parasitoids and adult parasitoids based on which stage of the host is attacked by that particular Eg: Trichogramma .Apanteles 3. Pathogens:are disease causing organisms that include Bacteria, fungus and virus. They are very species specific. a. Bacteria: Bacillus thuringiensis is a spore- forming bacteria that infects a variety of insects, including the larvae of moths, butterflies, flies, and beetles. This bacteria contains a toxic protein called crystal protein or Cry protein, when ingested by the insect pest, gets activated in the alkaline midgut of it , causing cell lying and eventual death of the inscet. b. Fungi: The most commonly used in insect control are Beauvaria bassiana Metarrhizium anisopliae. The spore of an entomogenous fungi germinates on the surface of the host’s integument. Once the host tissue is invaded, the fungus can complete its life cycle, but the survival and germination of spores is critical to the development of an epidemic. c. Virus: The insect pathogenic viruses are called inclusion viruses, a first multiply in the nuclei, but later continue to replicate in the cytoplasm of the pest. The disease eventually kills the insect.Eg: NPV (nuclear Polyhedrosis virus) use against a variety of pests, including cotton bollworm, tobacco budworm, com earworm.

Advantages of Biological Control:

1. It is specific to a particular species of insect pest and hence will not kill other useful insects like pollinators and natural enemies.. 2. Environment friendly 3. Biological control can be cost effective in the long run. it's a tactic that only needs to be applied once due to its self-perpetuating nature. 4. Most important of all, it's effective. Whatever pest population you want controlled will no doubt be controlled. Because the predator introduced will be naturally inclined to target the pests, very often you'll see the pest population dwindle.

Disadvantages of Biological Control:

1. Since it is species specific, it cannot control different species of pests at the same time. 2. It's a slow process. It takes a lot of time and patience for the biological agents to work their magic on a pest population, whereas other methods like pesticides work provide immediate results. 3. Predators can only survive if there is something to eat, so destroying their food population would risk their own safety. Therefore, they can only reduce the number of harmful pests. 4. While it is cheap in the long run, the process of actually setting up a biological control system is a costly endeavor. 5. It is not readily and easily available in the market. It has a very less shelf life.