CLASS HEXAPODA (INSECTS) (hexa, six + podus, feet)

CLASS HEXAPODA (INSECTS) (hexa six + podus, feet)

Members of the class Hexapoda are the most successful land animals. They  are called insects. The arthropods with three pairs of wing, one pair of antennae and three pairs of legs are called insects.

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EXTERNAL STRUCTURE AND LOCOMOTION

The body of an insect is divided into three tagmata: head, thorax, and abdomen.

1. Head: Head bears a single pair of antennae. mouth parts, compound eve and zero, two, or three ocelli.

2. Thorax: The thorax consists of three segments. These segments are ; the prothurax, mesothorax and metathorax. Following appendages are attached with the thorax:

(a) Legs: One pair of legs attaches at the ventral margin of each thoracic segment.

(b) Wings: A pair of wings is attached at the dorsolateral margin of the mesothorax and metathorax. Wings have thick hollow veins. These veins increase the strength of the wings.

(c) Spiracles: The thorax contains two pairs of spiracles. Spiracles are openings of the tracheal system.

3. Abdomen: Most insects have 10 or 11 abdominal segments. Following structures are present on abdomen:

(a) Lateral fold:Each abdominal segment has a lateral fold in the exoskeleton. This folds allows the abdomen to expand when the insect is gorged. It also expands the abdomen when abdomen is full of mature eggs.

(b) Spiracles: Each abdominal segment has a pair of spiracles.

(c) Genital Structures: Genital structures are used during copulation and egg deposition.

(d) Cerci : Cerci are sensory structures. They are present on each segment.

(e) Gills: Gills are present on abdominal segments of some immature aquatic insects.

INSECT FLIGHT

Flight is the most important form of insect locomotion. Insects were the first animals to fly.

Evolution of wing

There is a most popular hypothesis about the origin of wing. It suggests that the wing have evolved from rigid, lateral outgrowths of the thorax. These outgrowths protected the legs or spiracles. Later, insects started using these fixed lobes for gliding from the top of tall plants to the forest floor. Later the w ing developed the ability to flap, tilt and fold back  over. Then the evolution of limited thermoregulation started in insects.

Thermoregulation is the ability to maintain body temperatures at a level different from environmental temperatures. High body temperatures of 25° C or greater is needed or flight muscles to contract rapidly.

Mechanism of flight

There are two mechanisms of flight.

1. Direct or Synchronous flight

In this case muscles move the wings directly. One group of muscles is present on the bases oil the wings. They contract to produce a downward thrust. Second group of muscles is attached dorsally and ventrally on the exoskeleton. These muscles contract to produce an upward thrust. The synchrony of direct flight mechanisms depends on the nerve impulse to the flight muscles. This nerve impulse must come before each wing beat. Butterflies, dragonflies and grasshoppers are examples of insects with a synchronous flight mechanism.

2. Indirect or asynchronous flight mechanism

In this case a muscle does not move the wing directly. They change the shape of the exoskeleton for both upward and downward wing strokes.

(a)   Upward thrust: Dorsoventral muscles pullthe dorsal exoskeleton (tergum) downward. It produces the upward wing thrust.

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Fig: lnsect Flight (a) Muscle arrangements for the direct or synchronous flight mechanism. (b) Muscle arrangements for the indirect or asynchronous flightmechanism.

(b)   Downward thrust: Then. the longitudinal muscles contract. It pulls the exoskeleton upward. Therefore, the exoskeleton forms an arch. It produces downward thrust of wing. The exoskeleton have resilient properties. These properties enhance  the power and velocity of strokes. The thorax is deformed during a wing beat. It stores energy for the exoskeleton. There is a critical point midway into the down stroke. This stored energy reaches a maximum at the critical point. The resistance to wing movement suddenly decreases at the same time. The wing uses the stored energy in the exoskeleton and then complete the rest of the cycle. There is lack of one to one corn spondence between nerve impulses and wing beats. It causes asynchrony of flight mec anism. A single nerve impulse can cause approximately 50 cycles of the wing. The frequencies of 1,000 cycles per second are recorded in some midges. The asynchrony between wing beat and nerve impulses is dependent on flight muscles. These muscles are stretched during the “click” of the thorax. The stretching of longitudinal flight muscles during the upward beat of the wing initiates the contraction of these muscles. Similarly, stretching during the downward beat of the wing initiates contraction of dorsoventral fig muscles. Indirect flight muscles are called fibular flight muscles. Flies and wasps are examples of insects with an asynchronous flight mechanism.

Simple flapping of wings is not enough for flight. The tilt of the wing must be controlled. This tilt provides lift and horizontal propulsion. The muscles that control wing tilting are attached to sclerotization plates at the base of the wing.

Other Forms of Locomotion

lnsects can walk, run, and jump. Insects have three or more legs for walking. These legs stay on the ground at all times. It keeps their position stable. They keep fewer legs in contact with the ground during walking.

1. A running cockroach reaches speeds of about 5 km/hour. But it seems much faster when one person is trying to catch it. The apparent speed is the result of their small size and ability to quickly change directions.

2. Grasshoppers are jumping insects. They have long metathoracic legs. These legs have larger muscles. These muscles produce large propulsive  forces.

3. The flea jump and elastic energy is stored in the exoskeleton. Muscles of legs distort exoskeleton. A catch mechanism holds the legs in this cocked position. Then special muscles release the catches. It allows the stored energy to quickly ciond the legs. This action throws the Ilea for distance 100 times of its body length. It is equal to two football field jump of a human.

NUTRITION AND THE DIGESTIVE SYSTEM

Mouth parts

Insects have following mouth parts:

1.Labrum: 1.abrum is an upper lip like structure. It is sensory in function. It is not derived from segmental paired appendages.

2. Mandible: Mandibles are sclerotized chewing mouthparts.

3. Maxillae: The maxillae have cutting surfaces. They bear a sensory palp.

4. Labium: The labium is a sensory lower lip.

All of these mouth parts help in food handling. Some variations are present in these moot parts for sucking or siphoning plant or animal fluids.

Digestive system

The digestive tract consists of foregut, a midgut and a hindgut. There are gut enlargements for storage food. Some diverticula are present for the secretion of digestive enzymes.

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GAS EXCHANGE

Gas exchange requires a large surface area for the diffusion of gases. But water is lost from these surfaces area in terrestrial environments. Invagination of respiratory surfaces takes place in insects. It reduces respiratory water loss. Their respiratory surface is composed of trachea. Trachea are highly branched systems of chitin-lined tubes.

The trachea opens outside through spiracles. The spiracles have closure device. It prevents the excessive water loss. Spiracles lead to tracheal trunks. The tracheal trunk is bra cited and rebranched and it gives rise to smaller branches called tracheoles. Tracheoles open into intracellular spaces. Tracheoles are especially abundant in metabolically active tissues like flight muscles. No cells are more than 2 or 3 pm from a tracheoles.

Mechanism of respiration

Most insects have ventilating (exchange of gases) mechanisms. This mechanism move air into and out of the tracheal system. The flight muscles alternatively compress and expand the larger tracheal trunks. Therefore, they ventilate the tracheae. The carbon dioxide is dissolved in the haemocoel as bicarbonate ions (HCO3). Oxygen diffuses from the tracheae to the body tissues. It is not replaced by carbon dioxide. Therefore, a vacuum is created. This vacuum draws more air into the spiracles. This process is called passive suction. Periodically, the dissolved bicarbonate ions are converted back into carbon dioxide. It escapes through the tracheal system. Other insects contract abdominal muscles in a lump like fashion. It moves air into and out of their tracheal systems.

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CIRCULATION AND TEMPERATURE REGULATION

Blood circulation

The blood vessels are less well developed in insects. Blood distributes nutrients, hormones and wastes and amoeboid cells. It participates. in body defense and repair mechanism. Blood is not important in gas transport.

Thermoregulation

1. Ectotherms: Thermoregulation is a necessary for flying insects. All insects warm themselves by basking in the sun or resting on warm surfaces. They use external heat sources in temperature regulation. Therefore, the insects are as ectotherms.

2. Heterotherms: Other insects (e.g., some moths, alpine bumblebees, and beetles) can generate heat by rapid contraction of muscles. This process is called shivering thermogenesis.Thermogenesis can raise the temperature of thoracic muscles from near 0 to 30 oC. But some insects rely on a limited metabolic heat sources. Thus they have a variable body temperature. They are called heterotherms.

Insects are also able to cool themselves by moving in cool moist habitats. Honeybees beat their wings at the entrance of the hive. It circulates the cooler air outside through the hive and cools their hive.

NERVOUS AND SENSORY FUNCTIONS

Nervous system

The pattern of their nervous system is similar to other arthropods.

1. Supraesophageal ganglion: It is associated with sensory structures of the head.

2. Sub-esophageal ganglion: Connectives join the supraesophageal ganglion to the sub-esophageal ganglion. This ganglion connects the mouth parts and salivary glands. It has a general excitatory influence on other body parts.

3. Segmental ganglia: The segmental ganglia of the thorax and abdomen fuse to various degrees in different groups.

4. Visceral nervous system: Insects also possess a well-developed visceral nervous system. It connects the gut, reproductive organs. and heart.

Learning in insect

The insects are capable of some learning and have a memory. For example. the bees recognize flower like objects by their shape and ability to absorb ultraviolet light. If a bee is given nectar and pollen, it learns the odor of the flower. Bees that feed once at artificially scented feeders choose that odor in 90% of feeding trials. Odor is more constant than color and shape. Therefore, it is a very reliable cue for bees.

Sense organs

Sense organs of insects are similar to those to other arthropods. But they are usually specialized for functioning on land.

1. Mechanoreceptors: Mechanoreeeptors detect physical displacement of the body of body parts.

2. Setae: Setae are distributed over the mouth parts antennae, and legs. Touch, air movements, and vibrations of the substrate can displace setae.

3. Stretch receptors: Stretch receptors at the joints, on other parts of the cuticle, and on muscles monitor posture and position.

4. Hearing: Hearing is a mechanoreceptive sense. The air pressure waves displace certain receptors. All insects can respond to pressure waves with distributed setae. Some others have specialized receptors for hearing:

(a) Johnston’s organs are present in the base of the antennae of most insects like  mosquitoes. Long setae vibrate when certain frequencies of sound strike them. It vibrates the antennae of these Insects. Vibrating setae move the antenna in its socket. It stimulates the sensory cells. Sound waves in the frequency range of 500 to 550 cycles per second attract for mating behavior in male mosquitoes. The waves of this frequency are produced by the wings of females.

(b) Tympanal (tympanic) organs: Tympanal organs are present in the legs of crickets and in the abdomen of grasshoppers and some moths and in the thorax of other insects. Tympanal organs consist of a thin cuticular membrane. It covers a large air sac. The air sac acts asa resonating chamber. Sensory cells are present just under the membrane of sac. They detect pressure waves. Grasshopper tympanal organs can detect sounds Of 1,000 to 50,000 cps. Bilateral placement of tympanal allows insects to differentiate between the direction and origin of sound.

5. Chemoreceptors: Insects use chemoreceptor in feeding, selection of egg sites mate location and for social organization. Chemoreceptors are abundant on the mouthparts, antennae, legs and ovipositors. They take the form of hairs, pegs, pits and plates. They have one or more pores. These nerves enter into internal nerves. Chemicals diffuse through these pores and bind to the nerve endings.

6. Compound eyes

All insects can detect light. They use light in orientation, navigation, feeding or other functions. Compound eyes are well developed in most adult insects. They are similar in structure and function to arthropods. Although zoologists debate their possiblehomology (common ancestry). Their eyes have evolved from the eyes of crustaceans, horseshoe crabs and trilobites.

Structure of compound eye

Compound eyes consist of an up to 28,000 receptors called ommatidia. The ommatidia are fused into a multifaceted eye. Ommatidia have following parts:

1. Lens: The outer surface of each ommatidium is a lens. This, lens form one facet of the eye.

2. Crystalline cone: Crystalline cone is present below the lens. Ihe lens and the crystalline cone are light-gathering structures.

3. Rentinula cells: These are special cells of the ommatidium. Retinula has a special light-collecting area, called the rhabdom. The rhabdom converts light energy into nerve  impulses.

4. Pigment cells: Pigment cells surround the crystalline cone and rhabdom. Pigment cells prevent light from reflecting into an adjacent ommatidium.

Functions of compound eye

Many insects form an image. But image has no real significance for most species. The compound eye is used for detecting movement. Compound eye can detect movement of a point o light less than 0.1. This light successively reflects the adjacent ommatidia. For this reason, bees are attracted to flowers blowing in the wind. Similarly, the predatory insects select moving prey. Compound eyes detect wavelengths of light that the human eye cannot detect. Compound eye can detect the ultraviolet end of the spectrum. Compound eyes of some insects can also detect polarized light. Polarized light is used for navigation and orientation.


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Fig: (a) Compound eye:  (b) Structure of an ommatidium (c) Cross section through the rhabdom

7. Ocelli

Ocelli consist of  500 to 1, 000 receptor cells. These cells are present beneath a single cuticular lens. Ocelli are sensitive to changes in light intensity. Therefore, the regulation of daily rhythms.

EXCRETION

Malpighian tubules and the rectum are primary excretory structures    in   the   insects. Malpighian tubules end blindly in Hindgut the haemocoel: They open in to the gut at the junction of the mid gut and the hindgut. Microvilli cover the inner surface of their cells. Various  ions  are   actively transported into the tubules.

Water moves in by diffusion. Uric acid is secreted into the tubules. It is then transferred into the gut. Rectum reabsorbs water, certain ions, and other materials. Finally uric acid is eliminated through anus.

Excretory product: The excretion of uric acid has advantage for terrestrial animals. It minimizes water lass. But energy is wasted in conversion of ammonia into uric acid. Half of the energy is used in the processing of waste material. In aquatic insect, ammonia diffuses  into surrounding water.

CHEMICAL REGULATION

Hormones

The endocrine system controls many physiological functions of insects. These functions are cuticular sclerotization, osmoregulation , egg maturation, cellular metabolism, gut peristalsis  and heart rate.

Control of ecdysis by hormones: Ecdysis is under neuroendocrine control. Two endocrine glands are present in the subesophageal ganglion. Theseglands are the corpora allata and the prothoracic glands. These glands control different activities. Following processes are involved in the secretion and regulation of ecdysone:

1. Neurosecretory cells of the subesophageal ganglion synthesize ecdysiotropin.

2. Ecdysiotropin hormone is then transferred to corpora cardiaca.

3. The corpora cardiaca then releases thoracotropic hormone.

4. Thoracotropic hormone stimulates the prothoracic gland to secrete ecdysone.

5. Ecdysone initiates the reabsorption of the inner portions of the procuticle. It st initiates the formation of the new exoskeleton.

Other hormones are also involved in ecdysis. These hormones control the recycling of materials absorbed from the procuticle. Changes in metabolic rates, and pigment deposition.

Role of hormones in metamorphosis:The corpora allata releases small amounts of juvenile hormone in immature stages. The amount of juvenile hormone determines the nature of the next molt. Large concentrations of juvenile hormone causes molt to second immature stage.  Intermediate concentrations causes molt to third immature stage. Low concentrations causes molt to the adult stage. Thus there are decreases in the level of circaulating juvenile hormone. Low level of this hormone also causes the degeneration of the prothoracic gland. Therefore, molt is stopped in most of adult insects. But the level of juvenile hormone increases again after the final molt. Now it promotes the development of accessory sexual organs, yolk synthesis and the egg maturation.

Pheromones

The chemicals released by an animal that change the behavior or physiology of another member of the same species is called pheromones. Many different insects use phermones. Pheromones are much specific. An isomer of a pheromone is ineffective in initiating a response. Wind or water carry pheromones several kilometers away. Phermones fall on a chemoreceptor. A few pheromone molecules of another individual can produce enough response. Function of insect pheromones

1. Sex pheromones: Sex pheromones excite or attract members of the opposite sex. They accelerate or retard sexual maturation. Example. Female moths produce and release pheromones that attract males.

2. Caste-regulating pheromones: These are used by social insects to control the development of individuals in a colony. Example: The female bee feed the larva with royal jelly. The amount of royal jelly determines whether the larva will become a worker or a queen.

3. Aggregation pheromone: These are produced to attract individuals to feeding or mating sites. Example: Certain bark beetles aggregate on pine trees during an attack on a tree.

4. Alarm pheromones: These are used to warn other individuals of danger. It causes orientation toward the pheromone source and stimulates an insect to attack or flight. Example: A sting from one bee alarms other bees in the area.

5.Trailing pheromones: It is released by foraging (21, 2.45″ a) insects. These pheromones help other members of the colony to identify the location and quantity of food. Example: Ants often moves on a pheromone path to and from a food source.

REPRODUCTION AND DEVELOPMENT

Insects have high reproductive potential. It is one of the successes of insect. But reproduction in terrestrial environments is much difficult. Temperature. moisture, and food supplies vary with the season. The gametes dry quickly. Therefore internal fertilization requires active copulatory structures. A mechanism is also required to bring males and females together at appropriate times.

There are complex interactions between internal and external environmental factors. These interactions regulate the sexual maturity.

1. Internal regulation takes place by interactions between endocrine glands and reproductive organs.

2. External regulating factors are quantity and quality of food. For example, the eggs of mosquitoes become mature after the female takes a meal of blood. The production of number of eggs is proportional to the quantity of blood ingested.

The photoperiod indicates seasonal changes. Therefore, it controls the timing of reproductive activities in many insects. Population density, temperature and humidity also influence reproductive activities.

Fertilization: A few insects, including silverfish and springtails have indirect fertilization. The male deposits a spermatophore. The female picks up later. Most insects have complex mating behaviours. They can locate and recognize a potential mate for copulation. Pheromones may be involved in the mating behavior. The visual signals and auditory signals also play role in mating. These stimuli bring the male and female near each other. Then the tactile stimuli from the antennae and other appendages adjust the position of the insects for mating.

The insects have abdominal copulatory appendages. The male usually transfer sperm into sperm receptacle of female. Sperm receptacle is an out pocket of the female reproductive tract. Eggs are fertilized. The female laid the egg near the larval food supply. Some females have an ovipositor to deposit eggs in or on some substrate.

Insect Development and Metamorphosis

Different insects have developed different mechanisms of development. Some insects are produced in immature stages called larval instars. It is formed at the time of growth. Instar stores food for the transition to adulthood. The adult stage is associated with reproduction and dispersal. In these orders, insects spend a greater part of their lives in

juvenial stages. The developmental patterns of insects are classified into three or four categories.

1. Ametabolous metamorphosis (a, without + metabolos, change):

In this case, primary differences between adults and larvae are body size and sexual maturity. Both adults and larvae are wingless. The number of molts in the ametabolous development of a species varies. Molting continues after sexual maturity. Silverfish have ametabolous metamorphosis.

2. Paurometabolous metamorphosis (Or. pauros. small)

In this case, larvae undergo number of molts between egg and adult stages. The number of molt  is species-specific. The immature larva gradually changes into the adult form. The external wings are developed. The adult body size is attained. The genitalia develop during his time. Immature larva is called nymphs. Grasshoppers and chinch bugs show paurco eiabolous metamorphosis.

3. Hemimetabolous metamorphosis (Or. hemi, halt)

Some insects have a series of gradual changes in their development. Therefore. some zoologists use additional classification for insects, Their immature form is much different from t e adult form due to the presence of gills. This kind of development is called hemimetabolous metamorphosis. Their immatures are aquatic. This immature is called naiads (naiad, water nymph).

4. Holometabolous metamorphosis (holos, whole)

Following stages are formed during homometabolous metamorphosis:

(a) Larva: Their immature is different from the adult in body form. It has different behavior and habitats. Therefore, the immature are called larvae. The number of larval instars is species specific.

(b) Pupa: The last larval molt forms the pupa. The pupa appears inactive. But it is actually a time of radical cellular change. The characteristics of the adult insect develop during this stage. A protective case encloses the pupal stage. The last larval develops a cocoon. Cocoon is partially or entirely composed of silk. The eh sails and puperium are the last larval exoskeletons. They are retained through the pupal stage. Other insects like mosquitoes have pupae unenclosed by a larval exoskeleton. Their pupa may be active. The final molt occurs within the cocoon, chrysalis, or puperium.

(c) Adult: The adult open the cocoon with its mandibles and come out. This final process is called emergence or eclosion.

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INSECT BEHAVIOUR

Insects have many complex behavior patterns. Most of these are innate (inherited). For example, a newly queen develops in a honeybee hive . It search out and try to destroy other queen larvae and pupae in the hive. She was not taught to kill the previous queen. Therefore, her behavior is innate. Similarly, she has no experience to differentiate between the cell of queen and cells containing worker larvae and pupae. Some insects are capable of learning and remembering. These abilities play important roles in insect behavior.

SOCIAL INSECTS

Social behavior is present in many insects. It is particularly present in those insects that live in colonies. Different members of the colony are specialized structurally and behaviorally for performing different tasks. Social behavior is highly developed in the bees, wasps and ants and in termites. Each kind of individual in an insect colony is called a caste. Often three or four castes are present in a colony.

1. Queens: Reproductive females are called queens.

2. Workers: Workers may be sterile males and females (termites). Or they may be sterile females. Workers support, protect and maintain the colony. Their reproductive o gans are degenerated.

3. Kings or drones: Reproductive males are called kings or drones. They transfer sperm to the queen.

4. Soldiers: Soldiers are sterile. They possess large mandibles to defend the colony.

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Fig: Honeyhees (Order Hymenoptera). Honeybees have a social organization consisting of three castes. (a) A worker bee (b) A drone bee (e) A queen bee marked with blue to identify her. (d) The inner surface of metathoracic legs has setae, called the pollen comb

Social behaviour in honeybees

Honey bees have three of these castes in their colonies.

1. Queen: A single queen lays all the eggs.

2. Workers: Workers are female. They produce wax and construct the comb from this wax. They also gather nectar and pollen and led the queen and drones, They also gather nectar and pollen and feed the queen and drones. They care for the larvae. They also guard and clean the hive. These tasks are divided among workers according to age. Young workers work in the hive. The older workers bring nectar and pollen. The workers live for about one month.

3. Drones: Drones develop from unfertilized eggs. They do not work. They are fed by the workers. They leave the hive and mate with queen. Queen releases a pheromone. This pheromone controls the caste system. Workers lick and groom the queen and other workers. Thus they pick up caste- regulating pheromones from queen and pass it to other workers. This pheromone inhibits the workers from rearing new queens. The amount of caste-regulating pheromone in the hive decreases with the death or aging of queen. Now worker again feed the royal jelly to several female larvae. This food contains chemicals that promote the development of queen characteristics. The larvae that receive royal jelly develop into queens. The new queens start killing each her, Only one queen remains. This queen goes on a mating flight and returns to the colony. She lives in the colony for several years. .

Evolution of social behaviour

Many individuals leave no offspring in the evolution of social behavior. Thus these individuals sacrifice for the survival of the colony. This behaviour has puzzled evolutionists for many years. They explain it with the concepts of kin selection and altruism.

INSECTS AND HUMANS

Only about 0.5% of insect species adversely affect human health and welfare.

Beneficial insects

1. Many insects provided valuable services and commercially valuable products. These products are wax, honey, and silk.

2. Insects cause pollination of 65% of all plant species. Insects and flowering plants have coevolutionary relationships. It directly benefits humans. The annual value of insect-pollinated crops is 19 billion dollar per year in the United States.

3. Insects play role in biological control. For example: vedalia beetles are used for control of cotton-cushion scale. The scale insect, lcerya purchasi, was introduced into California in the 1860s. It destroyed the citrus industry in California in twenty year. The vedalia beetle was brought to the United States in 1888 and 1889. They cultured it on citrus tree. The scale insect was controlled in just a few years. Thus the citrus industry recovered.

4. Some insects live in soil. These insects play important roles in aeration, drainage, and turnover of soil. They promote decay processes. Other insects play important roles in food web.

5. Insects are used in teaching and research. They are used in the study of genetics. population ecology, and physiology. Insects also give pleasure to those who collect them and enjoy their beauty.

Harmful insects

Some insects are parasites and vectors of disease.

1. Parasitic insects: Parasitic insects are:

a)    Order Anoplura: Head body and pubic lice

b)    Order Hemiptera, bedbugs

c)    Order Siphonaptera, fleas.

2. Vector of the disease: Other insects transmit disease-causing microorganisms, nematodes and flatworms. Insect transmit diseases like malaria, yellow fever, bubonic plague encephalitis, leishmaniasis, and typhus.

3. Pest of domestic animals and plant: Some insects affect the health of domestic animals and their quality. Insects feed on crops and transmit plant diseases. These diseases are potato virus and asters yellow.

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  1. Kashaf Saif January 29, 2018

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