The Anatomy and Physiolology of Insects in Relation to the Plant Diseases

7/29/2019 The Anatomy and Physiolology of Insects in Relation to the Plant Diseases

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The Anatomy and Physiology of Insects in Relation to the

Dissemination of Plant Diseases

Because the insects play a vital role in the dissemination of plant pathogens, So it is

very important to understand the anatomy and physiology of different organs of insects and

how they aids in the dissemination of the plant pathogens.

Exoskeleton

Exoskeleton of an insect provides protection to the internal delicate structures of an

insects body, avoids desiccation and provides site of attachment to the different body organs.

It can be divided into three main parts;

1.

Epidermis: Cellular, living layer.2. Cuticle: Non cellular, secreted by epidermis made up of chitin. Can further be

divided into;

Exocuticle: Darker in color. Endocuticle: Lighter in color.

3. Basement membrane: Thin layer below epidermis.In some cases there is an other layer above cuticle called epicuticle is present. Again can

be divided into;

Cement layer Wax layer Upper epicuticle Lower epicuticle


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As far as the dissemination of pathogens with exoskeleton is concerned the dissemination

is totally mechanical, which is greatly inhanced by numerous cuticular processes found on an

insect body.

These cuticular processes can be of two main types;

Setae: Contain membranous articulation at their base and are moveable. Microtrichogen: Does not contain membranous articulation at their base and are

immoveable.

These cuticular processes along with body appendages greatly enhance the mechanical

transmission of pathogens to the healthy plants.

Mouth parts

Because the wounds produced by the feeding of insects provide an effective way of

transmission of pathogens, So it is very important to understand the anatomy and physiology

of different types of mouthparts and they help in the spread of pathogens.

There are six main types of mouthparts;

1. Chewing2. Rasping sucking3. Piercing sucking4.

Sponging

5. Siphoning6. Chewing lappingThese mouthparts are made up of eight essential structures which may be highly

modified or completely absent in these different type of mouthparts. These are;

Labrum Epipharynx Hypopharynx A pair of mandibles A pair of maxillae Labium


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1. Chewing mouthparts:

Represented by the mouthparts of grasshoppers, crickets, termites, caterpillars of

Lepidoptera and grubs of Coleoptera , which are important vectors of several destructive

diseases. These type of mouthparts are used to pinch off or eat out the part of plant tissue into

the mouth of insect, provides the site of entry to the pathogens into the plants body. Different

parts of an insect mouthparts has its own role;

1. Labrum - plate (sclerite) that serves as upper lip in insects with chewing mouthparts.Helps to pull food into the mouth.

2. Mandible - analogous to jaw. Used to chew, cut, and tear food, to carry things, tofight, and to mold wax. Move from side to side rather than up and down.

3. Maxillae - 2nd pair of feeding appendages, used for food handling and sensing. Morecomplicated than the mandibles but working in the same manner.

4. Labium - analogous to lower lip. They function to close the mouth below or behind.Evolved from paired maxillae-like structures that are fused along the center line.

5. Epipharynx- found on the ventral side of labrum and has the sensors of taste.6. Hypopharynx- functions as tongue.


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2. Rasping Sucking:

These type of mouthparts

found in thrips, responsible of several

destructive diseases in plants. The

mouthparts are cone shaped, below head

and bent backwards. Cone shaped portion

is made up of clypeus, labrum, maxillae

and labium. While there is a stylet

between this cone which is a main sucking

organ is composed of 1 hypopharynx, 1

mandible and 2 maxillae.

3. Piercing sucking:

Represented by the mouthparts

of plant bugs, sucking lice, biting lice,

members of Homoptera and Hemiptera.

We will only discuss about the mouthparts

of Homoptera and Hemiptera because they

are involved in the inoculation and

ingression of the plant pathogens.

The mouthparts are modified into beaklike

proboscis. Maxillae are doubly grooved and form the salivary canal and food canal, while the

mandibles are used to pierce the plant surface to enter the proboscis into the plant surface.

4. Sponging mouthparts:

Found in the adults of

specialized flies. The labium is modified

into elbow shaped proboscis and the

labrum into sponge like sucking organ

labella. There are several transverse

channels in labella which are filled in by

capillary action when labella is placed

over liquid food and sucked up by the


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flies. During the feeding of solid food by the flies the crop contents of the fore-gut of flies are

regurgitated by the flies on to the plant surface which contains bacteria that helps in the

digestion or dissolving of plant surface. Now these crop contents are again sucked up along

with the dissolved food. It has been found that these crop contents of the fore-gut also contain

plant pathogenic bacteria, So in this way plant pathogens can transfer into the plant body.

Some Dipterous contains prestomal teeth which are used to lacerate the plant surface.

5. Siphoning mouthparts:

These type of mouthparts are found in

moths and butterflies of Lepidoptera. Labrum

and mandibles are completely absent. Feeding

proboscis is formed by the galea of maxillae.

These type of mouthparts cannot penetrate the

plant body. The transfer of pathogens only

occurs mechanically.

6. Chewing lapping:

These type of mouthparts are found in honeybees

and many Hymenoptera. The transfer of pathogens having

these type of mouthparts also occurs mechanically.

Salivary glands

Every insect is provided with a pair of salivary glands located on each side of fore-gut

in thorax. They produce saliva which act as a solvent, converts sugar into starch and reduces

the viscosity of the food so it can pass the alimentary canal which is 1-2 in diameter.

Saliva of some species of insect is toxicogenic for the plants. It has been found that during

feeding by the insects on the virus infected plants takes virus particles into their body which

some how makes their way into the salivary glands and when these insects feeds on healthy


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plants these viruses transfer into them along with saliva. Spores of Ergot of fungi and soft rot

of bacteria spreads in this way.

Alimentary canal

Practically, in all cases of biological transmission of plant diseases alimentary canal is

directly involved. So, it is very important to understand the anatomy and physiology of

alimentary canal and they harbor plant pathogens.

Alimentary canal can be broadly divided into three main parts;

Stomodaeum or Fore-gut Mesenteron or Mid-gut Proctodaeum or Hind-gut

Stomodaeum:

It may be divided into;

Buccal cavity Pharynx Oesophagus Crop Proventriculus


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Buccal cavity: It is a just mouth opening. In ants it is modified into infra buccal cavity.

During the feeding of liquid food solid contamination gather here and thrown out in the form

of solid pellets it has been found that they pellets contain pathogenic spores of fungi. Fungus

cultivating ants also start new colony of fungus in this way.

Pharynx: Composed of strong dilateral muscles, which act as a pump during sucking in

sucking insects.

Oesophagus: Just act as a connecting tube between pharynx and crop.

Crop: It is a main storage organ of food. Some digestion may also take place in crop. In some

insects it is modified into lateral diverticulum. It has been found that the crop contents of

Diptera are liquid and contain plant pathogenic bacteria (Detail has been mentioned earlier).

Proventriculus: Its walls are highly seclarotized and contain spines to convert the food into

smaller particles. Also called as Chewing stomach or Gizzard.

Mesenteron:

It is also called as ventriculus. Food enters into ventriculus through cardiac valve. It is

the main digestion portion and also the digested food absorbed here. Epithelium of mid-gut is

modified into columnary cells, which secrete digestive juices. The food is covered into a

peritrophic membrane.

Gastric cecae: There are blind pouches in mesenteron called gastric cecae. Which no. and

location in mesenteron varies in different insects. It mainly works as secretary organ but has

different functions in different insects. It has been found that congenital transmission of

pathogens can take place of here. Because it contain many plant pathogens.

A striking modification of the alimentary canal occurs in Homoptera who contain

filter chamber. In order to get sufficient amount of nitrogenous food they had to suck large

amount of cell sap, which contains water and soluble carbohydrates. So, these filter chambers

filter the nitrogenous food and fatty materials from the cell sap, the rest sugar solution is pass

out the body in the form of honey dew. On this honey dew sooty mould develops and thenormal functioning of the leaf retarded.

Proctodaeum:

Digested food enters into the hind-gut through pyloric valve. Proctodaeum can be

divided into;


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Fore-intestine Hind-intestine

Fore-intestine: This can be divided into Ileum and Colon.

Some absorption may also take place in ileum. Malpighian tubules open into the

ileum which immerse into the haemolymph, gather the metabolic waste and transfer it to the

ileum. In some species of insects it has been found that they also produce a substance which

is used to woven cocoons for pupation. After that the waste material transfers into colon and

subsequently into hind-intestine.

Hind-intestine: It is composed ofRectum and Rectal pads.

Waste material enters into the rectum and from there it is ejected from the anus with

the help of rectal pads.

Organs of Reproduction

Organs of reproduction plays vital role in the congenital transfer of pathogens, So it is

very important to study them that how they can involve in the transfer of pathogens to the

next generation or to the healthy plant.


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Because the female insects are mostly involved in the transfer of pathogens, So we

will only discuss the organs of reproduction of only female insect.

Female reproductive system can be divided into two main parts;

Internal Genitalia External Genitalia

Internal genitalia: It can be divided into several parts;

Ovary: There are two pairs of ovaries in insects. Each of them are composed of two or more

Ovarioles (depends upon the species). The germ cells are produced in ovarioles in the

portion of Germarium just below the terminal filament. After that these germ cells transfers

into site of Vitallarium where they are nourished by the nurse or nutritive cells and

transforms into oogonium and then oocyte. The epithelium of ovarioles is composed of

follicle cells which may also provide nutrition to these cells. They also provide the protective

covering Chorion to the oocytes.

On the basis of presence or absence of nurse cells ovarioles can be divided into two types;

Planoistic: Nurse cells absent. Meroistic:Nurse cells present.

During feeding nurse cells gather around the germ cell and there protoplasm is

diffused into the germ cells. At this time some pathogens may also enter into the oocytes and

congenital transfer may occur.

After formation of egg they are transferred into the Lateral oviducts. In some

insects they may be modified into Calyx for temporarily storage of the eggs. Then these

enters into Median oviduct and from there they enters into Vagina through Gonopore

(regulates the flow of eggs).

Vagina: It acts as a pouch for copulation and also called as Bursa Copulatrix. It has been

found that many pathogens develop into the vagina and when eggs enters into vagina they

enters into it and transfers congenitally.

Spermatheca: They are also called seminal vesicles. In this the sperms stores after mating.

Many plant pathogens are also found in spermatheca and they enter into eggs during

fertilization.

Spermathical glands: They provide the fluid which transfers with the sperms.


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Accessory glands: They provide the gluing or adhesive substance for the eggs so that they

could remain attached to each other or to the substrate. They also called as Colletrial glands.

In some stinging Hymenoptera they also provide a toxic substance.

In some insects the vagina and anus are close together to ensure the congenital

transfer of pathogens from the alimentary canal. Rectum contains pouches in which

pathogens are present. So, when eggs pass through vagina they pushes the walls of these

pouches, results in the squeeze out of these pathogens at the same time as eggs pass through

vagina, and contaminates eggs.

External Genitalia: It is composed of copulatory organs and organs of oviposition.

In some insects the organ of oviposition is modified into a long ovipositor along

with 8th

and 9th

abdominal segment. It helps insects to lay eggs into the plant tissues. So,

ovipositional punctures made by these ovipositors provide an entry site to the pathogens into

the plant.

The Anatomy and Physiolology of Insects in Relation to the Plant Diseases

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