Introduction:

Insects contain segmented bodies as had their earlier wormlike ancestors. 20 segments were reported to exist in such ancestors. Nowadays, insects represent a reduction in this number, with some segment fusing to make three well-defined areas head, thorax and abdomen. The grouping of this segment into functional areas is termed as Tagmatisation.

Tagmatisation: 

The specialization and grouping of the adjacent metameric segments into area termed as tagma or tagmata is Tagmatisation.  

This regional specialization yield in supra-segmental organization like the head, thorax and abdomen. In ancient metamerically segmented animals, the structure and function of the segment which makes up the body are almost the same. In advanced Arthropods, groups of adjacent segment termed as tagmata are structurally marked off and specialized to carry out specific functions. This outcome in the division of labor leads to better efficiency.

Insects represent or show three distinguishing tagmata:

1) The head which consists of six segments.

2) The thorax which consists of three segments.

3) The abdomen which consists of at most eleven segments.

The Head:

The insect head bears the mouth portions and sense organs. It is transformed for feeding and detection of the external stimuli. The mouth portions are differed in form and the nature of the mouth portions generally pointing out the feeding habits of a specific insect. The mouth portions are, though, composed of some fundamental units, which might be modified in specific conditions to suit the mode of feeding and detection of the external stimuli.

Representative Head capsule of Insect:

The head is an anterior-like capsule such as heavily sclerotids body area having different shapes. It is assumed to have been made by the fusion of six segments. The head can be categorized into three kinds on the basis of the orientation by the mouth part.

 It is categorized into:

i) Hypognathous: The mouth portions encompass the similar orientation with the leg. This is the primitive arrangement and takes place in vegetarians. Example: grasshoppers.

ii) Prognathous: The mouth portion point forward, it takes place in carnivorous and burrowing species example: coleoptera (the weevils). 

iii) Opisthorhynchous: This is build-up of an elongate proboscis that shape downward and backward in between the legs example: Homoptera.

Eyes:

It is the primary structure which is positioned in the head; insects have two kind of eyes:

i) Simple eyes

ii) Compound eyes

The Antenna:

The antenna are the sense organs borne on the insect head, are paired segmented antennae (or feelers). They differ greatly in the form: 

1) Setaceous (threadlike)

2) Clavate (chibbed)

3) Lamellate (with leaf like folds)

4) Geniculate (elbowed)

5) Filiform (linear) 

6) Capitate (with head)

7) Moniliform (headlike)

8) Pectinate or hipectinate (Comblike)

9) Serrate (toothed)

10) Plumose (Feathery).

The tenure of one pair of antenna is a diagnostic characteristic of all insect and is features of each family, genus, species of insects and of great significance in recognition of sensory functions attributed to the antennae based on the kind of sensilla (that is, sensory receptors) present in them.

They might bear:

(a) Chemoreceptors, (b) Hygroreceptors, (c) Auditory, (d) Olfactory and (e) Gastatory

The antenna differs in sizes and forms, based on their functions and efficiency:

a) They are employed as sense organs that function as olfactory, tactile and auditory organ.

b) Sexual dimorphism might take place in certain insects through the possession of antenna particularly in social insects. Others are transformed in antenna into mating, breathing and feeding apparatus.

The Mouth Part:

The mouth portion is an organ essentially appendages and jaws, build-up of an unpaired labrum and followed through an epipharynx and hypopharynx in front and behind. Has paired mandible and maxilla and unpaired labium making the lower lib, this structures are altered in different insects on the dietary bases.

Two bases kinds were acknowledged:

1) Mandibulate type i.e. (biting and chewing type):

This is a broad-loped sterile hanging from the clypeus; it is mounted from the other mouth portion by two muscles occurring from the head and inserted medically into the anterior merging of the sclerite. This is though lowered by the other paired muscles inserted on the torma(e) that are thickened part of the sclerite positioned on the posterior lateral merging of the labrum.

2) Haustellate type (sucking mouthpart):

It is more advanced, lengthened and beak-like. This usually comprises modification of the fundamental mouth portion into lengthened stylets. Different modifications based on the diet have been noticed.  

• Rasping and sucking example: thrips.
• Piercing and sucking example: dipterans (mosquito) hemapterans (fleas and head louse).
• Sponging and lapping example: housefly.
• Sucking example: epidopterans.
• Chewing and sucking example: bees.

The Thorax:

The thorax comprises the chest area of the insect body. It is a somewhat short box-like area, specialized for participation in the locomotory activity. The legs and wings which are veritable organs of locomotion are joined to thorax. The thorax is categorized into three areas:

• The prothorax (in front)
• The mesothorax (in the middle)
• The metathorax (behind) the thoracic segment

Insect Legs:

The legs are basically found on the thoracic part, all insects' legs typically comprises of six segments that articulate with one other through monocondylic articulate or dicondylic articulation, such segments are:

1) Coxa: The coxa is usually short and instead stout; however it differs in shape among taxa. This is set in a coxa cavity and articulates by the thorax at the coxa methods of the pleural sulcus (that is, groove).

2) Trochanter: The Trochanter is minute and freely movable in the vertical direction on the coxa, however it is frequently rather fixed to the base of femur.

3) Femur: The femur is generally the biggest and strongest part of the leg. Its size is associated to the mass of the tibial extensor muscles in it, differing from a small, thick part in larval insects to the huge part in the hind leg of the jumping Orthoptera.

4) Tibia: The tibia usually is long and slender in the adult insects. Proximally, it is bent somewhat to the femur; let the shaft of the tibia to be flexed close against the femur for extra locomotory power in insects like grasshoppers.

5) Tarsus: The tarsus is a simple, complete segment in holometabolous larvae and basal hexapods like Protura and a few Collembola. In collembolans, the tarsus and tibia are fused to the single tibiotarsus. In most of the insects, tarsus is generally made up of 2-5 segments termed as tarsomeres.

6) Pretarsus: The Pretarsus, as well termed as the posttarsus or acropod, occurs from the distal end of the tarsal segment. In the Protura, Collembola, and larvae of most of the holometabolous insects, the pretarsus is a simple, claw like segment.

Insect Wings:

Insects owes certain part of their success as terrestrial organism to the possession of wings, being the only portion of organisms apart from the aves or birds which have this character, the wings are dorsolateral outgrowth of the body wall emanating among the northum and pleuron, they are thin plate-like growth of the integument and are strengthen through a frame-work of holochitinous tubes termed as VAILS.

The vail is membranous having small sclerites to which muscles for wing movement are joined.

The mobility of the wing is moreover improved by the ability of the thorax to change shape.  Insect wings differ in shape, number, size, variation and disposition.  

If insect are at rest, the wings are folded back on the body, most of the adult insects encompass two pair of wings that are independent of one other and therefore ineffective (not effective) most insect thus, tend to become functionally two winged, either through coupling the wing on every side, as a result that they function as one or by altering the second pair to form the sense organs.  Illustration: Halteres (Insect having two wings).

In most of the insects, wings are membranous and might carry small scales. In some though the forewing are thickened to make Elytra or they might become Leathery to make TAGMINA both of which are harder than the normal membranous wings, veins of wings differ in distribution with species however are quite constant by the species and so the character of veins have been employed lengthily in insects categorization. The whole system of wing on the wings is termed as or termed as Veination.

The Abdomen:

The abdomen is generally build-up of 11 segments. The 11th one being much reduced and exhibited by appendages in most cases, the number of segment might though be decreased due to the fusion or telescopic of the segment into one other. 

Throughout development other segment are added in the anamorphic insect example: Proturans.  However in Epimorphic insect particularly the holometabolous forms, no such growth is observed. The abdominal part has pronounced stergal, sternal and sclerite.  However the pleural plate is decreased to membranous materials on which the spiracle is borne.

The spiracle might take place on the stergal or sternal plate of a few insect. The pregenital part will not carry appendages apart from in a few immature forms and adult Apterygota in certain pterygote such segment might carry appendages that function as gills in the aquatic animals, lateral filament or pro-legs. The posterior segment are altered for ovipositor and mating (the no 8 to 10th abdominal segment). 

The reproductive opening of the male insect is generally on the 9th segment and it is concerned by copulation and transference of spermotheca to the female as the female opening of the oviduct is generally on or behind the segment 8 or 9. 

This is concerned mainly by oviposition and such structure might be withdrawn to the organism whenever not in use. The external genitalia of insect are much variable even in the species of the similar genera and this is strictly adapted to the habit comprised in copulation and egg lying.

The abdomen of several insects of more posterior segments tends to be hidden beneath those in front, of them, therefore decreasing the number of segments visible externally. Apart from in some primitive insects, like the Collembola and Thysanura, locomotory appendages are not present in the abdomen, however other structures like cerci, styles and those related with the external genitalia are generally present. Female adults bear ovipositors that assist in egg-laying and might be altered in shape and form for digging, piercing and stinging.

Spiracles:

Insects, such as many other terrestrial arthropods, respire through trachea that are branched, tube-like structures supplying the oxygen directly to the tissues and opening and closing as desired, so as to decrease water loss via them. Paired spiracles are positioned at the sides of the thoracic and abdominal parts.

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