All insects has three main body region, namely: the head, thorax and the abdomen. These main parts of an Insect body functions differently from each other.

The insect head was designed primarily for sensory purposes because of the fact that the eyes and the antenna of an insect is situated on the head.

The thorax on the other hand plays a very important role in insect movement and locomotion. Appendages like legs and Wing are attached to the thoracic region which enables an insect to fly and disperse.

The insect abdomen is considered to be the receptacle of the casing of the visceral organs of an insect. This is also essential in carrying out vital physiological processes such as respiration, circulation, thermoregulation, excretion, and reproduction.
Let us now examine these insect main body regions in detail.

The Insect Head

It is mentioned above that the insect head is where we find the sensory organs of the insect. Yes that’s correct. Insect’s eyes are located at the insects head.

Insect Eyes

We have two types of insect eyes namely:

Ocellus (plural: Ocelli)

The ocelli of an insect are also known as the simple eyes. The main function of the simple eye is to perceive the difference between light and darkness.

The stimulation of the ocelli prompts the other receptors or sense organs to facilitate reflex response. This type of insect eyes can also perceive ultra violet lights and infrared.

Insect’s Compound eyes

The insect’s compound eyes are made up of many hexagonal honey-comb shaped structures called facets; which enable an insect to see the details of its environment such as color, depth, height and etc.

Each of these facets is a cornea of a distinct eye or ommatidium which separately functions; proposed by Johannes Mueller in 1829; apposition eye for diurnal insects and superposition eyes for nocturnal insects.

Insect Antennae

The antenna of an insect serves as phonoreceptor, meaning this body part is well capable of perceiving sound waves.

It can also function olfactory perceiving smell of chemicals around them. The antennae has three major parts.

1. Scape – The scape is the basal segment of the insect antenna. It connects and pivot the the antennae on the head.
2. Pedicel – It is situated at the middle portion of the antenna. It contains the Johnston’s organ, which is capable of perception.
3. Flagellum – The flagellum is a series of smaller segments of equal size also called the clavola.

The pedicel and the flagellum are regions containing sense hairs and sensilla and special organ for olfaction and phonoreception.

Types of antenna:

• Serrate – sawlike

• Moniliform – beadlike

Setaceous – setalike

(dragonflies, damselflies, cicada, leafhoppers)

Filiform – threadlike

(grasshopper,crickets)

Pectinate – comblike

(male click beetle)

Bipectinate- bicomblike

(Atlas moth)

Lamellate – lamella like

(toy beetle, scarabaied)

Geniculate- elbowlike

(ants and bees)

Clavate- clublike

(butterfly)

Plumose- featherlike (mosquitoes)

(mosquitoes)

Capitate – head like

(beetles)

Aristate – arista like

(houseflies)

The insect mouth

The insect mouth is subdivided into the following sclerites: labrum or upper lip, mandible  or upper jaw, maxilla or lower jaw, labium or lower lip, tongue or hypopharynx. It has 2 appendages namely: maxillary palpus and labial palpus.

Types of insect mouth parts:

Mandibulate or chewing

The mandibles are tooth-like for cutting and grinding solid food; the maxillae are structured to assist in holding and moving the food into the mouth; mouth parts are present and conspicuous and are not elongated; most primitive type; found in : grasshoppers, beetles, mole crickets , katydid, locusts, cricket s, preying mantid, termites, dragonfly and damselflies, larvae of moths and skippers, chewing lice, silverfish, stoneflies, psocids, booklice

Chewing-lapping insect mouth

The maxillolabial (from maxilla and labrium) complex is lengthened and it ends in a spoon like structure called flabellum; the maxillary galeae and the labial palpi are brought together over the glossal tongue to form a tubular proboscis for sucking nectar from flowers; the mandibles are present like in chewing insects and are used for cutting leaves and other activities that involve solid food; found in honeybees, ants and wasps

Hemipteron-Homopteran type (piercing-sucking insect mouth)

The mouthparts become elongated into proboscis (beak); the mandibles and maxillae become elongated, fine needle-like structures called mandibular and maxillary stylets which can pierce /penetrate intact plants and animal tissues to suck liquid food (plant sap/juice or animal blood, depending on the host; found in  bugs, hoppers, mealybugs, psyllids, whiteflies, scale insects, spittle bugs, froghoppers, treehoppers, sucking lice in mammals, mosquitoes, horsefly, black fly, deer fly, stable fly, horn fly

Sponging insect mouth type

Sponging type of insect mouth has proboscis or an elongated composite structure formed of the labrum, hypopharynx and the labium.

It has a basal rostrum, median haustellum and the tip, the labella or lobes of the oral sucker; the insect through this mouthpart liquefies solid food then suck the resulting fluid; found in houseflies, blow fly and other muscoid flies 

Rasping-sucking type

 The mouth parts of this type are contained in a cone-like beak at the base of the head, directed downwards; it is asymmetrical because only the left  mandible is functional which is for piercing; the insect makes superficial wounds by rasping and then it will suck the sap/juice exuding from the injured part of the host plant; found in thrips.

Orientation of the insect head and mouth parts

• Hypognathous – mouthparts directed downwards (grasshopper)
• Prognathous – mouth parts is projected forward (carabid beetle)
• Opisthognathous – mouthparts are directed backwards (aphids, leafhoppers, cicada

The Insect Thorax

The Prothorax

Characterized by the saddle-like pronotum covering the rest of the prothorax laterally; pronotum  projects  posterly over the mesonotum in a triangular lobe fitting between bases of the folded tergum; the propleura is a small plate projecting from beneath the anterior margin of the notum just above the leg base; the prosternum is bounded by a pair of coxal cavities which open posteriorly.

The Pterothorax

Meso-meta nota is a rectangular plate limited anteriorly; the scutellum is formed by the posterior median elevation followed by a fold of the notum; the meso-meta sterna are united to form the shieldlike plate of plastron.; the metasternum is wider than the mesosternum.

The meso-meta pleura are clearly separated throughout their lengths by the intersegmental suture; the second thoracic spiracle is borne just at its base just above the coxal cavity.

Each pleuron is clearly divided by the pleural suture into a presutural area, the episternum and epimeron; chewing processes, the epipleurites and the trochantin are not pronouncedly developed in most Acrididae.

Thoracic Appendages

Thoracic appendages includes the wings and the legs of insects.

Insect Wings – It is composed of veins (cross and longitudinal), cells and the regions.

Wings (ptera = technical term for wings); appendage used for flying; majority of insects have wings but not all are winged.

Subclass Pterygota (pterygotes) –Includes all winged insects.

Div. Exopterygota – Includes insects whose wings develop externally as wing buds during the immature stage of the insects; adults would have fully developed wings.

Infraclass Paleoptera – Insects that has wings that have no folding mechanism; hence they cannot fold their wings over their body.

Infraclass Neoptera- Insects that has wings that have folding mechanism, thus, they can fold their wings over their body

Div. Endopterygota – Insects whose wings develop internally starting from the pupal stage; larva does not have wing buds; adults would have fully developed wings.

Subclass Apterygota (apterygotes)– Includes all the wingless insects

Insect wing longitudinal veins

• Costa (C) – the unbranched vein forming the thickened anterior margin of the wing
• Subcosta(Sc)-the simple or occasionally forked veins just above the costa
• Radius (R)-typically 5-branched vein with the 2nd branch very prominent and persistent in most insects and termed as the radial sector
• Media (M)- originally divided into an anterior branch and a posterior branch with both branches seen in most mayflies
• Cubitus (Cu)- a primitively 2-branched vein with the 1st branch often divided
• Vannal veins(V)- a series of veins arranged fan like in the posterior part of the wing
• Jugal veins (J) – usually appear as an irregular network on the posterior basal lobe of the jugum when it occurs in insects

Insect wing cross veins:

• Humeral crossvein (h)- this connects the costa and subcosta near the base of the wing
• Radial crossvein (r)-this extends from the radial vein to the first branch of the radial sector
• Sectoral crossvein(s)-connecting the two main branches of the radial sector
• Radio-medial crossvein (r-m)- extends from the radius to the media near the center of the wings
• Median crossvein (m)- connects the 2nd and 3rd forks of the media
• Medio-cubital crossveins (m-cu)- extends from media to cubitus

The Insect Wing Cells

These are the membranous areas enclosed by the veins; the so called basal cells are bounded by the main longitudinal  veins whiled the distal cells are bounded by branches of these main veins.

The Regions of the Wings

The basal angle or humeral angle, apical angle and the vannal angle; the main portion of the wing from costa to postcubitus is known as the remigium; immediately behind this is the vannal fold followed by the vannal region or vannus; behind this near the wing base a jugal fold is present in some insects.

The triangular region at the base of the wing is the auxillary region and forms the hinge of the wing with the body; there are at least three distinct sclerite present in this area horizontally over the body; these sclerites are altered considerably and are less distinct in mayflies and dragonflies.

Types of Insect Wings

Membranous – membranous wings:

The common type in the inner wing of almost all insects. Uniformly soft, flexible thin, mostly translucent or transparent wings, which may or may not have any form or covering.

No covering:

True flies, wasps, bees, ants, termites, dragonfly, damselfly, mayflies, stoneflies, hind wings of beetles, weevils , grasshopper

With coverings:

Butterflies and moths (with scales), caddisflies (w/ hairs)

Tegmina:

Slightly hard, leathery , parchmentlike in texture as in diploma paper; examples: forewings of grasshoppers, locusts, katydids, mantids, leaf insect  and walking sticks)

Elytra:

Hard , thick fore wings of beetles, weevils earwigs ; used as protective covering s of  the body of insects

Hemelytra:

Semi or half  elytra and half membranous ; the basal portion is elytron while the distal portion is membranous ; examples: true bugs

Haltere:

Two small knob like structures on each side of  the thorax in some flies acting as hind wings but are used instead  for balancing (e.g. Houseflies, soldier fly, crane flies )

Fringed wings:

Wing with central structure has its margins lined with very fine  feathery or hair like structures; fringe  means margin (found in thrips which is the smallest winged insect).

Insect Wing Coupling Mechanisms:

Simple coupling

The jugal lobe of forewing overlaps with the remigium of the hindwings (neuropterans)

Jugate coupling

Jugal lobe is fingerlike and clasps the hindwing

Frenate coupling

The anterior edge of the hindwing has 1 to several bristle called frenula (Lepidoptera)

Hamate coupling

Row of hook like setae called hamuli (honeybees, wasps)

The insect Legs

Parts of the Insect Leg

Subcoxa

True basal segment of the primitive leg, either reduced or in expanded to form the pleuron; the trochantin when present represent the remnant of the subcoxa which has remained in articulation with the coxa; the cicada shows the differentiation of the subcoxa into large sclerites including rudimentary pleurites with the small basal segement

Coxa

Functional base of the leg and has replaced the subcoxa in this function; frequently divisible into 2 lobes by an inflection of its wall where it articulates with the plueron; usually elongated amonf raptorial type of leg

Trochanter

2nd division of the leg which articulates with the coxa but usually fixed to the femur

Femur

The largest region of the leg and conspicuous in most insects which are powerful leapers; it is beset with spines

Tibia

4th division of the insect legs; almost always slender and frequently equals or exceeds the length of the femur; it has a row of tibial spines or a tibial spur

Types of Insect Legs

Fossorial leg

Stocky, strong and has strong spines that are effective in digging into the soil (mole crickets)

Raptorial leg

The tibia and tarsus may have complementary spines to catch and grasp the prey (preying mantid)

Clinging legs

Leg is adapted for clinging  or holding on its host’s hair  (lice)

Pollen-gathering leg

Has many hairs and modified as pollen basket  (bees)

Cursorial leg

Normally thin and long mainly for running or walking (termites, cockroaches and most legs of insects)

Saltatorial leg

Stronger and generally bigger hindlegs of insects like flea, psyllids, grasshoppers, locusts, katydids, flea beetles, leafhoppers, planthoppers, treehoppers, froghoppers 

Natatorial leg

Legs which are modified into broad and flattened legs which serve as paddles for many aquatic insects such as giant water bugs, diving beetles

The Insect Abdomen

The abdomen is the 3rd division of the insect body. It differs from the head and thorax on the following characteristics:

• Simplicity of feature
• Lack of segmental appendage in the adult
• Tendency of the segments to migrate anteriorly and become fused with the preceeding segment. This is a characteristics exemplified by the migration of the first abdominal sternite in the grasshopper to the sternum of the metathorax, and the appearance of the propodeum in Hymenopterans.

Segments of insect abdomen

The usual number of segments in insect’s abdomen is 10 to 11 and from embryological evidence it appears that the primitive number was no greater than 12.

The reduction in segments takes place at the posterior end of the body but in many of the higher insect, there is a tendency toward the elimination of the 1st segment.

Functions of insect abdomen

Progenital, genital and postgenital subsegments; the genital segments are primarily the 8th and 9th in the female and the 9th in the male since it is the appendages of these segments that form the principal parts of the external genitalia.

The ovipositor

the ovipositor of the grasshopper consists of three pairs of valves: the ventral valves (paraproct) , the dorsal valves (epiproct) and the inner valves (cerci).

The tympanum (organ for hearing)

In short horned grasshoppers (locusts), it is found on  the lateral side of the first abdominal segment; one tympanum is found on  each side

The male insect genitalia

the enlarged boatshaped 9th sternum of the male insect is divided by a transverse suture into a proximal and distal part which are not movable on each other.

The distal part is known as the subgenital plate; behind this is the pallium which is reflected to form the floor of the genital chamber.

If hooflike extensions are removed on the subgenital plate, it exposes the genitalia on the pallium.

The genitalia consists of the aedeagus, the two  endophilic plates, the aedeageal apodemes and the epiphallus.

The aedeagus is normally concealed beneath the hood of the pallium.

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