Unlike vertebrates, insects have no backbones. Instead, they have a tough outer support framework or exoskeleton called cuticle. The cuticle of each segment is formed into several hardened plates called sclerites, separated by infolds (sutures), which give it flexibility. The sclerite covering each segment can be divided into 4 main parts, namely, a dorsal region or tergum, a ventral region or sternum and on either side, a lateral region or pleuron. The sclerites are more developed in some parts of the insect than others e.g. pleural sclerites are more developed and pronounced on the thorax than the abdominal region. The uppermost layer of the insect cuticle is waxy in nature; it controls water permeability and prevents desiccation.

Like other arthropods, the insect’s body is segmented with jointed appendages. Generally, all insects have their bodies divided into three major regions namely – head, thorax and abdomen. Although the cuticle forms a continuous investment over the whole body of an insect, it usually remains membranous and flexible along certain lines so that the body is divided externally into a series of segments separated by intersegmental membranes.
The insect body comprises twenty (20) primitive segments, all of which may be apparent in the embryo. These segments are grouped into well defined regions – the head, thorax and abdomen. The head is a tagma made up of 6 segments and an anterior non-segmental acron that have all fused together into a head-capsule or cranium. The thorax consists of 3 segments, each of which carries a pair of legs. In addition, the 2nd and 3rd segments each carry a pair of wings. The thorax is connected with the head by the cervix or neck. The abdomen comprises 11 segments and a terminal non-segmental telson, but reduction and fusion in the posterior region often results in only 10 or fewer divisions being visible.
The insect’s head is an anterior tagma formed by the fusion of six segments namely preantennary, antennary, intercalary, mandibular, maxillary and labial segments. The head tagma is highly sclerotized to from a hard capsule or cranium. The head articulates with the thorax through the neck or cervix. Sclerites of the head include the Vertex, Frons, Clypeus, Gena, Occiput. Sutures of Head include the Epicranial suture (which is an inverted `Y') Epistomal suture, Clypeo labral suture, Post occipital suture. Posterior opening of the cranium through which aorta, foregut, ventral nerve cord and neck muscles passes is known as Occipital foramen. The appendages of the head are called Cephalic appendages and they include a pair of compound eyes, 0-3 simple eyes or ocelli, a pair of antenna and mouth parts.
Functions of the insect head include – food ingestion, sensory perception, coordination of body activities and protection of the coordinating centres.

Based on the inclination of long axis of the head and orientation of mouth parts there are three types of insect heads.
1) HYPOGNATHOUS (Hypo – below; gnathous – jaw)
This type is also called orthopteroid type. The long axis of the head is vertical. It is at right angles to the long axis of the body. Mouth parts are ventrally placed and project downwards. Found in grasshoppers
2) PROGNATHOUS (Pro- in front; gnathous – jaw)
This type is also called coleopteroid type. The long axis of the head is horizontal. It is in line with the long axis of the body. Mouth parts are directed forward. Found in ground beetles.
3) OPISTHOGNATHOUS (Opistho – behind; gnathous – jaw)
This type is also called hemipteroid type or opisthorhychous. Head is deflexed. Mouth parts are directed backwards and held in between the fore-legs. Found in Stink bugs.

One of the main features of an insect’s head is its antennae. All adult insects (except, at times, scale insects) have one pair. They usually are located between or in front of the eyes. The insect antennae are seg¬mented. The first two proximal segments are referred to as the scape and pedicel, respectively. The remaining antennal segments (flagellomeres) are jointly called the flagellum. Insect antennae vary greatly in form and complex¬ity.  They are primarily used as organs of smell, but in some insects they may serve additional functions such as detection of motion and orientation, sound, humidity, and even in mating.
Types of insect antenna
1. Aristate: are pouch-like with a lateral bristle. Example: House flies.
2. Capitate: are abruptly clubbed at the end. Example: butterflies.
3. Clavate: are gradually clubbed at the end. Example: carrion beetles.
4. Filiform: are thread-like in shape. Examples: ground beetles, cockroaches.
5. Geniculate: are hinged or bent like an elbow. Example: ants.
6. Lamellate: are antennae that end in nested plates. Example: scarab beetles.
7. Moniliform: are beadlike in shape. Examples: termites.
8. Pectinate: are comb-like in shape. Examples: fireflies
9. Plumose: are feather-like in shape. Examples: moths and mosquitoes.
10. Serrate: are saw-toothed in shape. Examples: click beetles.
11. Setaceous: are bristle-like in shape. Examples: dragonflies and damselflies.

The insect mouthpart consists of the labrum, mandibles, maxillae (plural maxilla) and labium. The labrum or upper lip is a flap like sclerite. The mandibles or jaws are highly sclerotized paired structures that are used for biting and chewing. The maxillae are paired structures with segmented palps. The labium or lower lip is made up of a pair of segmented palps.
Insect mouthparts all have the same basic structure but are modified in different insect orders in relation to their feeding habits. Functionally speaking, the insect’s mouthparts are of two main types namely –
1) Mandibulate (chewing): used for biting and grinding solid foods. Examples: Dragonflies, termites, adult lacewings, beetles, ants, cockroaches, grasshoppers, etc.
Larvae generally have chewing-type mouthparts regardless of the kind they will have as adults.
2) Haustellate (sucking): primarily used for sucking liquids. Types of haustellate mouthparts include –
a. Piercing-sucking mouthparts
These are stylate type of haustellate mouthparts. They are used to penetrate solid tissue and then suck up liquid food. Found in cicadas, aphids, and other bugs (order Hemiptera), sucking lice, stable flies and mosquitoes (order Diptera).
b. Siphoning mouthparts
These are non-stylate type of haustellate mouthparts. They are used to suck liquids. Examples: Butterflies, moths etc.
c. Sponging mouthparts
These are non-stylate type of haustellate mouthparts. Sponging mouthparts are used to sponge and suck liquids. Examples: House flies and blow flies.
There also are intermediate types of mouthparts: rasping-sucking (found in thrips) and chewing-lapping (found in honeybees, wasps and bumblebees). The chewing mouth type is more primi¬tive and generally stronger than sucking types. Some insects have different mouthparts as larvae and adults. Nymphs have mouthparts similar to those of adults. For some adult insects, the mouthparts are vesti¬gial or non functional.

The thorax is made up of three consecutive segments namely the prothorax, mesothorax and metathorax. Each of these segments bears a pair of legs. When the mesothorax and metathorax each bear a pair of wings they are collectively referred to as Pterothorax. If only one pair of wings is present, it is usually attached to the middle segment. The prothorax does not bear wings. The segment of the thorax is made up of four sclerites or body plates namely a dorsal body plate (Tergum or Notum), a ventral body plate (Sternum) and 2 lateral plates (Pleura). The notum of the prothorax, mesothorax and metathorax are called pronotum, mesonotum and metanotum respectively.
The thorax is concerned mainly with the function of locomotion.

Insects characteristically have three pairs of jointed legs on thoracic segments. True legs are found only in the adult stages of insects. However, pro-legs or pseudo-legs, which are fleshy body projections used for clinging, may occur on the larvae of some insect orders e.g. Order Lepidoptera. The prothoracic, mesothroacic and metathoracic legs are called forelegs, mid-legs and hind legs, respectively.
Each leg has six major components, listed here from proximal to distal: coxa (plural coxae), trochanter, femur (plural femora), tibia (plural tibiae), tarsus (plural tarsi), and pretarsus. The femur and tibia may be modified with spines. The tarsus appears to be divided into one to five "pseudo-segments" called tarsomeres.
Like the mouthparts and antennae, insect legs are highly modified for different functions, depending on the environment and habit of the insect. Leg modification in insects include –
1. Ambulatory legs: are used for walking. Examples: Bugs (order Hemiptera), leaf beetles.
2. Cursorial legs: are similar to ambulatory legs in structure. They are modified for running. Note the long, thin leg segments. Examples: Cockroaches, ground beetles.
3. Saltatorial legs: are hind legs adapted for jumping. Examples: Grasshoppers, crickets.
4. Raptorial legs: are fore legs modified for grasping preys. Examples: Mantids (order Mantodea), ambush bugs, giant water bugs and water scorpions (order Hemiptera).
5. Fossorial legs: are fore legs modified for digging. Examples: Ground dwelling insects like mole crickets and cicada nymphs.
6. Natatorial legs: are modified for swimming. Examples: Aquatic beetles and bugs.
7. Scansorial legs: are modified for clinging. Examples: Louse
8. Foragial legs: Legs of honeybee are adapted for various functions. The legs are equipped with a pollen comb to remove pollen from body, a pollen basket to store pollen, a spur on the apex of middle tibia to scrape pollen from the baskets and an antennal comb on the front legs to remove pollen from antenna.

Insects are the only flying invertebrates. Wings are present only in adult stage. Number of wings varies from two pairs to none. Certain primitive insects like silver fish and spring tail are primarily apetrous. Ecto parasites like head louse, poultry louse and flea are secondarily apterous. Wings are deciduous in ants and termites. There is only one pair of wings in the true flies. Normally two pairs of wings are present in insects and they are borne on the pterothoracic segments. Wing surfaces may be covered with fine hairs or scales, or they may be bare. Venation (the arrangement of veins in the wings) is different for each group of insects and is used as a means of identification.
The wings of insects may have the following modifications –
1. Tegmina (singular tegmen): are the leathery forewings of insects in the orders Orthoptera, Blattaria, and Mantodea. They help protect the delicate hind wings.
2. Elytra (singular elytron): are the hardened, heavily sclerotized forewings of beetles and are modified to protect the hind wings when at rest.
3. Hemelytra: are variations of the elytra. The forewings of Hemipterans are said to be hemelytrous because they are hardened throughout the proximal two-thirds, while the distal portion is membranous. Unlike elytra, hemelytra function primarily as flight wings.
4. Halteres: are an extreme modification among the order Diptera (true flies), in which the hind wings are reduced to knobs used for balance and direction during flight.
5. Hamuli: are tiny hooks on the front edge of the hind wings. They hold front and hind wings together during flight. i.e. they are used for wing coupling.
6. Frenulum: is a large bristle on the hind wings of some insects in the order Lepidoptera. It is also used for wing coupling.
7. Membranous wings: are thin and more or less transparent in appearance. Some may however be darkened. Found in the order Odonata, Neuroptera, Diptera, Hymenoptera and Isoptera.
8. Scales: some insect wings are covered with scales which make the wings colourful. Examples: Butterflies, moths and caddisflies.

The abdomen is a highly flexible tagma formed by the fusion of 9-11 segments. Each abdominal segment is made up of only two sclerite namely dorsal body plate (tergum) and ventral body plate (sternum). The first eight abdominal segments each bear a pair of spiracles. Abdominal appendages are genital organs and a pair of cerci at the tip of the abdo-men. They cerci may be short, as in grasshoppers, termites and cockroaches; extremely long, as in mayflies; or curved, as in earwigs.
The abdomen is concerned with the function of reproduction and metabolism.

EXTERNAL STRUCTURE OF AN INSECT EXTERNAL STRUCTURE OF AN INSECT Reviewed by DailyGgist_Official on August 19, 2018 Rating: 5

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