An insect's nervous system is a network of specialized cells (called neurons) that serve as an "information highway" within the body. These cells generate electrical impulses (action potientials) that travel as waves of depolarization along the cell's membrane. Every neuron has a nerve cell body (where the nucleus is found) and filament-like processes (dendrites, axons, or collaterals) that propagate the action potential. Signal transmission is always unidirectional -- moving toward the nerve cell body along a dendrite or a collateral and away from the nerve cell body along an axon.
Neurons are usually divided into three categories, depending on their function within the nervous system:
1. Afferent (sensory) neurons -- these bipolar or multipolar cells have dendrites that are associated with sense organs or receptors. They always carry information toward the central nervous system.
2. Efferent (motor) neurons -- unipolar cells that conduct signals away from the central nervous system and stimulate responses in muscles and glands.
3. Internuncial (association) neurons -- unipolar cells (often with several collaterals and/or branching axons) that conduct signals within the central nervous system.
Individual nerve cells connect with one another through special junctions, called synapses. When a nerve impulse reaches the synapse, it releases a chemical messenger (neurotransmitter substance) that diffuses across the synapse and triggers a new impulse in the dendrite(s) of one or more connecting neurons. Acetylcholine, 5-hydroxytryptamine, dopamine, and noradrenaline are examples of neurotransmitters found in both vertebrate and invertebrate nervous systems.
Nerve cells are typically found grouped in bundles. A nerve is simply a bundle of dendrites or axons that serve the same part of the body. A ganglion is a dense cluster of interconnected neurons that process sensory information or control motor outputs.
The Central Nervous System
Like most other arthropods, insects have a relatively simple central nervous system with a dorsal brain linked to a ventral nerve cord that consists of paired segmental ganglia running along the ventral midline of the thorax and abdomen. Ganglia within each segment are linked to one another by a short medial nerve (commissure) and also joined by intersegmental connectives to ganglia in adjacent body segments. In general, the central nervous system is rather ladder-like in appearance: commissures are the rungs of the ladder and intersegmental connectives are the rails. In more "advanced" insect orders there is a tendency for individual ganglia to combine (both laterally and longitudinally) into larger ganglia that serve multiple body segments.
An insect's brain is a complex of six fused ganglia (three pairs) located dorsally within the head capsule. Each part of the brain controls (innervates) a limited spectrum of activities in the insect's body:
Protocerebrum: The first pair of ganglia are largely associated with vision; they innervate the compound eyes and ocelli.
Deutocerebrum: The second pair of ganglia process sensory information collected by the antennae.
Tritocerebrum: The third pair of ganglia innervate the labrum and integrate sensory inputs from proto- and deutocerebrums. They also link the brain with the rest of the ventral nerve cord and the stomodaeal nervous system (see below) that controls the internal organs. The commissure for the tritocerebrum loops around the digestive system, suggesting that these ganglia were originally located behind the mouth and migrated forward (around the esophagus) during evolution.
Located ventrally in the head capsule (just below the brain and esophagus) is another complex of fused ganglia (jointly called the subesophageal ganglion). Embryologists believe this structure contains neural elements from the three primitive body segments that merged with the head to form mouthparts. In modern insects, the subesophageal ganglion innervates not only mandibles, maxillae, and labium, but also the hypopharynx, salivary glands, and neck muscles. A pair of circumesophageal connectives loop around the digestive system to link the brain and subesophageal complex together.
In the thorax, three pairs of thoracic ganglia (sometimes fused) control locomotion by innervating the legs and wings. Thoracic muscles and sensory receptors are also associated with these ganglia. Similarly, abdominal ganglia control movements of abdominal muscles. Spiracles in both the thorax and abdomen are controlled by a pair of lateral nerves that arise from each segmental ganglion (or by a median ventral nerve that branches to each side). A pair of terminal abdominal ganglia (usually fused to form a large caudal ganglion) innervate the anus, internal and external genitalia, and sensory receptors (such as cerci) located on the insect's back end.

The Stomodaeal Nervous System
An insect's internal organs are largely innervated by a stomodaeal (or stomatogastric) nervous system. A pair of frontal nerves arising near the base of the tritocerebrum link the brain with a frontal ganglion (unpaired) on the anterior wall of the esophagus. This ganglion innervates the pharynx and muscles associated with swallowing. A recurrent nerve along the anterio-dorsal surface of the foregut connects the frontal ganglion with a hypocerebral ganglion that innervates the heart, corpora cardiaca, and portions of the foregut. Gastric nerves arising from the hypocerebral ganglion run posteriorly to ingluvial ganglia (paired) in the abdomen that innervate the hind gut.
In comparison to vertebrates, an insect's nervous system is far more de-centralized. Most overt behavior (e.g. feeding, locomotion, mating, etc.) is integrated and controlled by segmental ganglia instead of the brain. In some cases, the brain may stimulate or inhibit activity in segmental ganglia but these signals are not essential for survival. Indeed, a headless insect may survive for days or weeks (until it dies of starvation or dehydration) as long as the neck is sealed to prevent loss of blood!

Nervous system

The nervous system of an insect can be divided into a brain and a ventral nerve cord. The head capsule, made up of six fused segments, each with a pair of ganglia, or a cluster of nerve cells outside of the brain. The first three pairs of ganglia are fused into the brain, while the three following pairs are fused into a structure of three pairs of ganglia under the insect's esophagus, called the subesophageal ganglion.
The thoracic segments have one ganglion on each side, which are connected into a pair, one pair per segment. This arrangement is also seen in the abdomen but only in the first eight segments. Many species of insects have reduced numbers of ganglia due to fusion or reduction.Some cockroaches have just six ganglia in the abdomen, whereas the wasp Vespa crabro has only two in the thorax and three in the abdomen. Some insects, like the house fly Musca domestica, have all the body ganglia fused into a single large thoracic ganglion.
Insects have nociceptors, cells that detect and transmit sensations of pain.This was discovered in 2003 by studying the variation in reactions of larvae of the common fruitfly Drosophila to the touch of a heated probe and an unheated one. The larvae reacted to the touch of the heated probe with a stereotypical rolling behavior that was not exhibited when the larvae were touched by the unheated probe.Although nociception has been demonstrated in insects, there is not a consensus that insects feel pain consciously.

The insect nervous system consists primarily of a brain (5), located dorsally in the head, and a nerve cord (19) that runs ventrally through the thorax and abdomen. The insect brain is a fusion of three pairs of ganglia, each supplying nerves for specific functions. The first pair, called the protocerebrum, connects to the compound eyes (4) and the ocelli (2, 3) and controls vision. The deutocerebrum innervates the antennae (1). The third pair, the tritocerebrum, controls the labrum, and also connects the brain to the rest of the nervous system.
Below the brain, another set of fused ganglia forms the subesopagheal ganglion (31). Nerves from this ganglion control most of the mouthparts, the salivary glands, and the neck muscles.

The central nerve cord connects the brain and subesophageal ganglion with additional ganglion in the thorax and abdomen. Three pairs of thoracic ganglia (28) innervate the legs, wings, and muscles that control locomotion.

Abdominal ganglia innervate the muscles of the abdomen, the reproductive organs, the anus, and any sensory receptors at the posterior end of the insect.

A separate but connected nervous system called the stomodaeal nervous system innervates most of the body's vital organs. Ganglia in this system control functions of the digestive and circulatory systems. Nerves from the tritocerebrum connect to a ganglia on the esophagus; additional nerves from this ganglia attach to the gut and heart.

Exoskeleton

Their outer skeleton, the cuticle, is made up of two layers: the epicuticle, which is a thin and waxy water resistant outer layer and contains no chitin, and a lower layer called the procuticle. The procuticle is chitinous and much thicker than the epicuticle and has two layers: an outer layer known as the exocuticle and an inner layer known as the endocuticle. The tough and flexible endocuticle is built from numerous layers of fibrous chitin and proteins, criss-crossing each others in a sandwich pattern, while the exocuticle is rigid and hardened.The exocuticle is greatly reduced in many soft-bodied insects (e.g., caterpillars), especially during their larval stages.
Insects are the only invertebrates to have developed flight capability, and this has played an important role in their success.These muscles are able to contract multiple times for each single nerve impulse, allowing the wings to beat faster than would ordinarily be possible. Having their muscles attached to their exoskeletons is more efficient and allows more muscle connections; crustaceans also use the same method, though all spiders use hydraulic pressure to extend their legs, a system inherited from their pre-arthropod ancestors. Unlike insects, though, most aquatic crustaceans are biomineralized with calcium carbonate extracted from the water.

General body plan

Insects possess segmented bodies supported by an exoskeleton, a hard jointed outer covering made mostly of chitin. The segments of the body are organized into three distinctive but interconnected units, or tagmata; a head, a thorax, and an abdomen.The head supports a pair of sensory antennae, a pair of compound eyes, if present, one to three simple eyes or (ocelli) and three sets of variously modified appendages that form the mouthparts. The thorax has six segmented legs (one pair each for the prothorax, mesothorax and the metathorax segments making up the thorax) and two or four wings (if present in the species). The abdomen (made up of eleven segments some of which may be reduced or fused) has most of the digestive, respiratory, excretory and reproductive internal structures.There is considerable variation and many adaptations in the body parts of insects especially wings, legs, antenna, mouth-parts etc.Insects have segmented bodies supported by an exoskeleton, a hard outer covering made mostly of chitin. The segments of the body are organized into three distinctive but interconnected units, or tagmata: a head, a thorax, and an abdomen.[8] The head supports a pair of sensory antennae, a pair of compound eyes, and, if present, one to three simple eyes (or ocelli) and three sets of variously modified appendages that form the mouthparts. The thorax has six segmented legs—one pair each for the prothorax, mesothorax and the metathorax segments making up the thorax—and, if present in the species, two or four wings. The abdomen consists of eleven segments, though in a few species of insects these segments may be fused together or reduced in size. The abdomen also contains most of the digestive, respiratory, excretory and reproductive internal structures.[9]:22–48 There is considerable variation and many adaptations in the body parts of insects especially wings, legs, antenna, mouth-parts etc.


Insect anatomy
A- Head B- Thorax C- Abdomen
1. antenna
2. ocelli (lower)
3. ocelli (upper)
4. compound eye
5. brain (cerebral ganglia)
6. prothorax
7. dorsal blood vessel
8. tracheal tubes (trunk with spiracle)
9. mesothorax
10. metathorax
11. forewing
12. hindwing
13. mid-gut (stomach)
14. dorsal tube (Heart)
15. ovary
16. hind-gut (intestine, rectum & anus)
17. anus
18. oviduct
19. nerve chord (abdominal ganglia)
20. Malpighian tubes
21. tarsal pads
22. claws
23. tarsus
24. tibia
25. femur
26. trochanter
27. fore-gut (crop, gizzard)
28. thoracic ganglion
29. coxa
30. salivary gland
31. subesophageal ganglion
32. mouthparts

Specification

Insects typically move about by walking, flying or occasionally sinking and swimming at the same time. Because it allows for rapid yet stable movement, many insects adopt a tripedal gait in which they walk with their legs touching the ground in alternating triangles. Insects are the only invertebrates to have evolved flight. Many insects spend at least part of their life underwater, with larval adaptations that include gills and some adult insects are aquatic and have adaptations for swimming. Some species, like water striders, are capable of walking on the surface of water.
Insects are mostly solitary, but some insects, such as certain bees, ants, and termites are social and live in large, well-organized colonies. Some insects, like earwigs, show maternal care, guarding their eggs and young. Insects can communicate with each other in a variety of ways. Male moths can sense the pheromones of female moths over distances of many kilometers. Other species communicate with sounds: crickets stridulate, or rub their wings together, to attract a mate and repel other males. Lampyridae in the beetle order Coleoptera communicate with light.
Humans regard certain insects as pests and attempt to control them using insecticides and a host of other techniques. Some insects damage crops by feeding on sap, leaves or fruits, a few bite humans and livestock, alive and dead, to feed on blood and some are capable of transmitting diseases to humans, pets and livestock. Many other insects are considered ecologically beneficial and a few provide direct economic benefit. Silkworms and bees have been domesticated by humans for the production of silk and honey.

Life cycle

The life cycles of insects vary but most hatch from eggs. Insect growth is constrained by the inelastic exoskeleton and development involves a series of molts. The immature stages can differ from the adults in structure, habit and habitat and can include a passive pupal stage in those groups that undergo complete metamorphosis. Insects that undergo incomplete metamorphosis lack a pupal stage and adults develop through a series of nymphal stages.The higher level relationship of the hexapoda is unclear. Fossilized insects of enormous size have been found from the Paleozoic Era, including giant dragonflies with wingspans of 55 to 70 cm (22–28 in). The most diverse insect groups appear to have coevolved with flowering plants.

Define Insect

•small air-breathing arthropod.
•Insects (from insectum, translation of entomon - threaded) are a class within the arthropods that have a chitinous exoskeleton, a three-part body (head, thorax, and abdomen), three pairs of jointed legs, compound eyes, and two antennae. ...
•An arthropod in the class Insecta, characterized by six legs, up to four wings, and a chitinous exoskeleton; Any small arthropod similar to an insect including spiders, centipedes, millipedes, etc; A contemptible or powerless person
•Startled Insects was a British New Wave instrumental jazz group. Their music was a unique mix of experimental synthesizer textures and ethnic rhythms.
•Insects are an alien race in The History of the Galaxy series of novels by Russian science fiction writer Andrey Livadny.

•A type of arthropod in the biological family of insecta; all insects have six legs.
www.pestworldforkids.org/glossary.html
1. Of or pertaining to an insect or insects. [1913 Webster]

2. Like an insect; small; mean; ephemeral. [1913 Webster]

Insect \In"sect\ ([i^]n"s[e^]kt), n. [F. insecte, L. insectum, fr. insectus, p. p. of insecare to cut in. See Section. The name was originally given to certain small animals, whose bodies appear cut in, or almost divided. Cf. Entomology.]
•An arthropod with six legs and a three sectioned body, a head, thorax and abdomen.
a.Any of numerous usually small arthropod animals of the class Insecta, having an adult stage characterized by three pairs of legs and a body segmented into head, thorax, and abdomen and usually having two pairs of wings. Insects include the flies, crickets, mosquitoes, beetles, butterflies, and bees.
b.Any of various similar arthropod animals, such as spiders, centipedes, or ticks. See Regional Note at lightning bug.

what is an insect

Insects (from Latin insectum, a calque of Greek ἔντομον [éntomon], “cut into sections”) are a class within the arthropods that have a chitinous exoskeleton, a three-part body (head, thorax, and abdomen), three pairs of jointed legs, compound eyes, and two antennae. They are among the most diverse group of animals on the planet and include more than a million described species and represent more than half of all known living organisms.The number of extant species is estimated at between six and ten million,and potentially represent over 90% of the differing metazoan life forms on Earth.Insects may be found in nearly all environments, although only a small number of species occur in the oceans, a habitat dominated by another arthropod group, the crustaceans.