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Insects, Characteristics of insects, Dominance of Insects – Factors responsible for dominance – Agricultural Free Online Studymaterial


  • Entomology is the scientific study of insects.
  • The word ‘Entomology’ is derived from two Greek words ‘Entomon’ meaning insects and ‘logos’ meaning  study 
  • It is a branch of zoology dealing with Origin & evolution of insects and their diversity & classification, body organisation and function, development, interaction with surroundings in which they live, past history and their economic importance.


  • Insect the word is derived from latin insectum meaning “cut into section
  •  insects are the largest group of invertebrate within the arthopoda phylum that have a chitinous exoskeleton a three part body ( head ,thorax and abdomen) three pairs of jointed legs, compound eyes and one pair of antennae. 
  • They are among the most diverse groups of animals on the planet, including more than a million described species and representing more than half of all know living organism.

Characters of Class: Insecta

  • Bilaterally symmetrical (இருதரப்பினரிடையேசமச்சீர்)
  • Body covered by chitinous exoskeleton which is periodically shed and renewed.
  • Body divisible into Head, Thorax and Abdomen
  • One pair of antenna, two pairs of wings and three pairs of legs 
  • Appendages are paired and segmented
  • Tubular alimentary canal
  • Open circulatory system (body cavity is blood cavity called haemocoel)
  • Excretion is by tubes (Malpighian tubules) 
  • Respiration by trachea or gills
  • Decentralised,ventral nervous system


  • Insects are the most successful life forms on earth since 500 million years.
  • It is estimated to be over one million insect species in earth Lives in every nook and corner, every crack and crevice of the earth much better than man kinds and still as thirty million insect species are to be identified.
  • Over a period of several hundred million years, insects have acquired many distinctive structural, developmental, physiological and behavioural perfection, which enabled them to occupy this dominant position.

The reasons for such dominance are

  1. Presence of exoskeleton:


    Insect body is covered with an outer cuticle called exoskeleton which is made up of a cuticular protein called Chitin. This is light in weight and gives strength, rigidity and flexibility to the insect body. The uses of exoskeleton

1)Covers the body         

2)Acts as point of attachment for muscles

3)Protects insects from enemies (predators and parasite)

4)Prevents water loss from the body

a) Prevention of water loss:

  • Lipids and polyphenols present in the Epicuticle act as water proofing. 
  • Wax layer with closely packed wax molecules prevents escape of water.
  • Spiracles are closed to prevent water loss.
  • In the egg stage shell development prevents water loss and desiccation of inner embryos.
  1. Conservation of water
  • Capable of utilizing metabolic water
  • Rectal resorption of water from faeces 
  • Terrestrial insects use less quantity of water to remove the nitrogenous waste (Uric acid) which is water insoluble. 
  1. Small size
  • Insects are small in size than other higher animals.
  • Reduce competition for food and shelter Eg. Fairy flies measures 0.21 mm in length (smallest insects in world)
  • Small size enables the presence of higher number in an unit area.
  • Higher number individuals means chance for development of new species
  • Less space, food, time and energy requirements for development and sustaining life.  
  • Energy Utilization maximum. 
  • Less gravitational effect. 
  • Muscular action and tracheal respiration more effective. 
  • Easy escape from enemies

Fairy flies

  1. Diversity in food habits and mouthparts
  • Some insects are carnivorous, saprophagous, and omnivorous while majority feeds on plants.
  • Even among plant feeders, some feed on leaves, others feed on plants sap etc.,
  • Insects having different types mouth parts to chew ,lap, suck, scrap and mine the food
  • This diversification in food habits helps to reduce competition

Dung roller  


          Tiger beetle    

            Blowflies     (Adult& maggot)

  1. Diversity in reproduction
  • Insects reproduce sexually and asexually Eg. Aphids
  • Presence of special types of reproduction
  • Polyembryony – development of many individual from a single eggs eg. Parasitic wasps
  • Parthenogenesis – reproduction without male or without fertilization eg aphids
  • Paedogenesis–reproduction by immature stages eg gall midges
  • Controlled reproduction eg honeybees & termites
  1. Higher Fecundity(Egg laying capacity of a female)
  • Possess higher reproductive rate than other organisms Eg Butterflies lay 300 eggs / female, Queen honeybee lays 2000 – 3000 eggs/day, Queen termite lays 6000 – 7000 eggs per day for 15 long years (60 eggs/ seconds)
  • Presence of polyembryony Eg.1000 individuals from one egg in endoparasitic wasp.
  • Pomace flies (drosophila) are well known for their higer fecundity and shoter life cycle(female lays – 100 eggs sex ratio 50:50,  if undisturbed 25generation, about 1.192×109flies produced

        Queen termite                            Queen Honeybee                 Pomace flies

(6)Diversity in life stages

  • Insects having different life stages and of each stage this prefer different niches hence food requirement is differ in reducing competition. Eg. Caterpillar and butterfly.
  • Insect undergoing dormant stages like egg & pupa during its lifecycle hence it enable to increase their survival rate.

More than 82 per cent of insects undergo complete metamorphosis

7) Higher mobility and stability

1) Presence of three pairs of legs and two pairs of wings make insects to move faster.

2) While moving, three legs operates like Tripod and three other legs work for stability this kind of locomotion is called “HEXAPOD LOCOMOTION”

(8)Diverse appendages

1) Insects possess legs, wings, compound eyes, simple eyes, which are put to proper use

2) In winged vertebrates one pair limbs is modified as wings but in insects two pair of wings in addition to the legs

(9)Decentralized nervous system

1) Ensures quick reflux action under emergency Eg. A gravid female abdomen will lay eggs.

(10) Diversity in respiratory system

1) Opening spiracle are attached to tracheae and divided into tracheoles

2) Respiratory and circulatory systems are independent and are efficient than higher animals.

(11)Capacity to withstand adverse conditions

1) Insects withstand extremes of cold, hot conditions because they are cold blooded

2) Some midges even live under dehydrated conditions.

(12)Adaptability to varied environment

1) Distributed throughout the world from artic, Antarctic, Himalayan peaks, hot springs, salt lakes, oceans, petroleum pools etc.,

Saharan ant – Sahara desert

Psilopa petroli – found in crude petroleum well

Ephydrid fly – living in great salt lake.

Short circuit beetle – founded in electric cable

2) Some insects mimic or camouflage the environment

Saharan ant                                                
  Psilopa petrol                                    
Short circuit beetle


       Stick insects or inchworms          Dry twigs Grass hopper Leaf blade Treehopper thorns

(13) Zenith of evolution                       

1) Insects came into being 500 million year ago man appeared only 1 million year ago.

2) Insect had enough time & opportunity to adjust with nature and perfected themselves during long evolution.

3) Hence insects are the more dominant organisms than other animals

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