Insects, Characteristics of insects, Dominance of Insects – Factors responsible for dominance
INSECT DEFINITION:
• 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.
• 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.
FACTORS RESPONSIBLE FOR INSECT DOMINANCE
• 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 reason for such dominance are :-
1. Presence of Exoskeleton
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| Exoskeleton cross section |
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.
2. 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
• 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
Cockroaches
5. 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.
Honeybee
6. Higher Mobility and Stability
• Presence of three pairs of legs and two pairs of wings make insects to move faster.
• While moving, three legs operates like Tripod and three other legs work for stability this kind of locomotion is called “HEXAPOD LOCOMOTION”
7. Diverse appendeges
8. Decentelised Nervous system
9. Capacity to withstand adverse conditions
10. Zenith of Evolution
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