Major points related to dominance of Insecta in Animal kingdom

Insecta in Animal kingdom: Major points related to dominance of Insecta in Animal kingdom -Entomology 

Measures of dominance

1.        More number of species: In the animal kingdom more than 85 per cent of the species belongs to insect group. Total number of insects described so far is more than 9 lakhs.

2. Large number of individuals in a single species:

e.g., Locust swarm comprising of 109 number of individuals, occupying large area.

3. Great variety of habitats: Insects thrive well under varied conditions.

3. Long geological history: Insects were known to occupy this earth for more than 350 million years, which is a good track record. This has given the insects great variety of adoptions under different conditions.

Reasons for dominance

1.    Capacity  for flight:  Insects  posess wings, which is the lateral extension  of exoskeleton.

Insects are the earliest animals and the only flying invertebrates. Flight is used for the following purpose. 

i.  To seek food, mate, shelter and oviposition sites

ii. To colonize in a new habitat and also to exchange habitat.

iii.  To escape from enemies and unfavorable conditions.

iv. To migrate (i.e. for long distance travel e.g. Locusts)

2. Adaptability or Universality: Insects are the earliest groups to make their life on the earth and to occupy vast habitats of soil and water.

i. Found in wide range of climatic conditions, from -50°C to 40°C.

ii. Psilopa petroli found in crude petroleum well.

iii. Ephydra fly living in great salt lake.

iv. Every flowering plant providing food for one or many Phytophagous insects.

v. Even the decomposing materials serving as food for many Saprophagous insects.

vi. Many Carnivorous insects are parasitic on other animals and insects.

3. Size: Majority of insects are small conferring the following physiological and ecological advantages.

i. Exploitation of numerous ecological niches inaccessible for other animals.

ii. Less space, food, time and energy requirements for development and sustaining life.

iii. Energy Utilization maximum.

iv. Less gravitational effect.

v. Muscular action and tracheal respiration more effective.

vi. Easy escape from enemies.

4. 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.

EXOSKELETON CROSS SECTION

Uses:  

i. Act as external armor

ii. Provides space for muscle attachment 

iii. Prevents water loss

5. Resistance to desiccation: Insects minimize the water loss from their body surface through the following processes.

I. Prevention of water loss:

i.   Lipids and polyphenols present in the Epicuticle acts as water proofing.

ii.  Rectal resorption of water from  faces.

iii. Terrestrial insects use less quantity of water to remove the nitrogenous waste (Uric acid) which is water insoluble.

6. Tracheal system of respiration: This ensures direct transfer of adequate oxygen to actively breathing tissues. Spiracles through their closing mechanism admit air and restrict water loss.

7. Reproductive potential: Reproductive potential of insect is high due to the following reasons:

i. Egg laying capacity (fecundity) is high. e.g., Queen termite lays 6000 - 7000 eggs per day for 15 long years.

ii. ii.   Development period is short. e.g., Corn aphid produces 16 nymphs per female which reaches the adulthood within 16 days. There by one generation is completed within a short period of 16 days, which favors greater genetic changes in the insect population, like quicker development of insecticide resistant strains.

iii. Careful selection of egg lying sites and protection of eggs.

vi.    Exhibits parental care like progressive provisioning (e.g. bees) and mass provisioning (e.g.

Wasps)

v. Presence of special types of reproduction other than oviparity and viviparity.

* Polyembryony: Development of many individuals from a single egg. e.g. parasitic wasps.

* Parthenogenesis: Reproduction without male or without fertilization, e.g. aphids.

* Paedogenesis: Reproduction by immature stages. e.g. certain flies.

8. Complete metamorphosis: More than 82 per cent of insects undergo complete metamorphosis (Holometabolous insects) with the following four stages.

i. Egg: Inactive, inexpensive, inconspicuous and embryo develops inside.

ii. Larva: Active, feeds, digests, grows and store food.

iii. Pupa: Inactive, internal reorganisation and resist adverse conditions.

iv. Adult: Active, reproduce and disperse.

As the larval and adult food sources are different, competition for food is less.

9. Defense mechanisms: By using the following defense mechanisms, insects escape from the enemies to increase their survival rate.

i. Behavioural: Thanatosis - insects pretends as if dead. e.g. some beetles.

ii. Structural e.g. hardened forewings of beetles known as elytra protect the beetles from predation of birds.

iii. Colourational: Presence of protective colours. e.g.Stick insects.

iv. Chemical: Presence of defensive chemicals. e.g. Bees producing venom.

10. Hexapod locomotion: Insects uses 3 legs at a time during locomotion, while the remaining 3 legs are static, which gives greater stability.

CLASSIFICATION OF CLASS INSECTA UPTO ORDERS

Insect is a six legged arthropod. Taxonomist A.D. Imms proposed a classification of insect.

Phylum : Arthropoda (with several classes)
Class: Insecta (Hexapoda)

Characters of class Insecta

1. Body is divided into three regions
2. In head a pair of antenna and a pair of compound eyes are usually present.
3. Thorax is the centre of locomotion with, 3 pairs of five jointed legs and two pairs of wings.
4. Excretion is mainly through malpighian tubules.
5. Tracheal system of respiration well developed.
6. Brain is divided into protocerebrum, deutocerebrum and tritocerebrum.

The class Insecta has two subclasses viz., Apterygota and Pterygota. 

	Apterygota				Pterygota
1.	Primarily wingless-evolved from wingless ancestors				Winged or secondarily wingless- evolved from winged ancestors. e.g. Flea, head louse, bed bug.
2.	Metamorphosis is totally absent or slight.				Present.
3.	Mandibular articulation monocondylic i.e., single	in	head	is	Dicondylic i.e., double.
4.	Pleural sulcus in thorax is absent.				Present.
5.	Pregenital abdominal appen- dages present.				Absent.

The subclass Apterygota has 4 orders namely
1. Thysanura - Silverfish (Thysan-fringed, Ura-tail)
2. Collembola- Springtail or snowflea (coll-glue; embol-peg)
3. Protura - Proturans or Telsontail (Pro-first, Ura-tail)
4. Diplura - Diplurans or Japygids (Di-two; Ura-tail)

The sub-class Pterygota has two division, namely Exopterygota and Endopterygota based on the wing development.

	Character	Exopterygota	Endopterygota
1.	Wing development	External	Internal
2.	Type of metamorphosis	Incomplete(Hemimetabola) or gradual (Pau- rametabola)	Complete (Holome- tabola)
3.	Pupal stage	Absent	Present
4.	Immature stage	Naiad or Nymph	Larva
5.	No. of orders	16	9

The class Insecta has 29 orders (4 in Apterygota and 25 in Pterygota)

EXOPTERYGOTA  

Group I. Paleopteran orders (1,2)

1. Ephemeroptera - Mayflies 
2. Odonata - Dragonfly, Damselfly

Group II. Orthopteroid orders(3-11)
3.  Plecoptera - Stonefly 
4.  Grlloblatodia - Rock crawlers
5.  Orthoptera-Grasshopper, locust, cricket, mole cricket
6.  Phasmida-stick insect, leaf insect
7.  Dermaptera-Earwigs
8.  Embioptera-Webspinners/Embids
9.  Dictyoptera-cockroach, preying mantis
10. Isoptera - Termites
11. Zoraptera - Zorapterans

Group III. Hemipteroid orders(12-16)
12. Psocoptera - Book lice 

13. Mallophaga - Bird lice
14. Siphonculata - Head and body louse
15. Hemiptera - Bugs
16. Thysanoptera - Thrips

ENDOPTERYGOTA

Group IV. Panorpoid complex (1-9)
1. Neuroptera-Antilions, aphidlion, owl flies, mantispid flies.
2. Mecoptera - Scorpionflies. 
3. Lepidoptera - Butterflies and moths.
4. Trichoptera - Caddisfly.
5. Diptera - True fly.
6. Siphonaptera - Fleas.
7. Hymenoptera - Bees, wasps, ants.
8. Coleoptera - Beetles and weevils.
9. Strepsiptera - Stylopids.