🧚🏻 Insect Dominance

Measures of dominance

• More number of species: In the animal kingdom more than 75 per cent of the species belongs to insect group. Total number of insects described so far is more than 9-15 lakhs. The most diverse order of insects is Coleoptera followed by Lepidoptera, Hymenoptera and Diptera. The structural, developmental, and protective characters acquired by insects made them dominant in the animal kingdom.

• Large number of individuals in a single species: e.g., Locust swarm comprising of 10⁹ number of individuals, occupying large area.
• Great variety of habitats: Insects thrive well under varied conditions.
• 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.

Adaptability or Universality

• Insects are the earliest groups to make their life on the earth and to occupy vast habitats of soil and water.
• Found in wide range of climatic conditions, from -50°C to 40°C.
• found in crude petroleum well.
• Ephydra fly living in great Salt Lake.
• Every flowering plant providing food for one or many Phytophagous insects.
• Even the decomposing materials serving as food for many Saprophagous insects.
• Many Carnivorous insects are parasitic on other animals and insects.

Factors responsible for abundance of insects

I. Structural perfections

Exoskeleton

• The insect body has an outer exoskeleton or body wall made up of cuticular protein called as chitin. This is light in weight and gives strength, rigidity and flexibility to the insect body.
• It is responsible for protection from:
  • Desiccation or water loss from the body
  • Physical or mechanical injuries and to maintain shape and size of the body
  • Providing area for muscle attachment
  • Giving strength to the body appendages

Resistance to desiccation

• Insects minimise the water loss from their body surface through the following processes. **I. Prevention of water loss: **

1. Lipids and polyphenols present in the Epicuticle acts as water proofing.
2. Wax layer with closely packed wax molecules prevents escape of water.
3. Spiracles are closed to prevent water loss.
4. In the egg stage shell development prevents water loss and desiccation of inner embryos. **II. Conservation of water **
5. Capable of utilizing metabolic water
6. Rectal resorption of water from faeces.
7. Terrestrial insects use less quantity of water to remove the nitrogenous waste (Uric acid) which is water insoluble.

Small size

• Majority of insects are small conferring the following physiological and ecological advantages.

1. It helps the insects to exploit different ecological niches inaccessible for other animals.
2. Less space (for shelter), food, time and energy requirements for development and sustaining life.
3. Energy Utilization maximum.
4. Less gravitational effect.
5. Muscular action and tracheal respiration more effective.
6. Easy escape from enemies.

Quicker speciation

• Because of hard exoskeleton, smaller size and short life cycle there is a chance of quicker species formation (more number of species at a faster rate).
• Changes that occur during the process of evolution through variation in their habitat or habits will be maintained or continued to several generations resulting in the development of more species from a genus.

Functional wings

• Insects posess wings, which is the lateral extension of exoskeleton.
• Insects are the earliest animals and the only flying invertebrates.
• Two pairs of wings that are present on meso and meta thoracic segments are mainly helpful for taking flight from one place to another for following purposes:

1. To seek food, mate, shelter and oviposition sites.
2. To colonize in a new habitat and also to exchange habitat.
3. To escape from enemies and unfavourable conditions.
4. To migrate (i.e. for long distance travel e.g. Locusts)

Hexapod locomotion

• Because of the presence of six legs on the three thoracic segments, though a pair of legs are lost, the insect will have equilibrium during all the phases of its locomotion.
• Insects uses 3 legs at a time during locomotion, while the remaining 3 legs are static, which gives greater stability.

Compound eyes

• Most of the adult insects and nymphs consists of compound eyes as visual organs which possess number of hexagonal units known as ommatidia, corresponding to the cornea of an individual eye or lens.
• Because of presence of number of ommatidia in the compound eyes, even if some or few ommatidia get damaged, the insect does not lose the power of vision.

Scattered sense organs

• The sense organs viz., visual organs, gustatory organs, organs of touch etc. are distributed on different parts of the body such as antennae, eyes, mouth parts in the head, legs with claws on thorax, tympanum, cerci in the abdomen etc.
• This scatteredness on all parts of the body prevents the chance of all being damaged.

Decentralized nervous system

• The nervous system is so decentralized that insects can be artificially stimulated to walk, fly, feed, mate or oviposit even if some parts of the body parts are removed or damaged.

Tracheal system of respiration

• Insects respire by means of thin elastic air tubes known as trachea which open outside, on the body surface through spiracles.
• This ensures direct transfer of adequate oxygen to actively breathing tissues.
• Spiracles through their closing mechanism admit air and restrict water loss.
• Presence of these trachea allows free supply of oxygen to the insect and make it to be an efficient terrestrial or aerial arthropod.

Enteronephric excretion

• In insects, excretion is mainly by means of malpighian tubules which open in between midgut and hindgut.
• This arrangement is well suited for water conservation as well as for the absorption of unwanted waste metabolites at a quicker rate.

II. Developmental characters

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

1. High fecundity: Fecundity is defined as the egg laying capacity of a female insects. It helps to increase the population at faster rate. e.g., Queen termite lays 6000 - 7000 eggs per day for 15 long years.
2. Method of reproduction: Insects can reproduce both sexually as well as parthenogenetically. This parthenogenetic reproduction coupled with high fecundity help insects to increase their populations to large numbers, when all the biotic and abiotic factors are favourable.
3. 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.
4. Controlled reproduction: Though insects possess high fecundity, there is also high degree of control over reproduction by reducing the number of females that can lay eggs. This character is mostly seen in social insects such as honeybees and termites.
5. Short life cycle: 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 favours greater genetic changes in the insect population, like quicker development of insecticide resistant strains.
6. Exhibits parental care like progressive provisioning (e.g. bees) and mass provisioning (e.g. Wasps)
7. Careful selection of egg lying sites and protection of eggs.
8. Specificity of food: There is diversity in food habits among different species of insects. As they differ in their preference for particular type of food, there will not be any competition among themselves. Less competition for food increases their chances of survival and further multiplication.
9. Zenith of evolution: During the process of evolution, insects have shown a high degree of specialization to the extent that there is division of labour, polymorphism etc., that make them to be efficient in their struggle for existence.
10. Complete metamorphosis: More than 82 per cent of insects undergo complete metamorphosis (Holometabolous insects) with the following four stages.
    • Egg: Inactive, inexpensive, inconspicuous and embryo develops inside.
    • Larva: Active, feeds, digests, grows and store food.
    • Pupa: Inactive, internal reorganisation and resist adverse conditions.
    • Adult: Active, reproduce and disperse
    • As the larval and adult food sources are different, competition for food is less.

III. Protective adaptations and devices

For protecting themselves from adverse environmental conditions or natural enemies, insects have developed or attained some adaptations including:

1. Morphological adaptations: The body colour and shape of some insects make them look like part of the plant, thereby protecting themselves from natural enemies. Eg: Stick insects and leaf insects.
2. Physiological adaptations:
   • Some insects produce or release poisonous or unpleasant odours from their body or possess warning coloration by imitating certain distasteful insects. Eg: Stink bugs have specialized exocrine glands located in the thorax or abdomen that produce foul smelling hydrocarbons.
   • Larvae of swallow tail butterflies have eversible glands called osmeteria, located just behind the head when disturbed they release repellent volatile and waves their body back and forth to ward of intruders.
   • Some blister beetles (Meloidae) produce cantharidin, a strong irritant and blistering agent.
3. Behavioral adaptations: It is a defence strategy adopted by some insects through feigning death or imitating the voice of dangerous insects or mimicry. By using the following defence mechanisms, insects escape from the enemies to increase their survival rate.
   • Behavioural: Thanatosis - insects pretends as if dead. e.g. some beetles.
   • Structural: e.g. hardened forewings of beetles known as elytra protect the beetles from predation of birds.
   • Colourational: Presence of protective colours. e.g. Stick insects.
   • Chemical: Presence of defensive chemicals. e.g. Bees producing venom.
4. Construction of protective structures: Some insects construct shelter with the available plant material for protecting themselves from adverse conditions, natural enemies and to store food material for use during the period of scarcity. Eg:
   • Cases / Bags in case of case worms/bag worms
   • Termatoria in case of termites,
   • Honeycomb in case of honeybees

- Insecta - Introduction: K.N. Ragumoorithi, V. Balasurbramani & N. Natarajan
- A General Textbook of Entomology (9th edition, 1960) – A.D. Imms (Revised by Professor O.W. Richards and R.G. Davies). Butler & Tanner Ltd., Frome and London.
- The Insects- Structure and Function (4th Edition, 1998) – R.F. Chapman. Cambridge University Press
- https://www.amentsoc.org/
- Researchgate
- Wikipedia