प्रारम्भिक कीट विज्ञान/१
नेपाली विकिपुस्तकबाट, स्वतन्त्र पुस्तकालय
FACTORS FOR INSECTS ABUNDANCE[सम्पादन]
Measures of dominance[सम्पादन]
- number of species:
- more than eighty per cent of the species belongs to insect group।
- Large number of individuals in a single species:
- e.g., Locust swarm comprises 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.
Reasons for dominance[सम्पादन]
- 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.
- To seek food, mate, shelter and oviposition sites
- To colonize in a new habitat and also to exchange habitat.
- To escape from enemies and unfavourable conditions.
- To migrate (i.e. for long distance travel e.g. Locusts)
- 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 -50C to 40C.
- Psilopa petroli 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.
- Size: Majority of insects are small conferring the following physiological and ecological advantages.
- Exploitation of numerous ecological niches inaccessible for other animals.
- 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.
- 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
- Act as external armour
- Provides space for muscle attachment
- Prevents water loss
- Resistance to desiccation: Insects minimise the water loss from their body surface through the following processes.
- Prevention of water loss:
- Lipids and polyphenols present in the Epicuticle acts as water proofing.
- Was 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 desication of inner embryos.
- 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.
- 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.
- Reproductive potential: Reproductive potential of insect is high due to the following reasons:
- Egg laying capacity (fecundity) is high. e.g., Queen termite lays 6000 - 7000 eggs per day for 15 long years.
- 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.
- Careful selection of egg lying sites and protection of eggs.
- Exhibits parental care like progressive provisioning (e.g. bees) and mass provisioning (e.g. Wasps)
- 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.
- 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.
- Defense mechanisms: By using the following defense 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
- . Hexapod locomotion: Insects uses 3 legs at a time during locomotion, while the remaining 3 legs are static, which gives greater stability