Here is a compilation of essays on ‘Animal Diversity’ for class 10, 11 and 12. Find paragraphs, long and short essays on ‘Animal Diversity’ especially written for school and college students.
Essay on Animal Diversity
Essay Contents:
- Essay on the Introduction to Animal Diversity
- Essay on Phylum Chordata
- Essay on Phylum Mollusca
- Essay on Phylum Nemathelminthes
- Essay on Phylum Arthropoda
1. Essay on the Introduction to Animal Diversity:
The diversity of life is one of the most striking aspects of our planet. Hence knowing how many species inhabit Earth is among the most fundamental yet elusive questions in science. However, the answer to this question remains obscure as efforts to sample the world’s biodiversity to date have been limited.
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Moreover, obtaining an accurate number is constrained by the fact that most species remain to be described and because indirect estimates rely on assumptions that have proven highly controversial. The higher taxonomical classification of species (i.e., the assignment of species to phylum, class, order, family and genus) follows a consistent and predictable pattern, from which the total number of species in any taxonomic group can be estimated.
The total number of species on Earth has been estimated to vary between 3 and 100 million. Recent estimates put the total number of species on Earth at 8.7 million, with 6.5 million on land and 2.2 million (about 25% of total) dwelling in the ocean depths. About 86 per cent of the species on land and 91 per cent of those in the seas have yet to be discovered, described and catalogued.
Among the estimated 8.7 million species, 7.77 million are animals, 298,000 are plants, 611,000 are fungi, 36,400 are protozoa and 27,500 are chromists (including e.g., brown algae, diatoms, water moulds, etc.). Of all the 1.72 million species described, approximately 63861 are vertebrates, 3,07,674 are plants and 1,000,000 (57.86%) are insects.
The identified animal species have been placed in ten phyla. Many of these species are pests of crops, domestic animals, and household goods and materials. In the economy of nature, these species have close association with a number of other animals that act as predators and exercise a check on their population increase. Animal pests belonging to these phyla are further placed under different classes, orders and families.
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Their classification is necessary not only for their identification, but also for the understanding of their feeding and breeding habits, their modes of multiplication, their seasonal activity and hibernation, the duration of their life-cycles and the number of generations in a year. All this information can be further utilized in determining the severity of pest outbreaks and their periodicity.
Practically all the pests and most of their enemies are included in the four phyla, viz., Chordata, Mollusca, Nemathelminthes and Arthropoda.
2. Essay on Phylum Chordata:
The chordates are bilaterally symmetrical animals, having a single dorsal tubular nerve-cord, a notochord and gill slits in the pharynx. However, in the adult stage, any one of the characters may be altered or may disappear. The lower chordates are worm-like creatures and are mostly aquatic. The higher chordates comprise the greatest number and are generally known as vertebrates.
Typically, their body has four regions, viz., head, neck, trunk and tail. The brain is enlarged and is enclosed in the skull or cranium. The cranium also forms a part of the internal axial skeleton which makes the framework and gives support to the body. With only a few exceptions, the sexes are separate. The chordates are divided into various classes and orders.
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The fishes as chordates were formerly included in a single class, Pisces, but are now placed in three separate classes, Cyclostomata, the lampreys and hagfishes; Chondrichthyes, the dogfish and sharks; Osteichthyes, the bony fishes that are mostly found in fresh, brackish or salt water. The animals belonging to all the three classes have elongated bodies and they breathe through gills. Their heart is two chambered and they possess scales on their body.
The class Amphibia includes frogs, toads, salamanders, etc. and they possess two pairs of limbs for walking or swimming. Their heart is three chambered and at some stage of development they possess gills. Depending upon the mode of life of the adults, respiration may be by the gills, lungs, skin, or the mouth-lining. The frogs and toads are very useful to the farmers as they eat a large number of insects harmful to the crops.
The class Reptilia includes lizards, turtles, snakes, crocodiles, etc. The body is covered with dry corniculate skin. The feet and the limbs are either very small or are absent. Respiration is through the lungs and the heart is imperfectly four-chambered. The reptiles are mostly terrestrial but some are aquatic. Lizards are useful, being insectivorous.
Crocodiles, even though prized for their hide, are a great danger to the life of the people near the rivers. The crocodiles belong to the order Crocodilia, which includes the true crocodiles (family Crocodylidae), alligators (family Alligatoridae) and the gavial (family Gavialidae). Together, they are called crocodilians and there are about 20 species in the world.
Crocodiles and alligators are distributed more widely in tropical and semitropical regions. The modern crocodiles are the largest living reptiles. They are what remain of a once abundant group in the Jurassic and Cretaceous periods. Having managed to survive virtually unchanged for about 160 million years, the modern crocodiles face a forbidding and perhaps short future in a world dominated by humans.
Crocodiles have relatively long slender snouts; alligators have short and broader snouts. With their powerful jaws and sharp teeth, they are formidable antagonists. The members of the group that attack humans are found mainly in Africa and Asia.
The estuarine crocodile, Crocodylus porosus Schneider, found in South Asia grows to a great size and is very much feared. It is swift and aggressive and, will eat any bird and mammal it can drag from the shore to water, where the prey is violently torn to pieces. The crocodiles are known to attack animals such as cattle, deer and people.
The class Aves includes birds, such as the crow, sparrow, pigeon, parrot and the domestic fowl. There are approximately 8000 to 25,000 species of birds which are placed in 170 families and 28 orders. In fact, there are 33 orders of birds, but 5 of these are extinct. Tinamiformes is the most primitive order and the Passeriformes is the most advanced.
Their body is covered with feathers, the forelimbs are modified to form wings used in flying, and they have well developed powers of hearing, seeing and producing sounds. They are warm-blooded animals, as their temperature remains constant. The heart is four-chambered and respiration is through lungs.
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The mouth projects into a beak or bill having an external horny sheath. Their legs also have a horny skin. They lay eggs on which they sit to hatch chicks and rear them till they can fend for themselves. Most birds are either useful or harmless, but a number of species are pests of crops or they are carriers of diseases in man and domestic animals.
The class Mammalia includes warm-blooded animals having mammary glands in which milk is secreted for the nourishment of the young ones. Parental care is highly developed in this group. Mammals have four limbs which are used for locomotion and the heart is four-chambered and respiration is by the lungs.
Their senses of smell, vision and hearing are highly developed and they produce characteristic sounds. In feeding habits, they may be herbivorous, carnivorous, insectivorous or omnivorous. There are approximately 4000 species of mammals in the world which are placed in 130 families and 19 orders.
The important orders of this class are:
(1) Primates:
It includes monkey, lemur, chimpanzee, gorilla, orangutan, man, etc.
(2) Carnivora:
It includes cat, tiger, lion and cheetah all of which belong to the family Felidae. In the family Canidae are included the wolf, jackal, fox and dog. Hyaena, civet and mongose are other close relatives and are placed in separate families. The bear belongs to the family Ursidae.
(3) Insectivora:
This order includes the insect-eating mammals, i.e., the shrew, hedgehog, mole, etc.
(4) Chiroptera:
This order includes flying foxes and bats, which are the only mammals that can fly.
(5) Rodentia:
It includes the squirrel, rat, mouse, porcupine, etc.
(6) Lagomorpha:
It includes the hare, rabbit, etc.
(7) Probascides:
It includes the animals having a long trunk, i.e., the elephant.
(8) Ungulata:
There are two suborders, viz., Artiodactyla and Perissodactyla. The former suborder includes even-toed animals, e.g., deer, antelope, sheep, goat, cattle, water buffalo, bison, yak, pig, etc. The latter suborder includes odd-toed animals such as the horse, ass, rhinoceros, etc.
(9) Cetacea:
It includes the marine mammals, namely the whale, dolphin, porpoise, sea cow, etc.
3. Essay on Phylum Mollusca:
The mollusks are quite a large group of animals and are widely distributed. They have an external slimy shell into which they retract at the time of danger or under adverse climatic conditions. A typical mollusk has a distinct head, a segmented soft body and a ventral muscular foot with which it crawls, leaving behind a trail of slime, which it uses as a protection against the rough surface.
Some mollusks are terrestrial whereas others are aquatic. There are a number of classes in this phylum, but two are most important. The class Pelecypoda or the bivalves comprises the clams, mussels and oysters, including the pearl oyster which can be cultured in sea water in protected areas.
The class Gastropoda includes slugs and snails which sometimes damage or disfigure economic and flowering plants. The snails usually have spiral shells into which the viscera is retracted under adverse environmental conditions, or when the animals are at rest. In slugs, the shell is either internal or absent.
4. Essay on Phylum Nemathelminthes:
The nematodes belong to the class Nematoda of the phylum Nemathelminthes. They are minute worm-like animals without a true body cavity. They are unsegmented and are bilaterally symmetrical. The body consists of two tubes, one outer and one inner. The outer is composed of cuticle, hypodermis and nerve muscle cells, acting as the skeleton.
The inner is the digestive tract, divided into a muscular oesophagus (pharynx), mid-intestine and a short rectum. The gonads are composed of a pair of tubes, one of which is often not developed fully. The female genital aperture or vulva opens typically near the centre of the body. The male genital aperture opens along with the anus into a common cloaca and is armed with copulatory spicules.
The nematodes are world-wide and are found in salt or fresh water and in soil. A few are parasites of man and animals. Their size varies from 20 mm to 30 cm in length. Most of the species found in the soil are either saprophytic or predatory, whereas some of them are parasites of plants.
The plant parasites may enter their host (endoparasites) or simply attach themselves to plant tissues and feed from the outside (ectoparasites). The mouthparts of these nematodes are armed with a hollow protrusible stylet or spear which is used for puncturing the plant cells and extracting the sap.
The spear is an important characteristic which is used for identification of various groups of nematodes. In order Tylenchida, it is known as stomatostyle and it develops in the pharynx. In the superfamily Dorylaimordea, it is formed in a cell in the left submedial wall of oesophagus and moves forward to its place in the phayrnx with each moult. This is known as odontostyle.
All plant parasitic forms begin as eggs, deposited by the female in plant tissue or in the soil, singly or in masses. The larvae are formed within the eggs and, on hatching; they go in search of suitable plant tissue, on which to feed. They undergo several moults, increase in size and become adults. In some species, the larva which emerges from the egg is the only stage that can move about and constitutes the infective stage, whereas in others, all stages (except the egg) can move about and infect the host plants.
The important plant-pathogenic genera of nematodes, viz., Meloidogyne, Heterodera, Tylenchulus, Pratylenchus, Anguina, Ditylenchus, etc., belong to the order Tylenchida. This order is characterized by the presence of a head composed of six fused lips, inconspicuous amphids (sensory organs on lips), a well-developed median oesophageal bulb, annulated cuticle and an excretory pore, almost in level with the nerve ring.
The three nematode genera, Xiphinema, Longidorus and Trichodorus belong to the order Dorylaimida. They are characterized by a smooth cuticle, a papillated head and a cylindrical oesophagus with 3, 5 or 7 oesophageal glands. These nematodes are known to be vectors of viruses. They are free-living, migratory and ectoparasitic.
The main characterisitics of the important genera are briefly discussed below:
i. Meloidogyne (Root-knot nematodes):
They show marked sexual dimorphism. Vermiform males have a well-developed oesophagus and no bursa (cuticular extension over the tail), whereas the females are swollen and flask-shaped, with two ovaries and an anus. The eggs are deposited in a mass within the root galls.
The first stage larva which is found inside the egg shell, and the second stage larva which is free, are the infective stages. On entering a young root, the larva establishes itself and begins to feed and grow, hardly showing any movement, except that of the head while feeding. During its development, the larva becomes swollen. It first becomes flask- shaped and after several moults, develops into an adult.
The nematodes produce galls on roots which may range from a string of beads on smaller laterals to a heavily knotted root mass. With the formation of galls, the flow of water and food through the roots is blocked and the plants show symptoms of starvation. These nematodes are known to attack over 1,700 species of plants and are most damaging in warm climates.
ii. Heterodera (Cyst-forming nematodes):
These nematodes resemble Meloidogyne in morphology. In addition, the cuticle of the female Heterodera forms a tough, resistant, brown sphere as in H. rostochiensis Wollenweber, or a pear shaped cyst containing embryonated eggs, as in H. avenae (Filipjev). These cysts may remain dormant in the soil for several years. The second stage larva constitutes the motile infective stage and the subsequent development takes place within the roots of the host plant.
Because of their attack, the roots proliferate into a shallow bushy system. The infested fields frequently have circular or oval patches of plants exhibiting poor growth. These patches increase year after year, if cropping continues.
iii. Tylenchulus:
The citrus nematode, Tylenchulus semipenetrans (Cobb) is an example of this genus. The female nematode is saccate with a well-developed stylet having basal knobs, and with the excretory pore located just anterior to the vulva. It is a partly exposed sedentary endoparasite. The young larva feeds on the outer cortical layer of the young roots.
As the larva destined to be a female develops through successive stages, its head penetrates deep into the rootlet, while the body outside the root enlarges and becomes saccate. At maturity, the female deposits eggs in a jelly-like mass. The males remain small and worm-like and they neither feed nor is their presence necessary for reproduction.
The signs of the affected trees are death of the terminal buds, chlorosis and dying of foliage, early wilting and die-back of twigs. Soil is seen adhering to infested roots at places where the sticky eggs are located.
iv. Pratylenchus (Root-lesion nematodes):
These nematodes are small, vermiform and have an annulated cuticle and a sclerotized lip region. Their styles are well-developed and bear basal knobs and, the median oesophageal bulb is ovate to spheroid. These are the most important nematodes associated with the destruction of plant roots.
They are endoparasites of fibrous roots on which they cause lesions, browning or necrosis. They move from root to root, feeding and laying eggs. The eggs may also be laid in the soil. All the active larval and adult stages are capable of entering the roots.
v. Anguina (Seed-Gall Nematodes):
These are obligate plant parasites, giving rise to galls in leaves, stem and flowers of a number of grasses. A. tritici (Steinback), which causes ear-cockles in wheat, is the best known species in this group. The hard round galls containing nematode larvae replace the seed in the ears. The nematode is characterized by a narrow, flattish head with irregularly shaped oesophageal glands. The ovary consists of cells arranged about a rachis and ending in a cap cell. The adults are slightly swollen and ventrally curved.
The second stage larvae, liberated from the galls in moist soil, infest wheat seedlings at an early stage. For some time, they live ectoparasitically between the young leaves and as the flower primordia develop, the nematodes enter the tissue. They moult successively to reach the adult stage. After pairing, the females lay hundreds of eggs in the galls. The eggs develop into second stage larvae and in this state, the nematode has been observed to resist desiccation for as long as 28 years.
The laboratory examination of the plant roots and the soil is necessary to make sure of the presence of injurious species of nematodes. The Baermann funnel technique with its various modifications is commonly used for nematode extraction, which essentially consists in extracting nematodes in water, from plant or soil samples. The filtrate is passed through folded muslin, making it settle to the bottom of the funnel stem. After some hours or overnight, a small quantity of water containing the nematode is, run off for examination and studied under a microscope.
5. Essay on Phylum Arthropoda:
This phylum includes the largest number of pests of crops and domestic animals. Nemathelminthes is the only other phylum which comes anywhere near it. The arthropods are also the largest group of the animal kingdom. The class Insecta, alone, constitutes a larger number of species than all the other classes and phyla of the animal kingdom.
As the very name implies, the arthropods possess jointed legs, each terminating into a claw, a characteristic not to be found in any other group of invertebrates. Moreover, these animals have a chitinous exoskeleton and their body is divided into segments which are discernible externally, but may be fused internally. Many of the internal organs, such as nerve ganglia, breathing-tubes and sets of muscles are also repeated segmentally.
There are five principal classes of this phylum:
i. Class Chilopoda:
Class Chilopoda includes centipedes or the hundred legged worms. They are flattened worm like creatures, with a distinct head bearing two antennae. They have one pair of legs on every segment of the body.
The first pair, just behind the head, has poison glands whose secretion has the property of paralysing the prey on which it feeds. Some of the larger tropical species which may be 45 cm long may give a painful bite to man. The common species in northern India is Scolopendra morsitans Linnaeus, which is approximately 10-15 cm long and has 21 segments.
ii. Class Diplopoda:
The class Diplopoda includes the millepedes or thousand-legged worms. These are also elongated worm-like creatures, but unlike the centipedes, their body is rounded and the segments are internally fused in twos. The large number of legs gives an erroneous idea that there are perhaps a thousand legs, but in fact, neither they nor the centipedes have the implied number of legs.
In millipedes, the antennae are short and there are no poison claws. The reproductive organs open on the anterior end of the body, close to the head. They generally feed on decaying vegetable matter, but a few of them may attack live plants in moist soil. The common species in northern India is Thyroglutus malayus Attems, and a large number of their colonies are seen above the ground in the monsoon season.
iii. Class Crustacea:
The class Crustacea is represented by a bulk of small zooplankton, such as cyclops, which serves as food for fish. Some of the larger forms include woodlice and other frill bugs and sow bugs which live in moist places and sometimes become pests of greenhouse plants and flowers. The still larger forms include crayfish , shrimps, crabs and lobsters, all of which live in the sea or in fresh water and are a source of food for man. The barnacles are sessile marine forms which get fixed to the bottom of the ships and impede their speed.
The crustaceans have two distinct regions of the body, viz. the cephalothorax (head and thorax fused) and the abdomen. They have two pairs of antennae, and at least five pairs of legs some of which are modified for swimming. They breathe by means of gills, except the small forms which breathe through the body wall. The sexes are separate and the genital openings are generally in pairs.
iv. Class Arachnida:
The class Arachnida includes scorpions, spiders, ticks and mites. Their body also has two distinct regions, cephalothorax and abdomen. They do not have antennae and compound eyes, but go about with the help of simple eyes. They have well-developed palps on the head and possess four pairs of legs on the thorax and use them for walking. They are terrestrial animals and breathe by means of air-tubes or book-lungs.
The sexes are separate and the reproductive organs are near the anterior portion of the abdomen and open through a single orifice. The arachnids are generally small, but some of them are quite large; the smallest is the mite measuring 0.09 mm (eriophyid mite), the largest is the scorpion measuring 160 mm. Some of the species have poison glands and a sting, with which they inject poison and cause severe pain. This class is divided into a number of orders.
The order Scorpiondia includes the true scorpions in which the abdomen is well segmented and there is a tail like prolongation at the end, terminating in a sting or the telson. The scorpions are widely distributed in temperate and tropical climates, and are found in rotten logs, under stones or bricks in deserted buildings. They feed on insects, spiders and other small animals, which are first paralysed with a sting.
The female produces 30-40 young ones, rears them for some time and then leaves them. She may even eat some of them. The yellow scorpion, Palamnaeus sp. is the commonest species in the plains of northern India and other adjoining dry areas. Its sting is very painful because of the neurotoxin it injects, but it is never fatal. The large black or dark brown Buthus sp. is more common in temperate climates. Its poisonous sting may even cause the death of a child.
The order Araneida includes the spiders which are universally predators and abound in the temperate and tropical climates. They are capable of secreting four types of silk through tubes located at the ventral surface of the abdomen. With the help of silken threads or webs, the spiders have the remarkable ability of floating in the air. With wind, they are carried over long distances and hence, many of the genera of spiders are cosmopolitan. They make various types of webs, each one of which is characteristic of the spider group.
The spiders have distinct cephalothorax and abdomen, which are joined by a short stalk. They breathe with the help of book-lungs. The sexes are separate, males being smaller than the females. They are carnivorous and predatory in habit. Courtship is generally exhibited, and, in many cases, the male is eaten up by the female after mating.
The black widow spider of North America is well known for this habit and is also feared, because its bite can be fatal to man. The female spiders lay eggs in silken sacs, which may be attached to some objects or may even be carried on the back. Except for a few species whose bite is painful to man, the spiders, as a group are very useful animals because they destroy a large number of insect pests.
The subclass Acari includes ticks and mites, the animals in which the cephalothorax and the abdomen are broadly joined and the segmentation is not distinct. The body is sac-like and the mouthparts are modified for piercing and sucking. This is the most important subclass of Arachnida, as a large number of them are serious pests of crops and farm animals. The mites and ticks are quite distinct biological groups and the former are generally microscopic, whereas the latter are quite large and have a leathery skin.
Mites have a worldwide distribution; they are rival to insects in their host range and varied habitats. They live in salt and fresh water, in organic debris of all kinds, and on plants and animals. In forests they greatly outnumber all other arthropods. They have been known to cause serious damage to livestock, agricultural crops, ornamental plants and stored products.
Although many mite species are injurious, some are beneficial as they act as predators of phytophagous mites. Many mites that live in soil and water assist in the breakdown and decay of organic matter. Some of them spin silk with which they make webs, whereas others, such as the itch mite, live in subcutaneous tissue in the skin of man and animals.
Most of the species deposit eggs, although some retain them inside till they hatch. A newly emerged larva is six-legged and after feeding and moulting transforms itself into nymph having eight legs. It feeds and moults one or more times and changes into an adult, when it becomes somewhat larger and acquires a genital pore.
Plant feeding mites belong to the families Tetranychidae, Eriophyidae, Tenuipalpidae, Tarsonemidae and Tuckerellidae. Important genera of plant feeding mites in different families are- Bryobia, Eotetranychus, Eutetra-nychus, Oligonychus, Petiobia and Schizotetranychus (Tetranychyidae); Acalitus, Aceria, Cecidophyopsis, Colomerus and Eriophyes (Eriophyidae); Brevipalpus, Cenopalpus and Tenuipalpus (Tenuipalpidae); Polyphagotarsonemus (Tarsonemidae); and Tuckerella (Tuckerellidae).
The red spider mite, Tetranychus cinnabarinus (Boisduval) of vegetables is the typical example of a phytophagous parasite and the mange mite, Sarcoptes scabei (Linnaeus) is a typical example of an animal parasite. Another parasitic mite is Acarapis woodi (Rennie) that lives in the thoracic tracheae of honey bees and causes acarine disease.
The ticks are blood suckers and when in need of a meal, they reach their specific hosts among the mammals, birds and reptiles. They are also important carriers of diseases. Texas fever is transmitted by the cattle tick; rocky mountain fever of man is caused by the wood tick; fowl spirochetosis is carried by the fowl tick.
There are two main types of ticks. The cattle tick, dog tick, and others like them, belonging to the family Ixodidae possess a dorsal shield or scutum and exhibit marked sexual dimorphism, and are commonly known as the hard ticks. The other type, belonging to the family Argasidae, is devoid of scutum and, therefore, it is known as the soft tick. In this group, sexual dimorphism is not marked, as for example, in the fowl tick, Argas persicus (Oken).
The life-cycle of various species of ticks is similar, although some complete it on one host and others on two, three or even many hosts. Consequently, they are known as one-host, two-host, three-host and many-host ticks. The cattle tick belongs to the one-host category, and the fowl tick to the many-host category.
The sheep tick, Ixodes ricinus (Linnaeus) is an example of the three-host species. The female usually lays eggs on the ground in sheltered places. On hatching, the six-legged larva rises on top of a grass blade, a crop plant or a bush and waits for its host to pass by and gets attached to it. Only a few, however, reach their hosts and the rest perish.
After feeding, the larva drops to the ground, moults and transforms into a nymph which, in turn, climbs the nearby plant waiting for the host. The nymph (four pairs of legs) after taking a meal of blood again drops to the ground to moult a second time to become adult.
The female also needs blood before the eggs can mature; therefore, it needs the host for the third time. After feeding and mating, the female drops to the ground where it lays eggs and thereafter dies. The tick is very prolific, laying about one thousand eggs; this biological adaption compensates the scant possibility of its reaching a host.
v. Class Insecta:
The fifth class is Insecta, to which the insects belong. Hexapoda is the original name of this class. The insects are those tracheate arthropods in which the body is typically divided into three parts, namely, head, thorax and abdomen. They possess one pair of compound eyes, one pair of antennae, two pairs of wings and three pairs of legs, and they have their reproductive apertures placed at the end of the abdomen.
A variable number of simple eyes or the ocelli (1-3) are present on the head. The sexes are separate, but in some species males may be rare or absent and reproduction may take place parthenogenetically. The females generally lay eggs and the young that hatch are different from the adults in body form. They grow and change form by a series of moulting, a process known as metamorphosis.
This is the largest group of the animal kingdom. Of all the species described, about 57 per cent are insects. Of the estimated number of species in the world, about 64 per cent are thought to be insects nearly half of all existing insect species feed on living plants. Thus, more than 400,000 phytophagous insect species live on approximately 300,000 vascular plant species.
According to recent estimates, the total number of insect species is considerably larger than was previously thought and may range from 4 to 10 million. Herbivory does not occur to the same extent in all insect groups. The members of some orders of insects are almost exclusively herbivores whereas in other orders herbivory occurs less frequently or is even absent.
The prominent among the herbivores are the Lepidoptera, Hemiptera, Orthoptera and some small orders such as Thysanoptera and Phasmida. A large proportion of the herbivorous insects belong to Coleoptera, Hymenoptera and Diptera, all three of which also include numerous species with predatory and parasitic habits.
Following biological attributes are associated with this predominance:
1. Small Size:
The great majority of insects are quite small, require little food and can easily seek shelter from adverse weather and enemies by entering crevices, hiding under the bark of trees and fallen leaves, and in the undergrowth. Minuteness, however, is not always advantageous and some of the Hymenoptera that are merely 0.2 mm long, if covered with water, cannot free themselves from the film of water owing to the surface tension.
Some of the species may be quite large and measure 120 mm. As their size becomes larger than 20 mm in diameter, the tracheal system of respiration limits the supply of oxygen required, causing them to become sluggish, which is to their disadvantage.
2. Strong Exoskeleton:
Combined with their small size, the chitinous exoskeleton gives the insects much strength without becoming too heavy. It gives them great physical strength and allows numerous modifications in the configuration of the body. In contrast, the body endoskeleton of the vertebrates is too heavy and weak.
3. High Mobility through Flight:
Insects are very mobile creatures and can easily seek food and mates, escape from their enemies, and disperse for fresh colonization. Apart from flying they can also run, jump or take long leaps with great agility and strength. A flea with 0.125 cm long legs can make a high jump of 20 cm and if a man with his one metre long legs were to show the same strength, he would have to jump 150 metres.
4. Efficient Water Conservation:
Insects resist desiccation by various modifications of the exoskeleton and also by efficient water conservation. They can retain metabolic water in the body and avoid liquid excretion, and void crystalline uric acid instead. Excessive transpiration from the body of an insect takes place only when the waxy coating on the outside melts at high temperature.
5. Rapid Reproduction:
Most of the insects feed on plants, weeds, animals, etc., which have a short seasonal growth. They are adapted for completing the life-cycle within that period and produce progeny prolifically.
For example, it has been estimated that if all the progeny of a single house fly female were to survive, within half a year she would produce enough flies to form a layer 15 metres thick all over the surface of the earth. Likewise, the female of an aphid in one year would produce a biomass equal to the entire population of India. However, in nature there are many adversities that check insects from multiplying to such an extent.
6. Adaptability:
No other group of animals is adapted to live in so diverse habitat as the insects. They live in all corners of the earth from the poles to the equator, in soil, in fresh water, in hot springs, on all sorts of plants and animals (dead or alive), in pools of crude petroleum, in argol, opium, pepper or strychnine.
Insects have no mechanism for regulating their body temperature and whenever the environmental temperature becomes too hot or too cold, they hibernate and become temporarily inactive.
Many of the species that are subjected to extremes of temperature or drought have the physiological adaptation of diapause in which their growth is arrested and the metabolic activity is reduced to the bare minimum and tolerance to the extremes of weather is increased manifold.
The most remarkable feature of diapause is that its initiation in many cases is triggered off by the day-length which naturally is precise to the day of the calendar. When the physiological changes required for the termination of diapause and the secretion of the ‘growth hormone’ have taken place, growth is resumed.
Quite often, the photoperiod provides the required stimulus for the termination of diapause. Thus, the life-cycle of the species synchronizes with the seasons and the availability of food.