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    ملخص لمادة مقدمة حشرات ... بيولوجي

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    ملخص لمادة مقدمة حشرات ... بيولوجي

    مُساهمة  Zoologist في الخميس فبراير 25, 2010 4:05 am

    Faculty of Science, Benha University.
    Dominance of Insects
    1. Number of Species
    • Over 800,000 insect species have been named and described.
    • The entire plant kingdom, contains between 400,000 and 500,000 species.
    • The "lower" invertebrates account for around 200,000 species, while vertebrates total only about 50,000 species.
    • Insects represent more than half (about 53%) of the 1.5 million species of living organisms known to science.
    • Discovery of a new species of vertebrate is a rare. Entomologists describe hundreds of these new species each year.
    • Two of every three living species may be insects.
    2. Number of individuals
    • A swarm of migratory locusts may contain up to 10 billion individuals, and have a total biomass of over 30,000 metric tons.
    • Some colonies of ants which live in a square mile may contain over a million queens and 300 million workers.
    • If insects were divided equally among each one of the earth's 6 billion human inhabitants, each of us would be allotted 1 x 1018 insects
    3. Distribution
    • Insects live everywhere from mountain tops in the Himalayas to tide pools at the seashore.
    • Some insects spend their entire lives in underground caves.
    • At least 40 species live on the polar ice in caps of Antarctica.
    • Some insects can survive in hot springs (50oC); others inhabit deserts where water is scarce and daytime temperatures exceed 60oC.
    • Insects live above and below the surface of water. One species of water strider lives nearly 2400 kilometers from the nearest land mass.
    • The petroleum fly, develops in pools of crude oil in the California oil fields.
    • The only place where insects are not plentiful is in the ocean depths.


    Reasons for Success
    The insects do have a unique combination of characteristics which, as a whole, have given them an unusual survival advantage.
    Exoskeleton
    • Insect's skeleton is located on the outside of its body.
    • This exoskeleton gives shape and support to the body's soft tissues, provides protection from attack or injury, minimizes the loss of body fluids in both arid and freshwater environments, and assures mechanical advantage to muscles for strength and agility in movement.
    • The exoskeleton can resist both physical and chemical attack.
    • It is covered by a layer of wax that prevents desiccation.
    • It may be as elastic as rubber or as hard and rigid as some metals.
    • Freedom of movement is ensured by membranes and joints in the exoskeleton.
    • Muscles that attach directly to the body wall have maximum strength and optimum mechanical advantage (leverage). An ant, for example, can lift up to 50 times its own body weight.
    2. Small Size
    • Most insect species are between 2 and 20 mm in length.
    • The smallest insect is a parasitic wasp of other insects' eggs (0.14 mm in length), nearly 1/3 smaller than some single-celled protozoa.
    • Small size is a big advantage to insects due to the minimal resources needed for survival and reproduction, and allows them to avoid predation.
    • Small size, together with adaptations in body shape and coloration, gives many species the ability to become virtually undetectable.
    3. Flight
    • Insects are the only invertebrates that can fly.
    • Flight help insects to escape from predators, transport easily from place to another and expand quickly into new habitats for new resources.
    • Efficient use of energy allows some insects to travel great distances or remain airborne for long periods of time.
    • More than 200 species migrate over long distances by air.
    • The migratory locust, can fly for up to 9 hours without stopping. Large swarms occasionally traverse the Mediterranean Sea.
    • The wings of a large insect can generate a considerable amount of lift. Green darner dragonflies are able to fly while carrying a load up to 15 times their body weight. Some of the tiny biting midges beat their wings over 1000 times per second.
    4. Reproductive Potential
    • In insect populations, females often produce large numbers of eggs (high fecundity), most of the eggs hatch (high fertility), and the life cycle is relatively short (often as little as 2-4 weeks). Together, these three characteristics enable insects to produce remarkably large numbers of offspring.
    • If we ignore all the limiting factors, in two years of continuous reproduction, a pair of some fly species would fill a ball nearly 8 million kilometers in diameter. This helps explain sudden pest outbreaks.
    • Many adaptations help maximize this reproduction potential. Most females, can store sperm for months or years within the spermatheca, a single mating can supply a female with enough sperm to fertilize all the eggs she will produce in her lifetime.
    • There are many insect species that reproduce asexually.
    5. Metamorphosis
    • Most insects undergo significant developmental changes as they grow from immatures to adults. These changes known as metamorphosis, may involve physical, biochemical, and/or behavioral alterations that promote survival, dispersal, and reproduction of the species.
    • In primitive insects, most of these changes occur gradually, does not include all body tissues (incomplete metamorphosis) the immatures and adults share many characteristics, they often live in similar habitats and feed on similar types of food.
    • More advanced insects, undergo complete metamorphosis -- a dramatic transformation in form and function between the immature and adult stages. larva molts into a transitional stage, called the pupa, and begins a period of massive internal and external reorganization.
    • Only 9 out of 28 orders undergo complete metamorphosis. In some cases, immatures and adults may consume different types of food, exploit different environmental resources, and even occupy different habitats.
    6. Adaptability
    • A combination of large and diverse populations, high reproductive potential, and relatively short life cycles, has equipped most insects with the genetic resources to adapt quickly in the face of a changing environment. Insects were among the first creatures to invade the arid environment, they were the first animals to use flight as an escape from predators, and they were the first organisms to develop a complex social life with division of labor and cooperative care of the young.
    • The most remarkable example of insect adaptation in this century has been the speed with which pest populations have developed resistance to a broad range of chemical and biological insecticides.

    Impact of Insects
    • Insects have a direct impact on agricultural food production by chewing the leaves of crop plants, sucking out plant juices, boring within the roots, stems or leaves, and spreading plant pathogens.
    • Insects feed on natural fibers, destroy wooden building materials, ruin stored grain, and accelerate the process of decay.
    • Insects have a profound impact on the health of humans and domestic animals by causing annoyance, inflicting bites and stings, and transmitting disease.
    • The economic impact of insects is measured not only by the market value of products they destroy and the cost of damage they inflict but also by the money and resources expended on prevention and control of pest outbreaks.
    • Economists generally agree that insects destroy around 10% of gross national product in large, industrialized nations and up to 25% of gross national product in some developing countries.
    Insects as Food
    • Insects represent an important food source for a wide variety of other animal species.
    • Insects were an important source of nutrition for our early human ancestors. Today, they are still collected and eaten by people of many cultures.
    Ecological Impact
    • Biogeochemical cycling of nutrients
    As consumers, scavengers, and decomposers, insects play a vital role in the biogeochemical cycling of nutrients. Insects help aerate the soil, improve its retention of rainwater, and enhance its fertility. Flies and dung beetles prevent the build-up of manure from large animals and speed up its decomposition by fungi and bacteria.
    • Parasites and predators
    As parasites and predators of other organisms, insects are part of a natural system of checks and balances that strengthens community stability and prevents explosive population growth from overrunning natural resources. So far, over 6000 insect species have been tested and released as biological control agents to fight insects and weeds that we regard as pests.
    • Pollinators of flowering plants
    Insects play an essential role as pollinators of flowering plants (angiosperms). Various flower types are adapted for luring different groups of insects -- not only bees and butterflies but also many species of wasps, moths, beetles, and flies.
    In the United States, beekeepers provide pollination services which cost growers about 50 million dollars annually, but they produce a net gain of over nine billion dollars.
    • Honey & Beeswax
    Since ancient times, honey bees (Apis mellifera) have been valued for the honey and beeswax they produce. Many doctors applied honey as an antiseptic to treat burns and lacerations. The term Apitherapy is familiar in many countries against many diseases.
    Beeswax is still used commercially in the manufacture of many cosmetics, candles, furniture waxes, and medicinal ointments.
    Bee pollen is a "complete" dietary supplement. Royal jelly is a glandular secretion that nurse bees feed to larvae of future queens. It is rich in vitamins and proteins. It has become a major ingredient in some expensive skin care products that promise to reduce wrinkles and retard aging.
    • Silk
    A silkworm, Bombyx mori, is the source of a unique natural fiber used to make silk cloth. Silk is the strongest of all natural fibers. It is comparable to steel or nylon in tensile strength, but considerably more elastic. It can be dyed, woven into fabrics that are warm in winter, cool in summer, resistant to wrinkling, and light in weight.
    Shellac
    Shellac, a product of a scale insect, was widely used as a varnish (protective coating) for floors, furniture, draperies, photographs, playing cards, and dried flower arrangements.

    • Cochineal
    Cochineal is a pigment extracted from a scale insect was highly valued for the intensity and permanence of its color; it is very expensive because of its scarcity. It is still used as a coloring agent in foods, beverages, cosmetics (especially lipsticks), and art products.
    Cultural Entomology
    • There is no question that insects have shaped and molded human civilizations. tsetse flies (Glossina spp.) prevent colonization of large parts of central Africa because they spread the trypanosome parasites that cause sleeping sickness in humans and nagana in cattle.
    • Over the course of human history, more soldiers have died from lice and mosquitoes than from bullets and bombs. These insects spread diseases that have affected entire armies.
    • Body lice are notorious for the spread of epidemic typhus, often called trench fever.
    • Malaria and yellow fever, both transmitted by mosquitoes, have also taken a deadly toll in tropical and subtropical climates.
    • The wartime impact of insect-borne disease has decreased ever since the discovery of modern chemical insecticides as DDT.
    Culture, Religion, and Aesthetics
    • In ancient Egypt, people considered the scarab beetle as a symbol of rebirth and immortality, and scarab-shaped gems and icons were a recurring motif in Egyptian jewelry and other decorative artifacts.
    • Many insects are valued simply for their beauty.
    • Insects appear in famous paintings of many artists.
    • Many people regard insect sounds as a form of music.
    Insects in Language and Literature
    • Our language still reflects many traditional associations: busy as a bee, annoying as a fly, mad as a hornet. We still recognize the ant as a symbol of hard work, and the butterfly as an object of beauty.
    • There are many references to insects in The Bible, Quran and many Islamic writings.
    • Cinematographers usually portray insects as villains or monsters, Insects are often included for their "shock" value, to set a "creepy" mood.
    Science and Technology
    • Scientists demonstrating that flies had to lay eggs on meat before maggots would develop; this finding destroyed the theory of spontaneous generation.
    • The study of insects, particularly their life cycles, physiology, genetics, and ecology, has contributed a great deal to our overall understanding of biology.
    • The geneticist's fruit fly (Drosophila melanogaster) has contributed much to our knowledge of genetics.
    • The first association between pathogens and disease grew out of Louis Pasteur's studies of silkworm diseases (1865-1870).
    • Our knowledge of physiology has been advanced by studying the processes of growth and development in assassin bugs.
    • Toxicology relies heavily on insects to assess the toxicity of products.
    • Engineers studied the legs of a grasshopper to help them design shock absorbers for jet fighter, and developing six-legged robots that is modeled after the distributed nervous system of insects.
    • Specialists in forensic entomology can often deduce how, when, or where a crime was committed. Insects can provide accurate information about the time of death, the number and location of wounds, and even whether the body has been moved from one place to another.
    Medical and Therapeutic Value
    • Insects were studied for their medicinal value.
    • During World War I, medics noticed that gunshot wounds infested with blow fly maggots seldom developed bacterial infections. Their observation led to the use of maggot therapy (sterile-reared fly larvae for cleaning necrotic tissue from deep wounds), and the later discovery of allantoin, a chemical secretion of the larvae that inhibits bacterial growth. Synthetically produced allantoin was commonly used as an antibacterial ointment until penicillin and other antibiotics became commercially available in the 1940's.
    • Honey bees (or their products) have been used for medicinal purposes. This practice, known as apitherapy. Bee stings are used as a treatment for patients who suffer from arthritis, multiple sclerosis, Parkinson's disease, and other auto-immune conditions.
    • Research has uncovered other substances with anti-viral, anti-fungal, and anti-inflammatory activity in a variety of other insects.
    • Some entomologists believe that pharmaceutical companies will find enough new drugs in the class Insecta to keep them busy for many hundreds of years.

    Insects as Hexapods
    • At one time, all six-legged arthropods were considered "insects". It has become difficult to justify grouping all "six-leggers" in a single class.
    • There are major differences in external structures, organ systems, and post-embryonic development that point to big differences between the "true" insects and three other closely allied groups: Protura, Diplura, and Collembola.
    • Many entomologists still consider these three taxa to be "primitive" orders of insects, or orders in a separate class (the Entognatha), or regard each taxon as a separate class of arthropods.

    EXTERNAL ANATOMY
    An insect body has a hard exoskeleton protecting a soft interior, it is divided into three main parts (the head, thorax and abdomen) each of which is in turn composed of several smaller segments.
    The Exoskeleton
    An insect's exoskeleton (integument) serves not only as a protective covering over the body, but also as a surface for muscle attachment, a water-tight barrier against desiccation, and a sensory interface with the environment. It is a multi-layered structure with four functional regions: epicuticle, procuticle, epidermis, and basement membrane.

    The Cuticle
    • The cuticle is a non-cellular material which lines the external surface of the body as well as lining the tracheae the anterior and posterior sections of the alimentary canal and parts of the reproductive system. It is flexible, elastic and white when first formed and stays this way in many larval forms, however in most adults it undergoes chemical processes which result in hardening and darkening and which are referred to as 'sclerotization'.
    • The cuticle can be divided into two layers, a very thin outer layer called the epicuticle which contains no chitin. Beneath this is the much thicker procuticle which can again be divided into an outer 'exocuticle' which lies immediately below the 'epicuticle' and an inner 'endocuticle' which consists of a large number of layers of protein and chitin fibres.
    • Below the cuticle lie , the 'epidermis' which is a single layer of secretary cells and the 'basement membrane.
    • The colors of the integument are produced either by pigment molecules, or by physical characteristics of the integument that cause scattering, interference, or diffraction of light.
    • Color patterns may change over time. This change may be rapid and temporary due to daily environmental conditions o slower, more permanent changes due to seasonal changes in the environment or hormonal influences

    The Head
    The head capsule houses the brain, a mouth opening, mouthparts, and major sense organs (including antennae, compound eyes, and ocelli). Embryological evidence suggests that the first six body segments fused to form the head capsule of most insects (The detailed structure of head is found in the practical notes).

    The Antennae
    • The antennae are a pair of sense organs. Although commonly called "feelers", the antennae are much more than just tactile receptors. They are usually covered with receptors that can detect odor or humidity Mosquitoes detect sounds with their antennae.
    • The first segment is known as the 'scape' the second segment as the 'pedicel' and the rest as the flagellum.
    • In most insects the antennae possess's a mechanosensory organ 'Johnston's organ'.
    The Eyes
    Insects possess two different sorts of eyes, the usually large and obviously visible compound eyes, and two varieties of ocelli or simple eyes.
    Compound Eyes
    Compound eyes are so named because the cornea is composed of a number of individual lenses (called omatidia), rather than a single lens as in ocelli. The number of omatidia varies greatly between species, so that while worker ants may have between 100 and 600 omatidia per single eye, adult male Odonata may have more than 28 000 per single eye.
    Simple Eyes
    Ocelli are present in most insects. Generally they consist of five separate parts the 'cornea', the 'corneagen layer', the 'retina', the 'pigment cells', and the 'central nervous connections'.
    Mouth parts
    • In the primitive form the insect mouth moves from side to side in a horizontal plane, rather than vertically as do ours. The Insect mouth consists of a number of parts which are called 1) The Labrum, 2) The Mandibles, 3) The maxillae, 4) The Labium, 5) The Hypopharynx or tongue. The basic chewing mouthparts have become highly modified, in other insects like butterflies and moths they are transformed into a long flexible straw, in the blood and sap sucking insects of various orders they have become a hypodermic needle and in most of the flies they evolved into effective sponge.

    Labrum Mandibles Maxillae Labium

    Thorax
    • The second (middle) part of an insect's body is called the thorax. It is almost exclusively adapted for locomotion -- it contains three pairs of walking legs and, in many adult insects, one or two pairs of wings.
    • Structurally, the thorax is composed of three body segments: prothorax, mesothorax, and metathorax. Each segment has a dorsal sclerite, the tergum. The ventral sclerite of each segment is the sternum. The side of each segment is called the pleuron.

    Legs
    • Most insects have three pairs of walking legs -- one pair on each thoracic segment. Each leg contains five structural components (segments) that articulate with one another by means of hinge joints:
    1. Coxa
    2. Trochanter
    3. Femur
    4. Tibia
    5. Tarsus
    Wings
    • Insects are the only invertebrates that can fly.
    • The wings may be membranous, heavily sclerotized, fringed with long hairs, or covered with scales. Most insects have two pairs of wings -- one pair on the mesothorax and one pair on the metathorax (never on the prothorax).
    • In most cases, a characteristic network of veins runs throughout the wing tissue. These veins are extensions of the body's circulatory system.
    Abdomen
    • An insect's abdomen is the third functional region of its body.
    • The front margins of each segment often "telecope" inside the sclerites of the preceding segment, allowing the abdomen to expand and contract in response to the actions of skeletal muscles.
    • In many adult insects, there is a spiracle (opening to the respiratory system) near the pleural membrane on each side of the first eight abdominal segments.
    • At the very back of the abdomen, the anus (rear opening of the digestive system) is nestled between three protective sclerites.
    • The insect's genital opening lies just below the anus: it is surrounded by specialized sclerites that form the external genitalia.
    • Other abdominal structures may also be present in some insects. These include: Cerci, Styles, Pincers, Median caudal filament, Cornicles, Abdominal prologs, Sting, Abdominal gills, Furcula, and Collophore (see practical notes).[left]


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