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VIRUSES   Leave a comment

DEFINITION:1 orig., venom, as of a snake 2 a) any of a kingdom (Virus) of prokaryotes, usually ultra microscopic, that consist of nucleic acid, either RNA or DNA, within a case of protein: they infect animals, plants, and bacteria and can reproduce only within living cells so that they are considered as being either living organisms or inert chemicals b) a disease caused by a virus 3 anything that corrupts or poisons the mind or character; evil or harmful influence 4 an unauthorized, disruptive set of instructions placed in a computer program, that leaves copies of itself in other programs and disks.

1981: US AIDS diagnosed. A new fatal, infectious disease was diagnosed in 1981. Called Acquired Immunodeficiency Syndrome (AIDS), it began appearing in major cities among homosexual men and intravenous drug users. Other high-risk groups were haemophiliacs and other recipients of blood or blood products, babies born of AIDS-infected women, bisexual men, and prostitutes and their customers. AIDS was soon recognized as a worldwide health emergency: a fatal disease with no known cure that quickly became an epidemic. It was especially widespread in Africa, the apparent land of its origin.

By 1983 the virus that causes the disease had been isolated. Some medicines, notably AZT (azidothymidine), slowed the disease’s progress for a few months or more; but the spread of AIDS continued relentlessly, with more than 3,000 new cases being reported each month by 1991.

The federal government had committed more than 1.6 billion dollars to research, while the homosexual community and other special interest groups sought more federal funding and greater assistance from the health insurance industry. Educational programs on safe sexual practices, such as the use of condoms, seemed the best means of slowing the epidemic. Meanwhile, more than 70,000 persons in the United States had died from AIDS by the end of the decade.

1981: WORLD AIDS identified. A strange, new, and deadly disease made its appearance in 1980. Physicians in such large cities as Los Angeles, New York, and San Francisco noticed that homosexual men were dying from rare lung infections or from a cancer known as Kaposi’s sarcoma. By 1981 the disease was identified and given a name: AIDS, or acquired immunodeficiency syndrome.

The virus that causes AIDS, human immunodeficiency virus (HIV), was identified by Dr. Luc Montagnier of the Pasteur Institute in Paris in research done during the years 1981-84. The results of Dr. Montagnier’s studies were released in 1984. Since its discovery, AIDS has become one of the world’s major health problems. Within certain populations it has become an epidemic: male homosexuals, haemophiliacs, and intravenous drug users in the United States, for example, and heterosexual men and women in Sub-Saharan Africa. Many people were infected through blood transfusions before HIV screening was introduced. An individual infected with the virus may not show the symptoms of AIDS for several years, but the condition is eventually fatal.

The search for a successful vaccine was pursued in laboratories around the world, with no success by the early 1990s. Meanwhile, the disease continued to spread to different parts of the world. Already rife in the United States, Europe, and sub-Saharan Africa by the mid-1980s, it quickly spread to Central and East Asia. The disease also began to spread to larger portions of the heterosexual community throughout the world.

The composition of a virus is relatively simple, and its size is extremely small. It cannot even properly be called an organism because it is unable to carry on life processes outside a living cell of an animal, plant, or bacteria. Yet its method of entering and “enslaving” a living cell is so ingenious that the virus is humankind’s deadliest enemy and resists the most advanced efforts of modern science to eliminate it.

Millions of people throughout the world suffer each year from viral diseases such as polio, measles, chicken pox, mumps, acquired immunodeficiency syndrome (AIDS), and the common cold. Viruses also produce such illnesses as foot-and-mouth disease in livestock, distemper in dogs, panleukopenia in cats, and hog cholera. The viruses that infect bacteria are called bacteriophages.

Structure and Composition

Nucleic acid, any of substances comprising genetic material of living cells; divided into two classes: RNA (ribonucleic acid) and DNA (deoxyribonucleic acid); directs protein synthesis and is vehicle for transmission of genetic information from parent to offspring.

Viruses are exceedingly small; they range in size from about 0.02 to 0.25 micron in diameter (1 micron = 0.000039 inch). By contrast, the smallest bacteria are about 0.4 micron in size. As observed with an electron microscope, some viruses are rod-shaped, others are roughly spherical, and still others have complex shapes consisting of a multi sided “head” and a cylindrical “tail.” A virus consists of a core of nucleic acid surrounded by a protein coat called a capsid; some viruses also have an outer envelope composed of fatty materials and proteins. The nucleic-acid core is the essential part of the virus it carries the virus’s genes. The core consists of either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), substances that are essential to the transmission of hereditary information. The protein capsid protects the nucleic acid and may contain molecules that enable the virus to enter the host cell that is, the living cell infected by the virus.

Cycle and Patterns of Infection

Outside of a living cell, a virus is a dormant particle. It exhibits none of the characteristics generally associated with life namely, reproduction and metabolic processes such as growth and assimilation of food. Unlike bacteria and other micro-organisms, viruses remain dormant in body fluids. Thus, great numbers of viruses may be present in a body and yet not produce a disease because they have not invaded the body’s cells. Once inside a host cell, however, the virus becomes an active entity capable of taking over the infected cell’s metabolic machinery. The cellular metabolism becomes so altered that it helps to produce thousands of new viruses.

The virus’s developmental cycle begins when it succeeds in introducing its nucleic acid, and in some cases its protein coat, into a host cell. Bacteriophages attach to the surface of the bacterium and then penetrate the rigid cell wall, transmitting the viral nucleic acid into the host. Animal viruses enter host cells by a process called endocytosis. Plant viruses enter through wounds in the cell’s outer coverings through abrasions made by wind, for example, or through punctures made by insects.

Virion, an entire virus particle the extracellular infective form of a virus consisting of an outer protein shell (capsid) and an inner core of nucleic acid (either ribonucleic or deoxyribonucleic acid); in some, the capsid further enveloped by a fatty membrane.

Once inside the host cell, the virus’s genes usually direct the cell’s production of new viral protein and nucleic acid. These components are then assembled into new, complete, infective virus particles called virions, which are then discharged from the host cell to infect other cells.

In the case of bacteriophages, the new virions are usually released by bursting the host cell a process called lysing, which kills the cell. Sometimes, however, bacteriophages form a stable association with the host cell. The virus’s genes are incorporated into the host cell’s genes, replicate as the cell’s genes replicate, and when the cell divides, the viral genes are passed on to the two new cells.

In such cases no virions are produced, and the infecting virus seems to disappear. Its genes, however, are being passed on to each new generation of cells that stem from the original host cell. These cells remain healthy and continue to grow unless, as happens occasionally, something triggers the latent viral genes to become active. When this happens, the normal cycle of viral infection results: the viral genes direct viral replication, the host cell bursts, and the new virions are released. This pattern of infection is called lysogeny.

Closely related to lysogeny is the process known as transduction, whereby a virus carries bacterial genes from one host to another. This transduction process occurs when genes from the original host become incorporated into a virion that subsequently infects another bacterium.

Viral infections of plant and animal cells resemble those of bacterial cells in many ways. The release of new virions from plant and animal cells does not, however, always involve the bursting of the host cell as it does in bacteria. Particularly among animal viruses, the new virions may be released by budding off from the cell membrane, a process that does not necessarily kill the host cell.

In general, a viral infection produces one of four effects in a plant or animal cell: in apparent effect, in which the virus remains dormant in the host cell; cytopathic effect, in which the cell dies; hyperplastic effect, in which the cell is stimulated to divide before its death; and cell transformation, in which the cell is stimulated to divide, take on abnormal growth patterns, and become cancerous.

Cold sore (or fever blister, or Herpes simplex), a virus infection of the borders of the mouth, lips and nose, or genitals; marked by watery blisters; may be due to illness, emotional upset, or other stress.

Viral infections in animals can be localized or can spread to various parts of the body. Some animal viruses produce latent infections: the virus remains dormant much of the time but becomes active periodically. This is the case with the herpes simplex viruses that cause cold sores.

Natural Defences, Immunization, Treatment

Fever, a condition in which the body temperature rises above normal.

Animals have a number of natural defences that may be triggered by a viral infection. Fever is a general response; many viruses are inactivated at temperatures just slightly above the host’s normal body temperature. Another general response of infected animal cells is the secretion of a protein called interferon. Interferon inhibits the reproduction of viruses in non infected cells.

Fever and interferon production are general responses to infection by any virus. In addition, humans and other vertebrates can mount an immunological attack against a specific virus. The immune system produces antibodies and sensitized cells that are tailor-made to neutralize the infecting virus. These immune defenders circulate through the body long after the virus has been neutralized, thereby providing long-term protection against reinfection by that virus.

Such long-term immunity is the basis for active immunization against viral diseases. A weakened or inactivated strain of an infectious virus is introduced into the body. This virus does not provoke an active disease state, but it does stimulate the production of immune cells and antibodies, which then protect against subsequent infection by the virulent form of the virus.

Active immunizations are routine for such viral diseases as measles, mumps, poliomyelitis, and rubella. In contrast, passive immunization is the injection of antibodies from the serum of an individual who has already been exposed to the virus. Passive immunization is used to give short-term protection to individuals who have been exposed to such viral diseases as measles and hepatitis. It is useful only if provided soon after exposure, before the virus has become widely disseminated in the body.

The treatment of an established viral infection usually is restricted to relieving specific symptoms. There are few drugs that can be used to combat a virus directly. The reason for this is that viruses use the machinery of living cells for reproduction. Consequently, drugs that inhibit viral development also inhibit the functions of the host cell. Nonetheless, a small number of antiviral drugs are available for specific infections.

The most successful controls over viral diseases are epidemiological. For example, large-scale active immunization programs can break the chain of transmission of a viral disease. Worldwide immunization is credited with the eradication of smallpox, once one of the most feared viral diseases. Because many viruses are carried from host to host by insects or contaminated food, insect control and hygienic food handling can help eliminate a virus from specific populations.

History of Virus Research

Historic descriptions of viral diseases date back as far as the 10th century BC. The concept of the virus, however, was not established until the last decade of the 19th century, when several researchers obtained evidence that agents far smaller than bacteria were capable of causing infectious diseases.

Mosaic disease, highly infectious virus disease affecting many plants including cucumber, potato, tomato, bean, and turnip; dwarfs growth and mottles leaves.

The existence of viruses was finally proved when bacteriophages were discovered by independent researchers in 1915 and 1917. The question of whether viruses are actually micro-organisms (similar to very tiny bacteria) was resolved in 1935, when the virus responsible for causing mosaic disease in tobacco was isolated and crystallized; the fact that it could be crystallized proved that the virus was not a cellular organism.

Bacteriophages are a valuable research tool for molecular biologists. Studies of bacteriophages have helped to illuminate such basic biological processes as genetic recombination, nucleic-acid replication, and protein synthesis.

GENETIC ENGINEERING (Part 1 of 2)   Leave a comment

DEFINITION: the branch of biology dealing with the splicing, and recombining, of specific genetic units from the DNA of living organisms: it is used to modify the existing genetic codes to produce new, or improved, species, valuable biochemicals.

1865: The birth of genetics. It was unfortunate for the biological sciences that Gregor Mendel was an obscure Austrian monk. His pioneering work in the field of genetics was being done at the time that Charles Darwin’s publications on evolution were beginning to create worldwide controversy, but Mendel’s work would remain unknown for years.

Mendel became an Augustinian monk in 1843, but his abilities in mathematics and the sciences were evident. His experiments on the principles of heredity were begun in about 1856 in what is now Czechoslovakia. By crossing various strains of peas with one another, Mendel found that traits were passed on from generation to generation in what he called “discrete hereditary elements” in sex cells, or gametes.

Mendel reported the results of his experiments to a local society for the study of natural science in 1865 and published his findings in the society’s journal. They were as good as buried there for the next 35 years. Although the journal found its way to libraries in Europe and North America, few paid any attention to his writings. When other botanists obtained results similar to Mendel’s, they began searching through earlier writings on the subject. Only then was Mendel’s 1865 research revealed. His “discrete hereditary elements” are now called genes, and the new science once called Mendelism is known as genetics.

1953: Discovery of DNA structure. The full name of DNA is deoxyribonucleic acid. It carries the codes of genetic information that transmit inherited characteristics to successive generations of living things.

DNA was discovered in 1869 by Friedrich Miescher. In 1943 its role in inheritance was demonstrated. In 1953 its structure was determined by an American biochemist, James D. Watson, and an English physicist, Francis H.C. Crick. Watson and Crick showed the structure to be two strands of a phosphoryl-deoxyribose polymer arranged as a double helix. Watson and Crick were awarded the Nobel prize in physiology or medicine in 1962.

1973: Biotechnology. Two American biochemists, Stanley H. Cohen and Herbert W. Boyer, inaugurated the science of genetic engineering and its associated field of biotechnology in 1973. They showed that it was possible to break down DNA into fragments and combine them into new genes, which could in turn be placed in living cells. There they would reproduce each time a cell divided into two parts.

Genetic engineering makes it possible to modify existing organisms or create organisms that already exist in the human body but that are difficult to isolate. For example, one early product was a genetically engineered form of insulin, used in the treatment of diabetes. Other genetically engineered products include interferons, which are used in the treatment of viral infections and showed promise in the treatment of various forms of cancer. Scientists hope that genetically engineered products will someday prevent or cure such genetic disorders as muscular dystrophy and cystic fibrosis.

Genetic engineering also opens the possibility of creating entirely new organisms. In 1980 the United States Supreme Court ruled that newly developed organisms could be patented, thus giving ownership rights to the companies that made them.

Chromosome, microscopic, threadlike part of the cell that carries hereditary information in the form of genes; among simple organisms, such as bacteria and algae, chromosomes consist entirely of DNA and are not enclosed within a membrane; among all other organisms chromosomes are contained in a membrane-bound cell nucleus and consist of both DNA and RNA; arrangement of components in the DNA molecules determines the genetic information; every species has a characteristic number of chromosomes, called the chromosome number; in species that reproduce asexually the chromosome number is the same in all the cells of the organism; among sexually reproducing organisms, each cell except the sex cell contains a pair of each chromosome.

Each human cell holds a vast storehouse of genetic information in some 100,000 genes, which code for individual biochemical functions, strung out along 46 chromosomes. Collectively, this storehouse forms the human genome. The techniques of genetic engineering allow scientists to identify specific genes, to remove any one of those genes from an organism’s chromosome, to clone or make a large number of identical copies of that gene, to analyse a copy in detail, to modify it, and to reinsert it into the genetic material of the organism from which it was derived or into the genetic material of a similar or very different organism.

The development of genetic engineering has had a great influence on science and business and has begun to radically alter medicine and agriculture. One of the first steps in shedding new light on human evolution and in controlling or altogether eliminating many diseases was taken in the early 1990s. Scientists mapped, or took apart, the smallest human chromosomes: the Y chromosome and chromosome 21. Breaking these chromosomes into small pieces allowed researchers to reproduce these segments in large quantities. Researchers believe that this, in turn, will lay the groundwork for mapping and eventually controlling all genes, including those that may be responsible for certain diseases.

Recombinant DNA, genetically engineered DNA prepared in vitro by cutting up DNA molecules and splicing together specific DNA fragments; usually uses DNA from more than one species of organism.

Smith, Hamilton O. (born 1931), U.S. microbiologist, born in New York City; with U.S. Public Health Service 1962-67; at School of Medicine of Johns Hopkins University from 1967, professor from 1973; received 1978 Nobel prize for research on effect of restriction enzymes on DNA molecules.

Nathans, Daniel (born 1928), U.S. microbiologist, born in Wilmington, Del.; professor school of medicine of Johns Hopkins University from 1967, director microbiology department from 1972; received 1978 Nobel prize for research on effect of restriction enzymes on DNA molecules.

Posted 2012/02/25 by Stelios in Education

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GENETIC ENGINEERING (Part 2 of 2)   Leave a comment

Genetic engineering had its origins during the late 1960s and early 1970s in experiments with bacteria, viruses, and small, free-floating rings of deoxyribonucleic acid (DNA) called plasmids, found in bacteria. While investigating how these viruses and plasmids move from cell to cell, recombine, and reproduce themselves, scientists discovered that bacteria make enzymes, called restriction enzymes, that cut DNA chains at specific sites. The 1978 Nobel prize for physiology or medicine was shared by the discoverer of restriction enzymes, Hamilton O. Smith, and the first people to use these tools to analyse the genetic material of a virus, Daniel Nathans and Werner Arber.

The action of restriction enzymes is the crux of genetic engineering. DNA is made up of two long intertwined helices. The backbone of each helix is constructed from a chain of organic compounds called nucleotides. DNA is the carrier of genetic information; it achieves its effect by directing the synthesis of proteins. DNA is composed of four different nucleotides, repeated in specific sequences that form the basis of heredity. Restriction enzymes recognize particular stretches of nucleotides arranged in a specific order and cut the DNA in those regions only. Each restriction enzyme recognizes a different nucleotide sequence. Thus, restriction enzymes form a molecular tool kit that allows scientists to cut the chromosome into various desired lengths, depending on how many different restriction enzymes are used. Each time a particular restriction enzyme or set of restriction enzymes is used, the DNA is cut into the same number of pieces of the same length and composition. At least 80 restriction enzymes are now known.

When restriction enzymes are used along with other enzymes that tie together loose ends of DNA, it becomes possible to remove a bit of DNA from one organism’s chromosome and to insert it into another organism’s chromosome. This allows scientists to produce new combinations of genes that may not exist in nature. For example, a human gene can be inserted into a bacterium or a bacterial gene into a plant.

So far, however, there are limits to this ability. Scientists are unable to start with only test tubes full of nucleotides to create a whole new organism. They must start with the complete genetic material of an already existing organism. Thus, genetic engineering allows the addition of only one or a small number of new characteristics to an organism that remains essentially the same. In addition, only characteristics that are determined by one or a few genes can be transferred. The technology of genetic engineering does not enable scientists to transfer behavioural traits, such as intelligence, that are a complex mixture of many genes and the effects of cultural conditioning.

In practice, genes to be inserted into bacteria are first recombined into a plasmid, which replicates and travels independently from its bacterial host. The modified plasmid is then inserted into another bacterium.

At the time that these methods became available, it was not known whether DNA could be replicated and expressed in a foreign organism. Experiments done in the mid-1970s with these techniques, however, showed clearly that a human gene could be reproduced by a bacterium along with its own genetic material and that the bacterium could make the protein coded for by the human gene. This opened the way to making large amounts of human hormones and other significant human substances in the laboratory, a much easier route than isolating these substances from blood or cadaver glands.

One of the first areas in which genetic engineering exerted an influence was the medical field. Genetic engineering techniques allowed the production of large amounts of many medically useful substances, particularly the class of biological compounds called peptides. Peptides are short proteins. Many of the most important chemical messengers in the body are peptides. These include hormones such as insulin, nervous system messengers such as the endorphins, and regulatory messengers from the hypothalamus that control production of hormones by the pituitary gland, the gonads, and the thyroid gland, among others.

Several biologically useful peptides were made and tested in clinical trials during the late 1970s and early 1980s. The first genetically engineered product to be approved for human use was human insulin made in bacteria. Insertion of the human insulin gene into bacteria was accomplished by the pioneer genetic engineering company Genentech. Testing, approval for medical use, and large-scale production of genetically engineered human insulin were carried out, and the first diabetic patient in the world was injected with human insulin made in bacteria in December 1980, making this the first genetically engineered product to enter medical practice. (Genetically engineered products are often identified by the prefix r, for “recombinant.” Thus, genetically engineered insulin is sometimes written, r-insulin.)

The interferons are another medically important group of peptides that became available in abundance only after the development of genetic engineering techniques. Interferon was useful for treating viral infections, and there were strong indications that it might be effective against some cancers. Before the advent of genetic engineering techniques, it took laborious processing of thousands of units of human blood to obtain enough interferon to treat a few patients. And this interferon was not very pure. With the insertion of the interferon gene into bacteria, large amounts of very pure interferon became available. This supply allowed trials of interferon in the early 1980s against more than ten different cancers, including the particularly virulent form of Kaposi’s sarcoma often found in persons with acquired immunodeficiency syndrome (AIDS). The human body makes more than 50 different varieties of interferon. It is thought that some types of interferon may be more effective against cancer than others.

Other medically useful human peptides that have been made widely available because of genetic engineering are human growth hormone, which is used to treat persons with congenital dwarfism and was formerly obtained from cadaver pituitary glands, and tissue-type plasminogen activator (t-PA), which is a promising new treatment for persons who suffer a heart attack.


Genetic engineering techniques have also been investigated as a means to produce safer new vaccines. The first step is to identify the gene in a disease-causing virus that stimulates protective immunity. That gene is isolated and inserted into a harmless virus, such as vaccinia, the virus used to immunize against smallpox. The recombinant vaccinia virus is used as a vaccine, producing immunity without exposing people to the disease-causing virus. In the case of viruses about which little is known, such as the AIDS virus, this extra margin of safety is crucial.

Recombinant techniques may be useful in making vaccines against organisms for which no vaccines could be made by traditional methods. These include possible vaccines against the tropical parasites that cause schistosomiasis and malaria.

Diagnosis, Therapy, and Research

Genetic engineering is also being used in the prenatal diagnosis of inherited diseases. Restriction enzymes are used to cut apart the DNA of parents who may carry a gene for a congenital disorder, and the DNA pattern of cells from the fetus is compared. In many situations the disease status of the fetus can be determined. Currently this technique is applicable to thalassemias, Huntington’s disease, cystic fibrosis, and Duchenne’s muscular dystrophy.

A future medical use of genetic engineering is for gene therapy. Persons who are born with a congenital disorder resulting from a defective gene could have a sound gene inserted into their cells, preventing the manifestations of the disease. The era of gene therapy began on Sept. 14, 1990, when the first therapeutic, genetically engineered cells were infused into a 4-year-old girl with adenosine deaminase (ADA) deficiency, an inherited life-threatening immune deficiency. The infused cells were lymphocytes from the girl’s own blood, into which researchers had inserted copies of a missing gene that directs production of ADA. On Jan. 29, 1991, gene therapy was used for the first time to treat cancer, when two patients with advanced skin cancer were infused with their own white blood cells after the cells had been genetically altered to produce a tumour-killing protein. Many obstacles must be overcome to achieve the promise of gene therapy, but its value could be immense.

Ribozymes, RNA molecules that act like enzymes to cut and splice themselves.

Genetic engineering has allowed discoveries that could not have been made any other way. One of the most important is the discovery of oncogenes, specific genes that play an important part in causing some cancers. The identification and isolation of oncogenes depended on being able to cut cancer-causing DNA into manageable segments and finding the specific segments that were responsible for transforming normal cells into cancer cells. Discovery of ribozymes RNA molecules that act like enzymes to cut and splice themselves gave scientists hope for a new way of destroying the expression of unwanted genes. (RNA, along with DNA, is a carrier of genetic information.)

Agricultural advances are also expected from genetic engineering. Some of the earliest recombinant organisms made were a soil bacterium that was induced to make a toxin against a worm that destroys corn roots, a bacterium engineered to make potato and strawberry crops more frost-resistant, and a tobacco plant bearing a bacterial gene that protects against herbicides. Work on agricultural applications of genetic engineering proceeds slowly because recombinant organisms must be released outdoors in test fields to find whether they work. They cannot be tested only in a contained laboratory. Government regulatory agencies and ecological scientists are wary of the possible adverse consequences of releasing recombinant organisms, particularly fast-reproducing bacteria, into a field plot. No one knows how likely it is that such organisms may escape the field, grow in some situation in which they are not wanted, and cause unexpected effects.

National Institutes of Health, established 1887 at Marine Hospital, New York, N.Y.; present name from 1948.

Public debate about the safety of recombinant organisms began in the 1970s. Government bodies were set up to screen proposed experiments and institute safety guidelines. Chief among them was the Recombinant Advisory Committee of the National Institutes of Health. For many years the public was concerned about the safety of laboratory research with recombinant organisms. As research has continued and no safety hazards have become evident, public concern has abated somewhat.

Assisted by William A. Check.

Posted 2012/02/25 by Stelios in Education

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INSECTS (Part 2 of 2).   Leave a comment


Insects belong to the phylum Arthropoda, one of the chief divisions of the animal kingdom. The name comes from two Greek words, arthron (“joint”) and podos (“foot”), and refers to the jointed feet. Arthropods also include spiders, lobsters, centipedes, and other animals. In this phylum, insects belong to the class Insecta. Each insect has two parts to its scientific name. For example, the housefly is Musca domestica. The first half of the name is that of the genus (a group of closely related species) to which the species domestica belongs. The many thousands of insect genera (plural of genus) are grouped under more than 900 families. These families, in turn, are grouped under as many as 30 orders.

To summarize, the housefly is classified as follows: kingdom, Animalia; phylum, Arthropoda; class, Insecta (Hexapoda); order, Diptera; family, Muscidae; genus, Musca; species, domestica. Each of these groups is often divided even further into subgroups (subphylum, subclass, suborder, and so on).

Ancestors of the Modern Insect

Insects appeared on Earth long before the advent of humans or the earliest mammals. The first insects probably evolved from primitive ringed worms. These insect ancestors were wingless and developed without metamorphosis, as do today’s silverfish.

The oldest fossils of ancestral insect forms are believed to be some 350 million years old. There are also fossil records, from later eras, of highly developed forms very similar to the mayflies, cockroaches, and dragonflies now in existence. Some ancient insects were truly huge; dragonflies, for example, had a wingspread of 2 feet (0.61 meter) or more.


Insects that attack humans or anything of value to humans are termed pests; many of these are mutually competitive with humans for the world’s food supply. Other insects are benefactors of humans, as they devour the carcasses of dead animals, pollinate orchards, manufacture honey, or simply serve as another link in the food chain of the animal kingdom, for humans eat the animals including fish and birds which, in turn, live upon the insects.


About 10,000 species of insects have been classified as pests. Some are disease carriers, afflicting and often killing humans. Many insects prey upon domestic animals; others eat human food, clothing, and other possessions. Still others, in their quest for food or lodging, destroy trees, wood, and paper.

Carriers of Disease


Following are the names of some insects and the diseases they carry, and what may happen to someone who gets the disease.




Tsetse fly

African sleeping sickness



Yellow fever



Liver damage




Rat flea

Bubonic plague


Human louse




Assassin bug

Chagas’ disease

Heart damage

Brain damage



As vectors, or transmitting agents, of disease organisms, insects have caused more deaths and have inflicted greater misery and hardship on humankind than all the wars of history. In their efforts to find food, insects wage their own war against the human race. Some feed upon humans directly. Notable among these are the true flies, including mosquitoes, horseflies, black flies, tsetse flies, and other two-winged pests.

Perhaps humankind’s worst enemy among the insects is the mosquito. More lives have been lost as a result of malaria, yellow fever, encephalitis, and other mosquito-borne diseases than from all the other insect-borne diseases combined.

The tsetse fly has been a serious deterrent to the development of much of tropical Africa, for the insect acts as a vector of trypanosomiasis (African sleeping sickness) among humans and of nagana, a serious disease of livestock.

Horseflies and stable flies also transmit disease through their bites. The common housefly is not a biter, but it can carry myriad disease organisms on the hairs and the sticky secretions of its body. The assassin, or kissing, bug transmits the highly fatal Chagas’ disease.

Bedbugs, fleas, and lice live on the blood of birds and mammals, including humans. The human louse lives on the blood of humans alone and transmits typhus, relapsing fever, and trench fever.

The flea is potentially one of humankind’s deadliest enemies; rat fleas, for example, carry the germs of murine typhus and bubonic plague, which was instrumental in wiping out the lives of one fourth of the population of Europe in four years.

Household Pests

Insect pests in the home are most commonly chewers. One of the most troublesome of these the clothes moth attacks furs, woollens, and materials made of hair.

The silverfish and the fire-brat eat sized or stiffened material, such as the paper and bindings of books and starched clothing and curtains. In some parts of the United States, termites do considerable damage to furniture and paper products, as well as to the timber frameworks of buildings.

Plant-Eating Pests

Most insects are herbivorous that is, they feed on plants. Virtually every part of a plant, from the flower to the root, is vulnerable to their attack. They do their damage in a variety of ways.

Insects with chewing mouth-parts are the most destructive plant eaters. A horde of grasshoppers, for example, can strip every blade of vegetation from a field in a few hours. The destruction caused by other chewing insects, such as beetles, can also be enormous.

Insects with sucking mouth-parts, though usually smaller and less conspicuous than the chewers, also do a great deal of damage to farm crops and to forest and garden plants. These insects pierce plant tissues and draw out the vital juices. These insects include the aphids, chinch bugs, cicadas, and scale insects.

Damage is also done to the host plant from within by many other plant pests usually as larvae. Some eat their way between the top and bottom layers of a leaf, giving it a blotched appearance. The leaf roller, the larval form of certain moths, rolls a leaf into a tube and spins silk to hold it together. The caterpillar then feeds on the leaf. Other insect pests tie several leaves together into a large nest.

Gall-flies cause swellings on buds, flowers, leaves, stems, bark, or roots of plants. Usually the female pierces the plant and lays an egg; the plant then grows a gall, or swelling, around the egg.

Insect Immigrants Upset Nature’s Balance

As long as a region is left in its natural state, no species of insect is likely to increase disproportionately in numbers. The balance of nature prevents this from happening. Every insect has natural enemies, such as the spider, the praying mantis, and many kinds of disease organisms, that help keep the number of insects down.

The balance of nature in the New World was upset when settlers from Europe brought their domestic plants with them. Many insects that were harboured by these plants escaped the natural controls that were present in their old environments and became pests. The widespread use of such insecticides as DDT, now largely discontinued, also disrupted the balance of nature in some areas.

Pests arrive in many ways and from many lands. The gypsy moth, for example, was brought to the United States for experiments in the 1860s. It escaped from the laboratory and before the end of the 19th century had cost millions of dollars annually in damage to shade trees. The Argentine ant, an enemy of field crops and stored foods, was a stowaway in a cargo that reached New Orleans, La., in 1891. The brown-tail moth, another shade-tree pest, reached New England from Europe in about 1897. The alfalfa weevil came to Utah in 1902 in soil adhering to imported plants. The corn borer was carried from southern Europe in 1909 in a shipment of broom-corn Two serious pests came from Japan the Oriental fruit moth, on cherry trees presented by the city of Tokyo to Washington, D.C., in 1913; and the Japanese beetle, on trees reaching New Jersey in 1916. Also in 1916, carloads of cotton-seed from Mexico brought in the pink boll-worm Four arrived in 1920: the satin moth, an enemy of shade trees; the Asiatic beetle, which destroys lawns; the Mexican bean beetle, which feeds on a variety of beans; and the Mediterranean fruit fly, which is highly destructive of fruits, nuts, and vegetables.


Until the middle of the 19th century Americans were helpless against the growing insect menace. Finally, in the 1860s, arsenic compounds were found to be effective in combating the Colorado potato beetle. This was the first successful control of insect pests by scientific means. In the Morrill Act, in 1862, Congress provided for the study of insect pests and other agricultural problems.

Six principal methods are used in the control of insect pests. These methods are chemical, mechanical, radiological, cultural, biological, and legal.

Chemical. The chemical substances used to destroy insects are called insecticides. These may be broadly classified as stomach poisons, contact poisons, fumigants, and sorptive dusts. The stomach poisons are more effective against the chewing insects; the contact poisons, against sucking insects. Fumigants are gaseous poisons that enter the insect’s breathing system. Sorptive dusts are dry chemical compounds that kill insects by absorbing fatty substances from the exoskeleton, thus causing vital body fluids to evaporate.

Mechanical. Mechanical methods of insect control often primitive and time-consuming are generally less effective than chemical methods. They can seldom be applied practically to large populations of insects or over wide areas. These methods include swatting, the use of traps and barriers, water control, and temperature control. Water control involves adjustment of the water level or the rate of flow in breeding places. Temperature control is sometimes effective against insects that infest enclosed storage facilities. Reducing the temperature to 40 or 50 F (4 or 10 C) will cause most insects to become dormant; raising the temperature to 130 F (54 C) for three hours is sufficient to kill almost any insect.

Radiological. Perhaps the most dramatic, wholesale destruction of insects can be accomplished by making them infertile. Sexual sterility in male insects is induced by treating them with the rays of radioactive cobalt. If a large number of a particular species undergo this process in the laboratory, the treated males though sterile will still mate with fertile females; but the eggs laid by these females will be sterile. Following continual releases of sterile males in a single area, the number of young can be gradually reduced over a period of several generations until the population of the insect is totally wiped out within that area.

Through this technique the screw-fly, a serious pest of cattle, was first eradicated from the island of Curacao in the West Indies in 1954. Radiological warfare was then used to bring the screw-fly under control in the south-eastern United States.

Cultural. The cultural control of insect pests is of special interest to the farmer. Methods include the destruction of plant residues and weeds, crop tillage, crop rotation, and the growing of insect-resistant strains of crops.

Four things that farmers can do to control insects are

1. destroy plant residues and weeds. This can kill insects that are hibernating so they will not reproduce the following year.

2. crop tillage. This means to plough plants that have finished growing so they go back down into the soil and replenish the land. If a farmer ploughs at the right time of year, many insects living in the soil are killed.

3. crop rotation. This means to change the type of crop grown in a certain field in different seasons. Insect numbers are kept down when a farmer switches to a crop that insects do not like to eat.

4. insect-resistant strains. These are crops that insects do not like to eat. Developing insect-resistant strains of food limits insect populations.

When the farmer destroys the crop residues and weeds, he also destroys hibernating insects that would otherwise reproduce the following season. By ploughing or cultivating at the right time of year, he can often eliminate large numbers of harmful insects living in the soil. Crop rotation is an important means of combating insect pests of field crops, for many such pests will feed on only a single species or a single family of plant. Thus, if a farmer grows a grain one season and a legume the next, populations of many grain pests (as well as legume pests) can be kept down or eliminated.

Insect-resistant strains of many crops have been developed. Many of these strains have been developed by means of genetic engineering techniques. Resistance to the European corn borer, the wire-worm, and the chinch bug, for example, has been obtained in a single corn hybrid through selective breeding.

Biological. The control of insects by biological means involves the application of the pest’s natural enemies. These enemies may be microbes, mites, or other insects. Scientists have succeeded in controlling harmful insects by first determining the major predators or parasites of that insect in its country of origin. Then the scientists introduced these natural enemies as control agents in the new country that the pest had infested. A classic example is the cottony cushion scale, which threatened the survival of the California citrus industry in 1886. The predatory ladybird beetle, or vedalia beetle, was introduced from Australia, and within two years the scale insect had virtually disappeared from California.

In eastern Canada in the early 1940s the vicious European spruce sawfly was completely controlled by the spontaneous appearance of a viral plant disease, perhaps unknowingly introduced from Europe. This event led to increased interest in plant diseases as potential means of pest control.

Legal. The legal control of insects concerns government regulations to prevent the spread of insect pests from one country or region to another. The Federal Plant Quarantine Act of 1912 began the fight against imported pests by providing for inspectors at ports of entry. These officials examine all plant products as well as passengers’ baggage. Infested material is destroyed or thoroughly fumigated. Aircraft are examined and may be fumigated as soon as they arrive in the United States from countries where insect pests are a potential threat.

By the time an immigrant pest is discovered in domestic plants, it is usually too late for eradication of the injurious insect. In some instances, however, control has been achieved. In 1929 the Mediterranean fruit fly was detected in Florida orchards; the insects threatened ruin to the fruit crop. State and federal entomologists united for battle, and all Florida was quarantined. Abandoned and run-down orchards were destroyed. Chemists developed new poison sprays. By the end of the summer not a “medfly” could be found in Florida. In 1956 a second such outbreak occurred; this too was put down after several months of intensive warfare.

In 1981 a serious spread of the medfly threatened California’s agricultural regions with economic disaster. The pest had been imported accidentally in 1980. An attempt to control the insects by importing sterilized males from Peru failed. The Department of Agriculture threatened to quarantine the state’s produce unless the infected areas were fumigated. Governor Jerry Brown finally authorized helicopter spraying of the pesticide called malathion in July 1981. The spraying halted the threat to the California crops.


Numerous species of plants depend upon insects to pollinate them. In visiting flowers for nectar, insects carry pollen from one flower to the pistil of another. In this way they fertilize the plant and enable it to make seeds.

Without insects there would be no orchard fruits or berries. Tomatoes, peas, onions, cabbages, and many other vegetables would not exist. There would be no clover or alfalfa. The animals that need these forage crops would be of poor quality, and humankind’s meat supply would suffer. There would be no linen or cotton; no tea, coffee, or chocolate.

The honeybee produces honey and wax. Silk is made by the silkworm larva. Shellac is secreted by an Oriental scale insect. Such insects as the dobsonfly are used in sport fishing as bait.

In many underdeveloped areas of the world grasshoppers, caterpillars, and other insects are necessary to humans as food. Insects are also important to humans as food for other animals. Freshwater fishes depend upon insects for food. Hundreds of species of birds would perish if there were no insects to eat.

Insects have also played a significant role in the biological laboratory. The Drosophila fly, in particular, has been valuable in the study of inherited characteristics. The European blister beetle, or Spanish fly, is helpful in the fight against human disease, for it secretes cantharidin, a substance used medically as a blistering agent.

Many insects are invaluable as predators on insects that are pests to humans. In the same way, plant-eating insects are often valuable for their destruction of weeds. Insects that burrow in the earth improve the physical and chemical condition of the soil.

As scavengers, insects perform the important function of eating dead plants and animals. The housefly, scorned as a disease carrier, is beneficial in its larval form the maggot. It feeds on decaying refuse and in this way makes the world somewhat cleaner and more habitable for others.

The Principal Insect Orders

In the following list are the principal orders within the two subclasses of the class Insecta. Several obscure orders with relatively few species are omitted. The orders of the most primitive groups are given at the beginning of the list; the most highly developed at the end. After the name of each order, its meaning is given. The suffix -ptera means “wing”; -aptera, “wingless”; -ura, “tail.”

Subclass Apterygota

(wingless, no metamorphosis)

Thysanura (“tassel tail”) silverfish, bristle-tails, and fire-brats; wingless, scaly, three long bristles at the end of the body.

Collembola (“glue bolt”) spring-tails; tiny, wingless; jump by means of a springlike appendage below the abdomen.

Subclass Pterygota

(winged, undergo metamorphosis)

The following 11 orders are sometimes known as the Exopterygota. These have incomplete metamorphosis.

Orthoptera (“straight wings”) cockroaches, grasshoppers, crickets, walking-sticks, mantids, katydids, locusts, and their allies; fore-wings leathery; hind wings folded fan-wise

Dermaptera (“skin wings”) earwigs; fore-wings short; abdomen ends in a forceps-like appendage.

Plecoptera (“braided wings”) stone flies; membranous wings fold flat over the back; aquatic nymphs breathe with gills.

Isoptera (“equal wings”) termites; social insects with a caste system; resemble ants but have a broad, rather than narrow, waist.

Psocoptera (“gnawers”) psocids, book lice, and their allies; winged or wingless; feed on books and museum specimens.

Mallophaga (“wool eaters”) biting lice; flat, with chewing mouth-parts; external parasites of birds and certain warm-blooded animals.

Ephemeroptera (“living but a day”) mayflies; night-flying, delicate, short-lived; with membranous wings and two or three long tail filaments; nymphs aquatic; adults do not feed.

Anoplura (“unarmed tail”) sucking lice; with piercing mouth-parts for feeding on blood; external parasites of mammals.

Thysanoptera (“fringed wings”) thrips; usually four minute narrow fringed wings; pests of cultivated plants, spread viral plant diseases.

Hemiptera (“half wings”) (includes the order Homoptera) true bugs, aphids, leaf-hoppers, scales, and their allies; mostly four-winged, with piercing or sucking mouth-parts; many are plant pests.

Odonata (“toothed”) dragonflies and damselflies; two similar pairs of long, narrow wings; dragonflies keep wings outstretched at rest, damselflies keep them together over the back.

The remaining orders are sometimes known as the Endopterygota. These have complete metamorphosis.

Neuroptera (“nerve wings”) lacewings, ant lions, snake flies, and dobsonflies; two similar pairs of large, membranous wings, usually folded roof-like over the body when at rest.

Mecoptera (“long wings”) scorpion flies; long-faced, narrow-winged; in some males tip of abdomen curls over the back as a scorpion’s does.

Trichoptera (“hair wings”) caddis flies; adults moth-like but with longer antennae and uncoiled proboscis; larvae aquatic, make fixed or portable cases in which they live and pupate.

Lepidoptera (“scale wings”) moths and butterflies; wings covered with minute, overlapping scales; coiled proboscis usually present.

Coleoptera (“sheath wings”) beetles and weevils; fore-wings hard, vein-less, and opaque, meeting in a straight line; hind wings membranous, translucent; the largest order of insects, numbering some 300,000 species.

Strepsiptera (“twisted wings”) males winged, females wingless; females of most species are parasites on other insects.

Hymenoptera (“membrane wings”) wasps, ants, bees, and their allies; many species useful to man; ovipositors in some females modified as a stinger.

Diptera (“two wings”) true flies, mosquitoes, and midges; two developed wings; mouth-parts variable; many species pupate inside the last larval skin.

Siphonaptera (“siphon wingless”) fleas; tiny, jumping insects with narrow bodies adapted for moving between the hairs of animal hosts, whose blood they suck; some species transmit disease.

Assisted by Thomas Park, Professor Emeritus of Biology, University of Chicago; former President, Ecological Society of America. Critically reviewed and updated by J. Whitfield Gibbons, Senior Research Ecologist and Professor of Zoology, Savannah River Ecology Laboratory, University of Georgia.


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