PARASITES.

Bed bug

An organism that lives on or within another organism, called the host, and that gains its sustenance from the host organism is known as a parasite. Parasites occur among all the major groups of living things. There are parasitic fishes for example, the lamprey, which attaches itself to other fishes and sucks their body fluids. There are many parasitic arthropods, including fleas, lice, biting flies, and mosquitoes.

Many worms are parasitic. Some live in their host's digestive tract and feed on the food that passes through. Some attach to the intestinal wall and suck the host's blood. Some, such as those that cause trichinosis, enter the host through the digestive tract and then burrow into the tissues of the entire body. Some also parasitism plants.

Many fungi are parasitic. The rusts are fungi that are responsible for many diseases of major food plants. Parasitic bacteria are responsible for diseases ranging in severity from acne and tooth decay to such major plagues as the Black Death.

The viruses are unique in that they are all parasitic. They are the smallest of the parasites and may enter the host through the respiratory system or may be spread through sexual contact.

Characteristics

As originally defined, parasites included any organisms that live by drawing food from a host organism. Defined in this broad way, parasitism included relationships that ranged from benign to harmful and even fatal to the host. The term parasitosis was later developed to describe those forms of parasitism that injure the host, and today the term symbiosis describes benign or even mutually beneficial associations between organisms.

Effects on the host. A parasite's effect on its host is determined by various factors. Many parasites, for example, do not reproduce in their hosts, or reproduce only to a limited degree. Such parasites, including many parasitic worms, produce eggs that enter another host before they develop. The damage done by such parasites depends in part on the number of parasites in the host, known as the host's parasite burden. Many hosts can carry a light parasite burden that is, they can support a small number of parasites and suffer no ill effects. A heavy parasite burden, however, may cause severe injury to the host.

In the case of parasites that may undergo unlimited reproduction in their hosts for example, the protozoans, bacteria, and viruses the factors determining the final effect on the host can be quite complicated. The ability of the hosts' natural defences to destroy the parasites often plays a major role. Very young, old, or weak hosts that have limited defences may be severely harmed by large parasite populations that are able to develop unchecked.

Varieties. Parasites are commonly described in terms of their relationships to their hosts. Parasites that remain on the outer surfaces of their hosts are called ectoparasites. Parasitic arthropods are usually ectoparasites. Endoparasites are parasites that live within the bodies of their hosts. The various parasitic worms that live within the hosts' digestive tracts are endoparasites. Many endoparasites actually dwell within the tissues of their hosts, not just in the cavities of the hollow organs. The bacterium Mycobacterium tuberculosis, the most common cause of human tuberculosis, lives within the cells of the lung tissues.

Bedbug, a small, flat, bloodsucking insect (Cimex lectularius), of reddish-brown colour, of order Hemiptera, family Cimicidae; is parasitic on humans.

Parasites may be permanent or temporary residents in or on their hosts. The bedbug is a temporary parasite. It crawls onto its host to feed and then returns to its hiding place, where it spends most of its life. The flatworm that causes a form of human schistosomiasis is a permanent parasite. Once it enters a host's body, it remains there until it dies.

Some organisms can live either as parasites or as free-living forms; they are called facultative parasites. For example, the free-living protozoan Naegleria fowleri, which occurs in streams and lakes around the world, can cause infection of the brain after it enters the noses of swimmers. Other organisms, called obligate parasites, can live only a parasitic existence. Plasmodium falciparum, an organism responsible for a form of human malaria, is an obligate parasite.

Autoecious parasites are parasites that complete their life cycles within a single host. Many parasites, however, have quite complex life cycles and may require more than one host. In some cases the immature stages of the parasite develop in one host, and maturation and sexual development occur in a second host. Hosts in which the immature stages of the parasite develop are referred to as intermediate hosts. Parasites that require two or more hosts to complete their life cycles are referred to as heteroecious.

Malaria, disease consisting usually of successive chill, fever, and "intermission" or period of normality.

The pattern of having more than one host can sometimes provide parasites with a means of spreading. The protozoan that causes malaria has two hosts: humans and certain other animals, and anopheles mosquitoes. Asexual reproduction occurs in infected humans and animals, and sexual maturation, fertilization, and reproduction occur in infected mosquitoes. The protozoans depend on the mosquito to transmit them from one human host to another.

Methods of transmission. An organism that transmits a parasite, as the anopheles mosquito does, is called a vector. Vectors need not transmit parasites by biting, however.

Some vectors transmit parasites when they are eaten by the hosts. Certain tapeworms that infect cats and dogs use fleas as vectors. When the cat or dog swallows a flea that is caught during grooming, the immature forms of the tapeworm emerge from the flea's body and mature in the cat's or dog's intestine. The mature tapeworm produces numerous eggs that then pass out of the animal's body with its faeces and contaminate the environment. If an immature, or larval, flea ingests the tapeworm's eggs as it feeds on the infected faeces, it becomes infected in turn. The parasite's life cycle is completed if the cat or dog catches and eats the mature infected flea. A situation such as this, in which a parasite (the tapeworm) is parasitic upon another parasite (the flea), is referred to as hyper-parasitism

Human Parasites

Parasitism in humans is widespread, but the type of parasite varies with geographic regions and social conditions. In areas where sanitation is poor, parasites that are spread by ingestion of faecal-contaminated food and water are common. In areas where housing is inadequate, parasitic insects may be common.

In parts of the world with adequate sanitation and housing, parasites transmitted by faecal contamination and biting insects are generally rare, but those transmitted by direct contact and through the respiratory system may still be common. The parasites that cause measles, mumps, and chicken pox, for example, can spread rapidly in crowded school environments.

Plant Parasites

Arthropod, animal of the phylum Arthropoda comprising invertebrates with external skeleton, segmented body, and jointed appendages.

In many respects the parasites of plants are similar to the parasites of animals. The arthropods, fungi, worms, bacteria, and viruses that parasitic plants may either grow on the plant's surface or invade the plant's tissues and, in the case of arthropods that suck plant fluids, may also transmit other parasites, particularly viruses.

Some plants have become parasites on other plants. The simplest form of plant parasitism is that in which the parasitic plant uses its host only for support. The strangler fig, a tropical tree that is grown as a common house-plant, slowly surrounds its host tree until the host dies. The fig then has access to the light above the forest canopy and can grow unhindered.

Other parasitic plants, such as the mistletoe, have a somewhat greater dependence on their plant hosts. Mistletoe grows on trees and uses them for support. In addition, though it makes some of its own food, the mistletoe sends modified roots into its host to draw out nutrients.

Dodder, a leafless parasitic plant introduced into U.S. from Europe with clover seeds; now a rapidly growing pest.

The most complete form of plant parasitism is that in which the parasite relies completely on the host for sustenance. Dodder, for example, is a parasitic vine that draws all its nutrients from its host.

Special Types of Parasitism

Entomologists, scientists who study insects, have described a type of parasitism in which one insect, usually a species of wasp, uses another insect to brood its young. This type of parasitism is called parasitoidism. The parasitoid wasp lays its eggs in or on the host insect, commonly a caterpillar. The wasp's larvae develop inside the host, feeding on its body, and emerge as full-grown adults. Parasitoidism is being used by some farmers as a means of pest control. Various parasitic wasps, for example, are used to help control agricultural pests.

Another unusual form of parasitism is brood parasitism, which is common among certain birds, particularly the cow-bird and the cuckoo. In this form of parasitism, the parasitic bird lays its eggs in the nest of another species. The host bird then raises the intruder's young as though they were its own.

A type of parasitism called social parasitism occurs among certain communal insects.

Some species of ants, for example, kidnap and enslave the workers of other ant species.

Assisted by Julius P. Kreier, Professor of Microbiology, Ohio State University, and author of 'Parasitic Protozoa'.

SEXUALITY

Human beings are born sexual. Humans develop a strong sense of being male or female. This sense of maleness or femaleness and the behaviour exhibited because of it is called human sexuality. The characteristics of being a man or a woman involve biological, psychological, and sociological behaviours as well as the reproductive capacity and sexual functions of genital organs

Human sexuality is an integral part of life from birth until death. Throughout history people from all cultures have acted in relationships in part on the basis of both physical and emotional feelings of sexual attraction. Sexual behaviour is also influenced by cultural traditions and laws about sex.

Cultures vary greatly in what kinds of sexual behaviour is permitted. In some societies children are discouraged from knowing anything about sexuality and some children are not allowed to understand sexual reproduction. Other societies encourage children to learn about sex. Parents sometimes exercise their right to educate their children about sex and encourage or discourage sexual activity.

Whatever the practice of the society, children grow up aware of the parental, religious, cultural, and social norms of sexual behaviour Human sexuality is also physically influenced by hormones, brain centres, networks of nerves, and sex organs.

Sexual Development

Uterus (or womb), female organ for holding and nourishing young during prenatal development.

Even before birth the female ovaries develop ova, or eggs. At puberty, under the influence of chemicals called hormones that are produced within the body, the ova ripen and are periodically released from the ovaries about once a month. When this event, termed ovulation, occurs, hormones are released to begin a thickening of the lining of the uterus (endometrium). Hormones control the discharge of the lining about once a month. This is called menstruation.

If a woman becomes pregnant, menstruation does not occur; the endometrium stays in the uterus and serves as the first nourishment for the developing embryo. Ovulation and menstruation begin in girls at puberty at about age 12 and continue until menopause, sometime in middle age. Puberty in boys begins when the testes start to produce sperm continuously at about age 13. This is a life-long process in males.

When ovum and sperm come together, the first complete cell of a potential new human begins to develop. This is called conception. Sexual arousal usually precedes conception and begins when hormones are given off by endocrine glands. This causes body fluids to shift to the pelvic area. In the male the fluid goes into the tube called the penis, and in the female through the walls of the tube called the vagina. The fluid also enlarges the labia and the clitoris in females. The testes in the male enlarge and draw close to the body.

Eventually the penis becomes erect and the vagina becomes lubricated.

Many body muscles become tense during sexual arousal and stimulation. Continued stimulation of the penis and the clitoris causes a spontaneous release of this tension in a pleasurable feeling called orgasm. In the male, this is most often accompanied by ejaculation. Ejaculate is called semen, a combination of sperm and internal fluids made primarily in the prostate gland. Ejaculation during sexual intercourse places the sperm in the vagina where the spermatozoa move through the opening of the cervix into the uterus and through the fallopian tube. If an ovum is present the sperm will be drawn to it. When a sperm cell penetrates the ovum, other sperm cells are prevented from entering.

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.

The combination of ovum and sperm make up a complete cell, containing 23 chromosomes from the sperm and 23 from the ovum. These chromosomes carry all the genetic information needed to determine all the inherited characteristics of the potential human being. Of the 23 pairs of chromosomes, one pair, called sex chromosomes, determine male or female sex. Male sperm cells carry either an X or a Y sex chromosome and 22 non sex chromosomes, or autosomes. Ova carry only X chromosomes and 22 autosomes. If an X-carrying sperm cell unites with the ovum, the cell will develop into a female. If a Y-carrying sperm unites with the ovum, the cell will develop into a male.

Future male or female body structures are thus determined at conception.

Zygote, in biology, a cell formed by the union of male and female gametes; a fertilized egg cell; earliest stage of development, preceding embryonic and fetal stages.

The new, fertilized cell, now called a zygote, divides until it becomes a ball of cells. It moves into the uterus where it implants itself in the endometrium as a blastocyst. The genital systems of humans appear by the fifth to sixth week of embryonic development.

Under the influence of sex hormones, the fetal body differentiates and develops sexual structures by about the seventh week. By the fourth month the foetus’s sex is unmistakably recognizable. This happens under the influence or lack of influence of male hormones testosterone and androgens. The absence of these hormones allows the fetal body to develop as a female. The presence or absence of these hormones is directed by the genes on the XX or XY chromosome pair.

Both male and female reproductive systems develop from the same structural origins. The male fetal structures have female counterparts. (It can also be said that female fetal structures have male counterparts.) Among these structures, called homologous pairs, are the testes and ovary.

Infants experience sexual arousal, which is a biological response, before birth. Young boys experience penile erections, girls vaginal lubrication. In childhood, sex play is a common and normal behaviour When sex play involves the rubbing or self-stimulation of genital organs, it is called masturbation. Masturbation is found in all cultures and is not physically harmful. Many religious groups, however, discourage it.

Sexual arousal expressed in sex play with other children is considered childhood sexuality, not early "adult" behaviour It is a normal exploration of the body and is often pleasurable. In Western culture it is sometimes followed by feelings of guilt because of disapproval from parents or religious authorities.

Adult patterns of sexual development begin at puberty and during adolescence. Hormones cause a rapid growth in height. Sex differences occur in bone and muscle density, breast development, and body and facial-hair patterns. The vagina gradually lengthens and the uterus enlarges. The penis and testes increase in size. Both male and female voices lower.

Girls' bodies begin to grow at about age 12. They usually complete much of their growth rather quickly. Boys begin later, at about age 13, and grow for a longer period of time. Every person is unique in growth rate and development of adult sexual characteristics. It is considered within normal range for the process to begin as late as 16 years.

Feelings of sexual desire begin at puberty. This is often accompanied by fantasy, daydreams, or infatuation. Feelings of sexual attraction can generate social interaction that begins the romance and dating process. At this point, or sooner, boys begin to experience erotic dreams accompanied by orgasm and ejaculation. Girls also experience orgasms during sleep.

Dream, illusion or hallucination of real experiences that occur during sleep.

Normal sleep patterns involve rapid eye movements in about 90-minute intervals. It is during these periods that dreaming, effective rest, slight erections of the penis, and slight engorgement of the labia and clitoris in the female occur. These are biological responses and are not psychologically induced.

Feelings of sexual attraction and attentiveness are a part of the search for self-understanding. Parents and society often guide young people about what is permissible and avoidable sexual behaviour They often caution against the risk of pregnancy, sexually transmitted diseases, and emotional injury. Adults have a desire to protect youths from adverse experiences. Even if sexual feelings are strong, humans can consciously decide whether or not to act on those feelings.

Adolescence is a time for physical, psychological, and social self-discovery. It is a period when children learn about themselves in relation to other people and the community in which they live. Conflict between adolescents and their parents can occur as the search for self-identity leads to a desire for more independence from the family. Parental response during adolescence often includes fear for the health and safety of offspring. Dating is a way in which young people learn about both themselves and the people that attract them.

There is a general expectation in many societies that people will choose one marriage partner sometime in young adult years.

Gender Orientation

People grow up knowing themselves as male or female. The human behaviour associated with being a man or being a woman is called gender identity. Among the influences on gender identity are body development, sexual organs, socialization as a boy or a girl, brain hormones that determine our knowledge of our male or female nature, and pubertal hormones that affect both sexual structure and sexual function. Gender identity is related to physical appearances, feelings of arousal and attraction, and desires to dress and act socially in ways considered male or female.

People have both male and female hormones in their bodies. The balance of these hormones allows us to be one sex or the other. Hormonal imbalance in a foetus can cause abnormalities in physical sexual development prior to birth. Cultures promote acceptable behaviours for roles based on sex, called gender roles. Many of these roles are partly determined by the person's function within the family and economy. In Western culture the family represents a unit based on love, nurture, economic interdependence, and preparation of the young for adult life. For centuries physical work outside the home was perceived as a part of the male role. The female role was to give birth and direct the maintenance of the home. Work-related gender roles have changed. Families often need the earnings of both parents for financial survival. Women work out of necessity, desire, or both. Job qualifications are no longer gender-specific but focus instead on skill, knowledge, and experience.

Sexual Orientation

Homosexuality, the manifestation of sexual desire toward a member of one's own sex.

What determines whether or not two persons are attracted to one another? The answer to this question is sexual orientation or sexual identity. In the 1940s Alfred Kinsey studied sexual practices in the United States and devised a scale for sexual orientation. The scale ranges from heterosexuality, or basic sexual attraction to the other sex, to homosexuality, or basic sexual attraction to the same sex. Midway on the scale is bisexuality, which means sexual attraction to both sexes. Kinsey concluded that most people do not exhibit exclusively heterosexual or homosexual behaviour Many adults, however, label themselves as one or the other. About 10 percent of the people in Kinsey's study identified themselves as homosexual. Whether homosexuality was condemned or accepted, every civilization throughout history has included homosexual men and women.

There have been many attempts to explain the origin of sexual orientation. Some believe there is evidence that it is biological. Others believe it is learned behaviour Sexual orientation is very complex and most likely a combination of many factors. There are many religious and cultural attitudes about sexual orientation. Many religions specifically allow, encourage, or condemn various sexual behaviours This may involve an attempt to encourage people to conform to heterosexuality to ensure the continuance of the reproductive family.

Sexual Fantasy

Sexual fantasy or daydreaming is another aspect of human sexuality. This commonly used outlet for sexual feeling can be pleasurable, humorous, and even satisfying. It can also include imagined hostility and behaviours that, if acted upon, would be harmful. Some people feel that portrayals of sex and violence on television and in cinema and music, encourage more sex and violence in real life. There are recorded fantasies of sexual experience with every possible object, animal, or person. During fantasy, the person becomes sexually aroused but usually has no intention of acting out the fantasy. Attempts to act out fantasies are often expressed in art, literature, and the theatre Fantasy portrayed in explicit pictures or words is called pornography.

Attitudes About Sexuality

Western cultural attitudes about sexuality have been greatly influenced by religious attitudes. These came from times of the Old Testament, the Christian church, and the Middle Ages. The Enlightenment, the Puritans in the United States, and Victorian attitudes all have had a profound influence on current sexual attitudes. One attitude, for example, forbids sex outside of marriage. Despite this, society encourages through the media acceptance of various forms of sexual expression in order to entertain. This encouragement often elicits strong reaction from religious communities. Cultural expressions of sexual attitudes are found in religion, novels, films, paintings, music, television, theatre, and formal education. All of these things influence sexual development.

A major role of sex education is to teach the positive nature of sexuality. This aspect is often neglected because priority is given to the perceived danger of pregnancy for young people and of sexually transmitted diseases. The appropriate content of a sex education program is determined by the students' age level from kindergarten to adult. The purpose of sex education classes is to teach communication and decision-making skills and the anatomy and physiology of the sexual reproductive systems. Classes often include discussions of rape, sexual abuse, abortion, contraception, masturbation, sexually transmitted diseases, pregnancy, childbirth, dating, marriage, and family life. Many religious groups and both public and private schools have sex education programs.

In some communities there are organized groups of adults who oppose sex education in the public schools on the basis of values. It is said that family values about sex are personal and private, that teachers will change that private nature and make statements contrary to family values and beliefs. While the right of parents to teach their children their own values about sex is recognized, there is still debate about the extent to which special interest groups can control sex education in public schools.

Sexuality and Civil Rights

Discrimination, prejudiced or prejudicial outlook, treatment, or action.

Many people concerned with discrimination in society on the basis of sex or sexual orientation become involved with specialized groups that monitor and lobby in government for the rights of all women and men as well as such specific groups as homosexuals, also called gay people. Civil rights groups work toward equal opportunity in employment, housing, and business dealings. There are also concerns about how sexual privacy laws in many states dictate what kinds of sexual practices are allowed for consenting adults or for married people. Many people express a desire to act in any way that is not destructive.

The courts deal with issues of the right to mutually consenting behaviour, the sale of pornography, and various forms of discrimination in the workplace.

Sexuality and Health

There are many health concerns related to sexuality. For youths concerns often arise when their bodies begin to mature. Uneven growth, lumps, unfamiliar feelings, and aches can create fear of abnormality. A physician can usually reassure the person of normality or have abnormalities corrected.

Sterility, in biology, the inability to produce offspring; one cause is the production of non functioning sex cells.

Infertility is the inability to get pregnant or to impregnate for a period of time. A permanent state of infertility is called sterility. In men, the most common cause of infertility is low sperm count. In women, failure to ovulate or blocked fallopian tubes commonly cause infertility. Other causes include infections caused by sexually transmitted diseases that scar the tubes, or severe malnutrition, as a result of anorexia nervosa.

Infertility is often treated with drugs, microsurgery or artificial insemination.

AIDS (acquired immunodeficiency syndrome), incurable disease caused by a virus that damages the human body's immune system; believed to be transmitted through sexual contacts, blood transfusions, or contaminated needles used for intravenous drug injections; often fatal; high percentage of victims are homosexuals or drug abusers.

Sexually transmitted disease is a serious public health issue worldwide. Acquired immune deficiency syndrome, or AIDS, is usually transmitted through sexual intercourse, but it can also be transmitted at birth or with blood products. It is fatal. Other sexually transmitted diseases also require medical treatment.

Sexual abuse and molestation are health concerns that involve deliberate sexual acts, often between adults and children. The adults can be parents, older siblings, other relatives, babysitters, neighbours, or strangers. Sexual abuse and molestation occur frequently. When the child or teenager is being stroked, touched, kissed, or sexually approached in any way by an adult, there is an assumed impropriety. Adults who choose children as the object of their sexual desire are called paedophiles Sexual behaviour between close relatives is called incest. Children who grow up with sexual advances from older, more powerful strangers or relatives can get both help and understanding from school or social work professionals.

Sexual therapy. For many reasons, many of them psychological, people can become unable to function sexually. Some experiences in peoples' lives can cause attitudes about sex that are disturbing. There can also be physical reasons for being unable to function in a sexually satisfying way. This is called sexual dysfunction. For men it commonly results in either the inability to have an erection or unplanned immediate ejaculation after the penis becomes erect. The most common sexual dysfunction in women is the inability to have an orgasm or painful constriction of the muscle at the opening of the vagina when intercourse is attempted (vaginissimus). These problems can occur occasionally or be prolonged and cause anxiety. Sexual dysfunctions are often treated by a therapist in what is called sex therapy.

Assisted by Mary Lee Tatum, teacher and consultant for family life and sex education programs in Virginia.

BIBLIOGRAPHY FOR SEXUALITY

Bell, Ruth and others. Changing Bodies, Changing Lives (Random, 1980).

Boston Women's Health Book Collective. Our Bodies, Ourselves, rev. 2nd ed. (Simon & Schuster, 1976).

Calderone, Mary and Johson, Eric. The Family Book About Sexuality (Harper, 1981).

Kelly, Gary. Learning About Sex: The Contemporary Guide for Young Adults (Barron's Educational Series, 1976).

Madaras, Area and Lynda. The What's Happening to My Body? Book for Girls (Newmarket, 1983).

Madaras, Lynda and Saavedra, Dane. The What's Happening to My Body? Book for Boys (Newmarket, 1984).

Nilsson, Lennart and others. A Child Is Born (Delacorte, 1977).

BATS

Because they fly, bats are often mistaken for birds. Bats are mammals, however, not birds. They have soft fur and large ears. The young are born alive and get milk from their mothers.

Bats are completely specialized for flying. The bones in a bat's wings are similar to those in a humans arms and hands. A bat holds objects with its mouth and with the hooks on its wings. It cannot walk or run. Its legs and feet are only strong enough to hold on when it hangs upside down from a perch, which is how bats rest or sleep. To start flying a bat drops from its perch. If it is on the ground it usually crawls to some height, using its wing hooks and feeble legs, until it can drop into the air. It can, however, take off from a position on the ground if necessary.

Bats can chase insects through thick forests on the darkest night without striking a branch or twig. To learn how they do so, experimenters sealed the eyes of several bats with gum and released them in a large room where many ropes hung from the ceiling. The bats flew about with their usual bullet speed without once hitting a rope.

When their ears and mouths were sealed, however, they blundered about helplessly. Further study showed that a flying bat gives a continuous cry, so high-pitched that people cannot hear it. The high-frequency waves of the cry, like radar waves, are reflected by all obstacles in the bat's path and echo back to its sensitive ears. The animal instantly responds to the signals and avoids the obstacles.

Habits of Brown and Red Bats

About 900 kinds of bats are known, more than any other kind of mammal except rodents. One kind or another can be found in all but the coldest climates. In North America the little brown bat is the most abundant. It ranges from Alaska southward. (There is also a big brown bat in the United States.)

The little brown bat is only 3 1/2 inches (9 centimetres) long and weighs about 1/2 ounce (15 grams). It hunts insects in the air, and it may catch beetles, crickets, and other insects while crawling on the ground. It spends its daylight hours in caves and hollow trees, under eaves and roofs, and in attics.

Between May and July the female brown bats gather in dark hiding places in colonies of a hundred or more. They drive away all the males, and each one gives birth to a single, naked, pink, blind baby. There is no nest. When the mother goes out she hangs the baby up by its feet. The young reach full size and can fly when they are three weeks old.

The red bat is common from Canada southward. It is thought by many to be a beautiful little animal, about 4 1/2 inches (11.5 centimetres) long, with soft, fluffy fur. Males are orange red, frosted with white; females are a delicate chestnut.

Red bats live in forests and spend the daylight hours hidden among the leaves. Females bear from one to four young. For a few days the babies cling to the mother's breast as she flies about seeking food.

Bats may live ten years or longer. Owls are among their few enemies. During the winter the little brown bats hibernate in caves. Red bats migrate in winter to the southern limits of their range.

Harmful and Helpful Bats

In the tropics some bats are huge. The flying foxes of the Malay region may have a wingspread of 5 feet (1.5 meters). These and other large tropical bats are fruit eaters and do great damage to crops. Flying foxes are found in Madagascar, tropical Asia, eastern Australia, and on South Pacific islands. If fruit is lacking they live by fishing. They skim over the surface of bodies of water and catch fish with their feet. Many other bats also catch fish. All bats skim the surface of water to scoop up drinks.

Vampire bats are common in parts of Central and South America but are never found in the United States. They puncture the skins of animals and sleeping humans and lap up blood as it oozes out. For some unknown reason this process is painless, and the victim is usually unaware of it.

Some vampire bats spread disease among cattle and horses. These bats get their name from the legendary vampires, which were believed to come out of their graves at night to suck blood from the living.

Insect-eating bats are extremely useful to humans. A brown bat may eat half its weight in insects in one night. In some parts of the southern United States, huge roosts, or shelters, have been erected for them to get their help in fighting insects.

The guano, or droppings, that bats deposit year after year while occupying a roost is a valuable material for fertilizer. One deposit in Carlsbad Caverns is more than a quarter of a mile (half a kilometre) long, 100 feet (30 meters) wide, and 100 feet deep. Some deposits are mined and sold.

It is not true that bats carry common lice or bedbugs. Bat parasites are peculiar to bats and will not attack humans. It is also not true that bats willingly entangle themselves in women's hair.

The insect-eating bats of the United States and Canada usually do their best to avoid humans. On rare occasions, however, a bat suffering from rabies, or hydrophobia, will bite a human. Any person bitten by a bat should consult a doctor immediately for tests and for treatment if needed.

If a bat flies into a room through an opening, it may be easily caught and removed. Shut the door so it cannot get into the rest of the house. Soon it will be fluttering in a corner of the ceiling, trying to escape. Then toss up a large piece of soft cloth. The bat will become entangled in it and will fall with it to the floor. Open the cloth outdoors to release the bat.

Bats belong to the order Chiroptera, a name that comes from Greek words meaning "hand-winged." They are divided into two suborders: Megachiroptera and Microchiroptera. Megachiroptera, including fruit bats and flying foxes, are fruit- or flower-eating natives of the Old World tropics. Most of this suborder depend heavily on vision to avoid obstacles.

The suborder Microchiroptera is comprised of small bats that generally prey on insects, but a number of them feed on fruit, flowers, blood, and small animals. They orient themselves by sound.

The scientific name of the little brown bat is Myotis lucifugus; the big brown bat is named Eptesicus fuscus; the red bat is named Lasiurus borealis. The common vampire bat is Desmodus rotundus.

AIDS (ACQUIRED IMMUNODEFICIENCY SYNDROME)

HIV

The disease known as AIDS is a complicated illness that may involve several phases. It is caused by a virus that can be passed from person to person. AIDS impairs the human body's immune system the system responsible for warding off disease and leaves the victim susceptible to various infections.

AIDS was first conclusively identified in the United States in 1981, when 189 cases were reported to the Centers for Disease Control. Within a decade the disease had spread to virtually all populated areas of the world. In the United States alone there are about 65,000 new cases every year. The origin of the AIDS virus is uncertain, but it may have originated in Central Africa.

The first AIDS patients in the Americas and Europe were almost exclusively male homosexuals. Later patients included those who used unsterilised intravenous needles to inject illicit drugs; haemophiliacs (persons with a blood-clotting disorder) and others who had received blood transfusions; females whose male sexual partners had AIDS; and the children of such couples. However, since 1989, heterosexual sex was found to be the fastest growing means of transmission of the virus, with 90 percent of new cases originating from heterosexual sex.

Public awareness of the disease gradually built up as high-profile victims began to die: actor Rock Hudson (1985), clothes designer Perry Ellis (1986), choreographer Michael Bennett (1987), photographer Robert Mapplethorpe (1989), and Oscar-winning director Tony Richardson (1991). When basketball superstar Magic Johnson announced in 1991 that he had contracted the AIDS virus, the feeling spread quickly that anyone, not just particular groups of people, could be at risk. This was again confirmed as tennis legend Arthur Ashe announced in 1992 that he had been infected with the virus for several years.

The AIDS virus. American researchers initially named the virus that causes AIDS the human T-lymphotropic virus, type III or HTLV-III. After researchers discovered in the late 1980s that there were several forms of the AIDS virus, the original virus was renamed the human immunodeficiency virus type 1, or HIV-1.

The virus enters the bloodstream and destroys certain white blood cells, called T lymphocytes, that play a key role in the functioning of the immune system. The virus can also infect other types of cells in the body, including the immune-system cells known as macrophages. Unlike T lymphocytes, however, macrophages are not killed by the virus. In fact, research has suggested that macrophages may carry the AIDS virus to healthy brain cells, to the lymphatic system, and to other healthy cells in the body.

What happens after infection. Most people recently infected by the AIDS virus look and feel healthy. In some people the virus may remain inactive, and these people act as carriers, remaining apparently healthy but still able to infect others. After a few years, some people may develop AIDS-related complex, or ARC. Its symptoms may include fever, fatigue, weight loss, skin rashes, a fungal infection of the mouth known as thrush, lack of resistance to infection, and swollen lymph nodes. Sometimes the symptoms of ARC disappear, but the condition frequently goes on to become AIDS. Though it can take up to 20 years after the virus is contracted for AIDS to fully manifest itself, the average time is one to two years.

The AIDS virus causes so much damage to the immune system that the body becomes susceptible to a variety of opportunistic infections infections that are less harmful to people with normal immune systems but take advantage of the breakdown in an AIDS sufferer's immune system to produce devastating and eventually lethal diseases. Among the most frequently occurring opportunistic infections are tuberculosis and a type of pneumonia caused by the micro-organism Pneumocystis carinii. AIDS sufferers are also more likely to develop certain tumours, particularly Kaposi's sarcoma, a rare form of cancer. The AIDS virus may also attack the nervous system and cause brain and eye damage. The average life expectancy for an AIDS victim from the onset of symptoms is one to five years.

How AIDS is spread. AIDS is transmitted by direct contamination of the bloodstream with body fluids that contain the AIDS virus, particularly blood and semen from an HIV-infected person. The virus is usually transmitted through various forms of sexual intercourse, the transfusion of virus-contaminated blood, or the sharing of HIV-contaminated intravenous needles.

The AIDS virus cannot penetrate intact bodily surfaces, such as skin, and quickly perishes outside the human body. Consequently, AIDS is not spread by casual physical contact or by sneezing. The virus has been found in tears and saliva, but it exists there in such low concentrations that transmission from these body fluids is extremely rare. There are no known cases of AIDS transmission by insects such as mosquitoes or by domestic animals. Studies show that the virus is usually passed to an infant close to or during delivery, rather than moving across the placenta during pregnancy. Recently infected mothers can transmit the virus to their children via breast milk. The United States Congress approved guidelines recommending that health care workers who perform invasive procedures be tested for the AIDS virus but the testing and disclosure of results would be voluntary; no restrictions would be placed on those who tested positive.

There are several ways to reduce the spread of AIDS through sexual contact. These include practising abstinence no intercourse or practising safe sex. Practising safe sex means either participating only in a monogamous, or mutually exclusive, relationship in which both people are free of HIV infection, or using latex condoms whenever engaging in intercourse.

Detection and treatment. Usually, when the AIDS virus enters the bloodstream, the body's immune system produces antibodies to battle the micro-organism Blood tests can detect these antibodies and therefore can indicate exposure to the virus. However, these tests occasionally give false readings and only begin to give accurate results within two weeks to three months after infection, during which time an infected person may pass the virus to others. Scientists do not know exactly how the AIDS virus damages the immune system, nor do they understand why the natural antibodies developed to destroy the virus are ineffective.

By 1987 the drug azidothymidine (AZT) had proved effective in slowing the reproduction of the HIV-1 virus in humans, but it is highly toxic and cannot be taken by many patients. In 1989 researchers determined that lower doses of AZT would be effective and less harmful for patients that have early symptoms of AIDS and for children with AIDS.

Dideoxyinosine (DDI) was approved in the United States in 1991 for the treatment of HIV infection. This drug is a useful replacement for AZT and is used in children and other patients for whom AZT is too toxic. In 1992 zalcitabine, or DDC, became the third drug approved to treat people infected with the AIDS virus. It was, however, approved for use only in combination with AZT to treat adults with advanced HIV infection.

Several other drugs and treatments have recently been approved or become available experimentally for the treatment of P. carinii pneumonia, Kaposi's sarcoma, and other AIDS-related conditions. Several vaccines against AIDS are being developed and tested.

BIBLIOGRAPHY FOR AIDS

Cozic, C.P., and Swisher, Karin, eds. The AIDS Crisis (Greenhaven, 1991).

Hein, Karen, and others. AIDS: Trading Fears for Facts, updated ed. (Consumer Reports Books, 1991).

Tiffany, Jennifer, and others. Talking with Kids About AIDS (Parent AIDS, 1993).

INSECTS (Part 1 of 2)

Carpenter ant

The total number of people, plants, and animals in the world is smaller than the total number of insects in the world.

Although over 800,000 insects have been described and named, there are still so many different kinds of insects on earth that scientists have named fewer than half of them. There are so many insects around us because insects are able to stay alive under many different conditions.

The world's most abundant creatures are the insects, whose known species outnumber all the other animals and the plants combined. Insects have been so successful in their fight for life that they are sometimes described as the human race's closest rivals for domination of the Earth. Entomologists, the scientists who study insects, have named almost 1,000,000 species perhaps less than one third of the total number.

Many kinds of insects are very adaptable. This means that they can live in almost any kind of weather and eat almost any type of food and still survive.

The most adaptable insects are usually small, require little food, and produce many young in a very short time. It is hard for humans to get rid of these insects. One example of a highly adaptable insect is the common cockroach, which can be found all over the world.

Insects thrive in almost any habitat where life is possible. Some are found only in the Arctic regions, and some live only in deserts. Others thrive only in fresh water or only in brackish water. Many species of insects are able to tolerate both freezing and tropical temperatures. Such hardy species are often found to range widely over the Earth. Few insects, however, inhabit marine environments. Small size, relatively minor food requirements, and rapid reproduction have all aided in perpetuating the many species of insects.

Many insects are parasites. Parasites cannot stay alive by themselves. They must live off the body of another organism, called the host.

The parasite stays alive by using the host's body for food, water, warmth, and protection. Sooner or later, the parasites take so much from the host that they cause the host to die. An insect parasite may spend all or only part of its life inside a host.

Certain parasitic insects spend much of their lives on or within the body of an animal host, where all the comforts of life food, moisture, warmth, protection from enemies are optimal. Other kinds of insects spend all or some part of their lives securely enclosed in a food plant.

Rain, wind, cold, or human activity can quickly endanger insects. When insects are in trouble, they can:

• fly, swim, or run away,
• use their mouths or legs for fighting,
• squirt poison at their enemies,
• make themselves look bigger or smaller,
• blend into the ground or plants around them,

or

• use protective armour or spines.

Some species have become remarkably versatile in order to meet the changing demands of the environment. Various water bugs and water beetles are able to fly and swim, as well as crawl. Many types of insects, such as the bees, ants, and wasps depend on a complex social structure and defensive behaviour Non-predatory species frequently have special defences, such as an unpleasant taste or odour, venomous spines, or camouflage.

Insect numbers are kept down by sudden weather changes and by the creatures that eat them. Birds, fish, bats, spiders, and many other forms of life depend on insects for their food. In some areas, a very cold winter will kill many insects that would otherwise multiply in the spring.

Although they are adaptable and versatile as a group, insects are often unable to adjust to unusual weather conditions. Excessive rain, an unusually early frost, an extended drought these and other weather extremes can quickly wipe out or drastically reduce insect populations in a region. Because insects are an important item in the diet of many other animals birds, reptiles, amphibians, and fish, as well as other insects the number is constantly held in check.

The total of all factors unfavourable to insect survival is overwhelming; thus, in some species, out of hundreds of eggs laid by a single female, seldom do more than a few individuals reach adulthood. The survival of some species is enhanced by the large numbers of eggs laid.

INSECT STRUCTURE AND FUNCTION

Despite their diversity, all adult insects share some basic external and internal anatomical features. Insects are distinguished from other members of the animal kingdom by having six legs; one pair of antennae; a ringed, or segmented, body; and three well-defined body regions. It is from the joined body rings, or segments, that insects derived their name, for the Latin word insecta means "segmented."

Many creatures closely resemble insects and are often mistaken for them for instance, spiders and scorpions, which have eight legs; centipedes, which have dozens of legs; and mites and ticks, which have sac-like bodies unbroken by segments. The name bug refers to certain insects with piercing and sucking mouth-parts but is also commonly applied to insects in general.

External Anatomy

The three main sections of an insect body are the head; the middle section, or thorax; and the hind section, or abdomen. The body is covered with a horny substance containing chitin. The protective armour plate also serves as an external skeleton, or exoskeleton, for the support of the internal organs.

The head bears the antennae, the mouth-parts, and the eyes. The thorax has three segments; on each is a pair of legs. In winged insects the thorax also bears one or two pairs of wings. The abdomen typically has 11 segments, though no more than 10 are visible; it contains a large part of the digestive system. In females the ovipositor, or egg-laying organ, is located at the tip of the abdomen.

Internal Organs

The nervous system of the insect includes a brain and a pair of parallel nerve cords, which extend along the length of the underside of the body. Along the nerve cords are a series of nerve masses, called ganglia. Each ganglion controls certain activities and is more or less independent of the others.

Insect blood is usually green, yellow, or colourless Few insects have red blood. The fluid is not enclosed in a system of arteries, veins, and capillaries but fills the body cavity. It is circulated by a tube that extends down the length of the body along the centre of the back. The tube has valved intake openings along its sides and is open at the anterior, or front, end. By means of muscles, it draws the blood through the side openings and pumps it forward into the head cavity and out again into the body. The pulsations of the tube can be easily seen in light-coloured caterpillars.

Air enters the body through breathing pores, called spiracles. A pair of spiracles is usually found on each of two thoracic segments and on several abdominal segments. From the spiracles, large air tubes called tracheae and smaller ones known as tracheoles carry air to all parts of the body. Some water insects breathe by means of gills. Other aquatic insects have a snorkel-like tube that leads to the water's surface. Certain internal parasites and very primitive insects breathe directly through the body wall.

Mouth-parts

The mouth-parts of an insect can tell us about the kinds of food an insect eats.

Moths, cicadas, and butterflies have long slender tubes that they use to suck nectar out of flowers and into their mouths.

Caterpillars, grasshoppers,beetles, and crickets have chewing mouth-parts that may seem more complex than even a human mouth. The lower mouth-parts hold the food,and the upper mouth-parts chew the food.

Mouth-parts vary with feeding habits. For example, the mouth of a chewing insect, such as the grasshopper, has several parts. There is an upper lip, the labrum, and a lower lip, the labium. Between these are two pairs of jaws, which work sideways. The upper jaws, or mandibles, are for crushing; the lower pair, the maxillae, manipulate the food. On the maxillae and on the labium are two pairs of sensory structures called palpi. On the floor of the mouth is the tongue-like hypopharynx, which secretes digestive juices.

The sucking type of mouth is a modification of the chewing type. The butterfly's coiled proboscis, or sucking tube, is a modification of the maxillae.

Sense Organs

The sense organs of insects are as varied as they are intricate. In some of these creatures the visual organs are capable of nothing more than distinguishing night from day. Others have eyes as efficient and sensitive as those of the vertebrates.

Insects have two different kinds of eyes: simple eyes and compound eyes.

Simple eyes are not paired, are very small, and act mainly as light detectors. They often act as helpers to the compound eyes, so the insect can react more quickly to any changes in the amount of light that is present. This is very important because many insects use light to tell them where they are. If you covered the simple eyes of a honeybee, it would still see with its compound eyes, but it would not be able to react as quickly to changes in light.

Insect eyes are of two general types simple and compound. Simple eyes, also called ocelli, are usually located in small clusters on the sides of the head or on the frons, or forehead. Although small, they may easily be seen by means of a magnifying glass. Ocelli are found in both immature and mature insects, but they appear to be more important in the mature forms. Individually these organs can do no more than detect light; however, the sensations received by several ocelli can together produce in the insect's brain an image of the surrounding area as the creature turns its head from side to side.

A compound eye is actually a group of many eyes clustered together. These tiny eyes look like lenses or facets of the compound eye. Each one points in a slightly different direction and sees only a very small part of the world. The insect brain is able to put all of these tiny pictures together like a puzzle and form one overall picture. The more facets an insect has on its compound eye, the better it sees. Compound eyes are better than our eyes at seeing movement, which is very important since this may warn the insect that an enemy is near.

Compound eyes, like the sight organs of higher animals, are present in pairs, with one eye on each side of the head. They are most common in adult insects. Some certain mayflies, for example have two pairs of compound eyes.

The eyes are called compound because each one is composed of many lens-like facets. Each of these facets of which there are, for example, some 25,000 in a single dragonfly eye receives a separate image. The total effect of these images is a composite picture in the insect's brain. The eyes of many insects bees, for example are sensitive to ultraviolet light, but insect eyes are generally less sensitive to colours at the red end of the spectrum.

The antennae are vital structures, because organs of taste, touch, smell, and hearing may be located in them. The loss of the pair of antennae usually leaves the insect in a shocked and helpless state. Their appearance and structure may vary greatly, even between insects of the same order.

The hearing organs of insects are well developed in many species and are found on various parts of the body. The ears of katydids and crickets are located on the tibiae of the forelegs. The typical grasshopper's ear is clearly visible as an oval plate on the first abdominal segment.

The "Voices" of Insects

Insect sounds are produced by specialized structures to attract the opposite sex, to communicate with other members of a group, or to frighten enemies. Wings or mouth-parts may be rubbed together. Legs may be scraped against wings or bodies.

The grubs of certain wood-boring beetles produce sound by rubbing their legs together. The male cicada vibrates miniature "drum-heads" on the lower surface of its abdomen. The song of the female mosquito comes from the vibration of special bands stretched across its breathing organs.

Growth and Development

As they grow, the bodies of some insects go through major changes. Insect bodies are very different from human bodies. Whereas humans have an internal skeleton, the insect's skeleton is a thick, hard, outer layer called an exoskeleton. Since this exoskeleton does not stretch, the insect must replace it with a larger covering when its body needs to grow.

There are different kinds of growth and development in insects. Some insects go through many changes from egg to adult, and some go through very few. Below are the types of development and examples of insects that grow that way.

DEVELOPMENT	CHANGES	EXAMPLES
No metamorphosis	Little change in appearance from birth to adult	Silverfish Cockroaches
Incomplete metamorphosis	Young look like adults, but body parts do not work as they will in the adult	Grasshoppers Crickets Cicadas
Complete metamorphosis	Insect goes through many very different changes before becoming an adult	Butterflies Ants Bees

The development from egg to adult is most interesting, especially in those insects that go through the complex changes called complete metamorphosis. The growth of insects is quite different from that of vertebrates because the insect skeleton is an external covering rather than an internal framework. Except for the pliable fold between the plates of chitinous cuticle making up the exoskeleton, there is no place where expansion can occur; thus the growing insect must periodically shed, or moult, its covering. The new skin, already formed, then expands and begins to harden.

The offspring of all insects undergo a varying number of such growth intervals before maturity. Adult insects do not grow at all. With the exception of the subimago (subadult) stage of the mayfly, only adults have functional wings. Primitive species such as silverfish mature with little change in appearance except their size. These kinds of insects are known as ametabolous insects. The immature insects of such species are called simply the "young."

Immature grasshoppers, cicadas, the true bugs, and a number of other types resemble the adults in many respects but lack functional wings. Such young, called nymphs, are hemimetabolous or are said to exhibit incomplete metamorphosis. A variation of such development occurs in dragonflies, mayflies, and caddis flies. The nymphs of these forms are aquatic and have a way of life quite unlike that of the adults.

Bees, beetles, butterflies, and moths are some of the insects that go through all the changes of complete metamorphosis. They are said to be holometabolous. The young are called larvae (singular, larva). In the inactive stage immediately preceding adulthood they are called pupae (singular, pupa).

The larva hatches from an egg. Often larvae are mistaken for worms. They may be smooth-bodied, like the maggots of the fly, or hairy, like some caterpillars (literally, "hairy cat"), or they may be vicious-looking, like the grub of the tiger beetle. Larvae are classified into five forms, based on their shape: eruciform (caterpillar-like), scarabaeiform (grub-like), campodeiform (elongated, flattened, and active), elateriform (wire worm-like), and vermiform (maggot-like). Larvae differ from adults in many respects. The mouth-parts may be completely different. The mouth is always well developed, for this stage is the hungriest period of the insect's life. Eyes, if present, are usually simple rather than compound. Certain structures found in the larva may be absent in the adult. Caterpillars, for example, have several additional legs, called prolegs, along the abdomen.

Near the end of its larval stage, the insect must find a place in which to pupate, or turn into a pupa. Beetle larvae, as well as certain caterpillars, may hollow out cells in the soil. Some caterpillars may spin silken cocoons about their bodies; some may spin bands to hold themselves against twigs or leaves. Some caterpillars hang upside down from silken pads. Hairy caterpillars pluck out their hairs to line the walls of their cocoons.

The pupal stage is a time of tissue transformation. During this period different kinds of mouth-parts, legs, eyes, and, perhaps, breathing organs must replace those of the larva. When the changes are completed, the creature bursts out of its old skin to become a fully developed insect. In this final, sexually mature state, it is also known as an imago.

HABITS AND BEHAVIOR

Each species of insect, in its struggle for survival, has developed complex behaviour mechanisms and habits. These involve every activity of daily life including egg laying, nest building, self-defence, and the search for food.

Egg Laying and Care of the Young

Most species of insects are of two sexes, but in some the white-fringed beetle, for example males are unknown. In certain insects the sex of the offspring depends upon whether or not the egg has been fertilized. The un-mated females of some parasitic wasps produce males only, while mated ones produce the two sexes in about equal numbers. The queen honeybee can lay either fertilized or un-fertilized eggs, according to the needs of the hive. Un-fertilized eggs produce drones, while fertilized eggs produce females.

The adult female instinctively places her eggs in a place suitable for their hatching and for proper development of the young. Parasitic wasps and flies place their eggs directly on the host. The horse botfly glues her eggs to the hairs of the horse where they can be licked off and thus be transferred to the horse's stomach; the larvae live on the lining of the stomach and intestines. If a caterpillar feeds on only one species of plant, then the egg from which it will hatch is unerringly placed upon that plant.

Often insects' eggs are hidden in special protective materials. They may be encased in frothy secretions which dry to form a hard covering, or clusters of eggs may be coated with hairs or scales from the adult insect's body. The eggs of many species are inserted directly into plant tissues by means of saw-like or spear-like ovipositors.

The young of some insects are born alive. Such insects are called viviparous (from Latin vivus, "alive," and parere, "bring forth"), to distinguish them from egg-laying insects, which are called oviparous (from Latin ovum, "egg"). Aphids sometimes lay eggs and sometimes produce live young; female aphids also bear young for many generations without mating. This is called parthenogenesis (from the Greek words meaning "virgin birth"). A few insects reproduce in the larval or pupal stages. This is known as paedogenesis (from the Greek words meaning "birth from young").

Nests

Nest building as an adult activity is peculiar to ants, wasps, and bees. Carpenter ants live in galleries, which they chew out of tree trunks, logs, and fence posts. Mound-building ants construct cities in the soil, with thousands of chambers and passageways. The great paper apartment houses of the paper wasps and the honeycombs of the bees are considered to be marvels of engineering.

Nesting species must feed their larvae. Ants forage for food for their young. Some species raise fungus gardens and cultivate aphid "cows," whose liquid excrement, or honeydew, they eat. The mud dauber wasp lays its eggs in tubes of mud. It then stocks the tubes with paralysed spiders and seals the tubes. After the larvae hatch, a sufficient food supply is at hand until they pupate.

How Insects Spend the Winter

Each species of insect usually passes the winter in one particular phase of development. Some butterflies winter as pupae, caterpillars, or eggs. The monarch butterfly migrates long distances southward in the fall; some survive for a return flight in the spring.

In the winter some insects may come out of hibernation during brief periods of mild weather. Snow scorpion flies and spring-tails are often found on snow or ice. Honeybees in well-protected hives use their body heat to maintain a hive temperature that permits them to remain somewhat active and to feed on stored sweets. They leave the hive when the temperature rises to about 55° F (13° C).

Extreme heat or drought brings about a period of inactivity called estivation. The eggs of mosquitoes do not hatch and the nymphs and adults of many aquatic insects become dormant when the breeding ponds and marshes dry up.

Protection from Enemies

Insects have developed many methods of self-defence to avoid being devoured by their enemies. Flight, concealment, motion, armour and weapons, and even grotesqueness are some of these methods. Certain insects are specially adapted for hiding. Vast numbers hide beneath stones or the bark of trees. The flattened bodies of cockroaches and bedbugs enable them to disappear into narrow cracks.

The most interesting means of concealment are mimicry and protective colouration. The walking-stick looks like a twig. Certain moths blend so well into the bark of the tree on which they rest that they cannot be distinguished from the tree. Some harmless insects resemble stinging species in shape or colour and so are avoided by predators. Certain moths and flies mimic bees.

Armour and weapons are well developed in many insects. The tough, horny covering of the beetles amounts to a solid shell of armour Sharp jaws and beaks, poisoned stingers, and spines serve as effective weapons. The extreme hairiness of some caterpillars makes birds and other predators avoid them, and in some caterpillars the hairs have venomous spines.

Stink glands in some insects repel attackers in the same way as those in a skunk. When disturbed, the bombardier beetle ejects an irritating gas from its tail. The gas may be fired repeatedly and audibly. Grasshoppers exude a fluid popularly known as "tobacco juice." The flavour of some insects is so bitter or sour that would-be predators avoid eating them.

Response to Environment

The reactions that insects have to different stimuli are called tropisms. Here are some different types of tropisms and the reactions given by insects.

TYPE OF TROPISM	INSECT IS ATTRACTED TO OR REPELLED BY...
Chemotropism	Certain chemicals, usually related to a smell made by the insect's food or mate
Phototropism	Light, either natural or man-made
Geotropism	Gravity
Thigmotropism	Touch, usually from a similar insect
Thermotropism	Heat
Hydrotropism	Water
Rheotropism	Currents, or flow, of water
Anemotropism	Currents, or flow, of air

Insects are not able to reason. They are guided by instinct and by physiological reactions to their environment. Such reactions are called tropisms, from the Greek word tropos, meaning "turn." All tropisms involve turning toward or away from a stimulus.

Through chemotropism, chemical stimuli help insects find places to lay their eggs. The carrion beetle, for example, deposits eggs on decayed meat drawn to it by odour Butterflies and bees are attracted to flowers by odour as well as colour

The scent glands of various insects help them attract a mate. Insects also avoid certain substances by chemotropic reactions. Clothes chests made of cedar or camphor wood have long been used for storing woollens and furs because these woods contain substances repellent to clothes moths.

Moths are attracted to artificial light and moonlight but avoid sunlight. This is called phototropism.

Some moths use the sun as a point by which to guide their flight, always keeping the light source at an 80 degree angle from its eye. When a light bulb is on, a moth will use the light as its guide, but when it flies at an 80-degree angle to the light bulb, its circular path will lead it directly into the bulb.

Many insects seem to be attracted to or repelled by light (phototropism). Moths are attracted to artificial light and moonlight but avoid sunlight. Butterflies react in the opposite way. Cockroaches in a dark room hide when a light is turned on.

Response to gravity (geotropism) may govern the way various boring insects react. Thermotropism, or attraction to heat, may draw parasites to their warm-blooded hosts. Thigmotropism is reaction to touch. Some insects avoid all contact with others; some thrive in close contact. The swarming of bees may be due to an attraction to one another's bodies. Attraction to water (hydrotropism), adjustment to currents of streams (rheotropism), and adjustment to air currents (anemotropism) may explain the behaviour of a wide variety of insects. However, no single stimulus governs all of their complex activities.