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FLOWERS (Part 1 of 2)   Leave a comment

 

DEFINITION:1 a) the seed-producing structure of an angiosperm, consisting of a shortened stem usually bearing four layers of organs, with the leaf like sepals, colourful petals, and pollen-bearing stamens unfolding around the pistils b) a blossom; bloom c) the reproductive structure of any plant 2 a plant cultivated for its blossoms; flowering plant 3 the best or finest part or example [the flower of a country’s youth] 4 the best period of a person or thing; time of flourishing 5 something decorative; esp., a figure of speech 6 [pl. ] Chem. a substance in powder form, made from condensed vapours [flowers of sulphur].

Most plants pass on life to future plant generations by seeds. It is the work of a flower to make seed. All its beauty serves this one purpose. Colour and perfume attract insects and hummingbirds to aid in the flower’s pollination. Some flowers are so formed that they admit certain insects and no others. The chief seed-making parts are the stamens, pistil, and ovary. Many interesting flower shapes have developed that protect these parts.

The Chief Parts of a Flower

Bracts, small, sometimes scale like, leaves in a flower cluster; develops into large leaves.

A flower’s beauty and perfection of form may be enjoyed more fully if one understands its structure and how each part helps in the work of seed making. A typical flower has four sets of organs. From the outside to the centre, they are: sepals, petals, stamens, and pistils. The leaf like sepals make up the calyx, or “cup.” The petals form the corolla, or “little crown.” The calyx and the corolla together form the perianth. When present, the bract is a small leaf below the flower. Awns are stiff bristles that terminate some flower parts.

The flower rises from the axil of the bract; that is, the angle between the bract and the stem. Bracts are sometimes the most conspicuous feature of a flower and may be mistaken for petals. This is true of dogwood, poinsettia, and Indian paintbrush. Sometimes one great bract forms a hood, called a spathe, as in the jack-in-the-pulpit, the calla lilly, and the skunk cabbage. The top of the stem, to which the parts are attached, is the receptacle. The stem is also called a pedicel.

Sepals and Petals

The sepals are the lower, or outermost, part of the flower. They fold over the tender, closed bud and protect it from cold and other injuries while it is developing. Usually sepals are green. In many flowers, however, they are as colourful as the petals and increase the flower’s attractiveness to insects. Tulips, irises, and the yellow pond lily, or spatter dock, are examples.

The petals attract insects and hummingbirds to help in the work of pollination. By their fragrance and colour they advertise their sweets the nectar in the heart of the flower. This is the reward the flower offers its helpers. Glands at the base of the petals secrete nectar. Oil in the petals gives the flower its perfume.

Many flowers have petals of the same size and shape arranged in a circle. They are said to be regular. The wild rose is typical. The petals of the morning glory and petunia are joined, forming a funnel-shaped corolla. Each portion is regular in shape, but the petals are united. Such flowers are sympetalous. Many irregular flowers are pollinated only by a certain kind of insect. The snapdragon can be sprung open only by the heavy bumblebee.

The simplest flowers have no sepals or petals at all. The small flowers of grasses consist commonly of three stamens surrounding a single pistil. They are said to be naked. Some flowers are apetalous; that is they have no petals.

Some flowers are tiny but grow in showy clusters. In the largest family of flowering plants, called Compositae, tiny florets are set so closely together in a solid head on a receptacle that one mistakes them for a single flower. A dandelion is a composite. In other composite flowers, such as the daisy and sunflower, perfect seed-producing flowers are found only in the centre The rim is made up of ray flowers. Garden flowers in this group are the aster, zinnia, dahlia, chrysanthemum, and marigold. The family includes many weeds, such as rag weeds and thistles.

Stamens and Pistils

Inside the ring of petals are the stamens. Each stamen has a stem, which is called the filament. At the top of the filament is the anther. The pollen grains form in sacs, usually two in number, inside the anther.

Ovary, in flowering plants, the receptacle in which fertilized seed germs develop.

Finally, inside the ring of stamens is the pistil. It is shaped like a vase, with a neck and oval base. The neck is known as the style. On top of the style is a stigma, which has a sticky surface. Its purpose is to catch and hold the pollen. The base of the pistil is the seed case, called the ovary. Inside the ovary are one or more eggs, the ovules, which become the embryo plant. Some flowers the lotus, buttercup, and strawberry, for example have many pistils. The pistils may be separate from one another or they may be closely united. A simple pistil, or one of the segments of a compound pistil, is called a carpel.

How Flowers Are Attached to the Base

The parts of a flower are attached to the receptacle, or base, in three different ways. If they are attached at the base of the ovary, the flower is hypogynous, meaning “growing on the lower side of the ovary.” The tiger lily is an example of this type. In the second form the receptacle is cup-shaped and encloses the ovary. The sepals, petals, and stamens are attached to the rim, surrounding the pistil but free from it. The flower is said to be perigynous, meaning “around the ovary.” The cherry blossom is perigynous. In a third type the ovary grows fast to the receptacle, and the parts grow from its top. The flower is epigynous, meaning “growing upon the ovary.” An example is the apple blossom.

How Fertilization Takes Place

When ripe pollen from an anther of the same kind of flower catches on the stigma, each pollen grain sends out a tiny threadlike tube. The tube grows down through the style and pierces one of the ovules in the ovary. This process is called fertilization. Each ovule must receive the contents of the pollen tube before it can develop into a seed. It usually takes the tube from two to five days to reach the ovule, but the time may vary from a few hours to six months.

Insects Pollinate Flowers

A seed cannot grow until pollen is transferred from the stamen to the pistil. This transfer is called pollination. Since flowers cannot go after pollen, they depend upon some carrier to bring it to them. Flowers are pollinated by flies, moths, wasps, bees, and sometimes by hummingbirds. The flowers attract these helpers by their colour, fragrance, and nectar. Some flowers open in the evening and invite night-flying insects to their banquet table. Such flowers are usually white or pale yellow, the colours that show best at dusk.

To reach the nectar, insects must crawl over the pistils and anthers into the heart of the flower. Their bodies become covered with pollen dust. As they move from flower to flower, they transfer the pollen of one to the stigma of another. Flowers that require the help of insects are called entomophilous, meaning “insect-loving.” Some flowers can be pollinated only by a single kind of insect. The fig, yucca, and red clover are examples.

Certain flowers depend upon the wind to bring pollen to them. They are called anemophilous, or “wind-loving.” Most trees, the grasses, sedges, and many other plants depend upon wind pollination. Wind-pollinated flowers have no sepals or petals, for the wind has no need for nectar and fragrance. They are dull in colour They produce enormous quantities of pollen. The wind scatters pollen indiscriminately, so that only a small percentage falls on the stigmas of the same kind of flower.

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Posted 2012/03/17 by Stelios in Education

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FLOWERS (Part 2 of 2)   Leave a comment

Avoiding Self-Pollination

Self-pollination, transfer of pollen from the stamen of a flower to the pistil of the same flower, as distinguished from cross-pollination.

A few kinds of flowers are self-pollinating; that is, they can be fertilized with their own pollen. In most cases, however, nature takes great care to prevent self-pollination. Cross-pollination usually produces more vigorous plants. This requires the transfer of pollen from one plant to the stigma of another plant of the same species.

Flowers avoid self-pollination in several ways. In some cases the stamens and pistils mature at different times. In other flowers the stamens are shorter than the pistils and hence do not deposit pollen on their own stigma. Wind-pollinated flowers usually bear the stamens and pistils in separate flowers. Alders, birches, walnuts, and hickories bear catkins with pistillate flowers on some branches and catkins with staminate flowers on other branches. Corn has the pistils and stamens on different parts of the same plant. The tassel bears the staminate flowers; the ear bears the pistillate flowers. These are known as monoecious (of the same household) plants. A few trees, such as cottonwoods and willows, carry the separation even further, with the staminate flowers on one tree and the pistillate on another. These are known as dioecious (of two households) plants.

How Fruit Develops

After fertilization of the ovule has taken place the petals, sepals, stamens, and usually the upper part of the pistil fall off. Now, as the ovules grow into seeds (embryo plants), the ovary, or seed case, also changes. In some plants it turns into a fleshy covering, called fruit. The ovary wall separates into two layers. The inner layer becomes a hard shell, called a stone, or pit, which encloses the seed. The outer layer forms the pulpy portion of the fruit. The peach, plum, cherry, and apricot are examples. In the case of berries the entire ovary becomes a fleshy mass in which the seeds are embedded. In the apple, pear, and quince, the ovary and its seeds become the core of the fruit. The pulpy part, which is eaten, is the modified calyx.

The ovaries of many plants develop into so-called dry fruits capsules, pods, nuts, and acorns. Like the fruits and berries, they protect the seeds and help scatter them when they are mature. Another kind of dry fruit is the achene. In this case the ovary wall becomes a coating of the single seed. It does not open at maturity, as the pods and capsules do, to release the seed. Achenes are developed by flowers that produce but one ovule, such as the individual flowers of the composites. The style of the pistil sometimes remains attached to the achene as a long, feathery tail that carries the seed away on the wind. The most common flower with seeds that are readily scattered by the wind is the dandelion, regarded by most people as a weed.

The Origin of Flowers

At least 250,000 species of flowering plants are known. All of them descend from a primitive ancestor that no longer exists. The most primitive modern flowers are the members of the buttercup order, Ranales. A step higher is the rose order, Rosales.

The simplest flowers are the least skilful in making seed. Many stamens mean a great deal of pollen is wasted. A large number of pistils means that many will fail to become pollinated and produce seed. All members of the buttercup order, which includes the little buttercup itself and the splendid magnolia and water lilies, and all the roses have many pistils and stamens. The most highly specialized and most successful flowers are the composites.

Two Kinds of Flowering Plants

Angiosperms (or Angiospermae), class of flowering, vascular plants of the division Magnoliophyta having seeds in an enclosed ovary.

Flowering plants belong to the phylum Tracheophyta, or vascular plants. Thus far the flowers and seed making up only one group of this phylum, the angiosperms, have been described. These are flowers that enclose their seeds within an ovary.

Another group of flowering plants, called gymnosperms, has naked, or exposed, seeds. These plants include the conifers, or cone-bearing trees, such as the pine, fir, spruce, cypress, and cedar. Cones take the place of flowers.

Cones are of two kinds staminate and pistillate. They are usually borne on different branches of the same tree. The staminate, pollen-producing cones are small and last only a few weeks in the spring of the year. The pistillate cones are the large familiar ones. The ovules, usually two in number, are located on the upper surface of each scale. The ovule consists of an embryo sac surrounded by a covering that later becomes the seed coat. In the covering is a tiny opening called the micropyle (little gate).

In late spring the pistillate cones stand upright with the scales opened wide to catch the windblown pollen. When pollen lodges between the scales, they close. Thus protected within the closed cone, the pollen sends out a pollen tube that enters the ovule through the micropyle. When the seeds in the cone are fully grown, it again opens, releasing the matured seed. All gymnosperms are wind-pollinated.

Posted 2012/03/17 by Stelios in Education

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CORN (Part 1 of 2)   Leave a comment

 

5000 BC: Cultivation of maize. The primary grain in use in North America prior to the European discovery was maize, now called corn in some countries. It was probably grown first by the inhabitants of Mexico. After the arrival of Europeans in the Americas, corn was sent to many parts of the world and is in use nearly everywhere today, often as feed grain for animals.

Maize is unique from other grains in that botanists do not know how the plant evolved. In the Old World, no evidence exists of maize in archaeological remains, and no mention of it is made in ancient writings. It is believed to have evolved solely in the Western Hemisphere.

In the United States, Canada, and Australia the term corn refers to maize, or what is sometimes known as Indian corn. The rest of the world calls this grain maize. (This grain is known in South Africa, however, as mealies.) In England the word corn refers to wheat, and in Scotland and Ireland it refers to oats. This article uses the word corn to refer to maize.

Upon returning from the New World, Christopher Columbus and other explorers introduced corn into Europe, where it was previously unknown. Since that time corn has spread into all areas of the world suitable to its cultivation. Corn was served at the first Thanksgiving Day feast in America in 1621. In modern times, it has become a popular snack for movie viewers in the form of popcorn.

After wheat and rice, farmers the world over use more land for corn than for any other grain crop. More than 319 million acres (129 million hectares) of corn are planted worldwide each year. Most of the corn grown is the coarser kind called field corn. It is not grown for people to eat. Farmers feed it to pigs, cattle, and other livestock. Out of every 100 bushels grown, farmers store half in silos or in bins for feeding livestock. For this reason the value of the corn crop cannot be measured by what is sold as grain. Most of the yearly crop “goes to market on four legs” as pigs and cattle. Thus a large part of the multi billion-dollar corn harvest never reaches the grain market.

Where Corn Grows Throughout the World

Out of every four bushels of corn grown in the world, farmers in the United States produce one. Many states grow corn. Most of it, however, is raised in the famous Corn Belt. This vast fertile region extends across the north-central plains from western Ohio to eastern Nebraska. The top-ranking corn-producing states are Iowa, Illinois, Nebraska, Minnesota, Indiana, and Wisconsin. Corn will grow wherever it has suitable soil, freedom from frost and cold nights, and plenty of hot sun when it is maturing. It also needs ample soil moisture during the hot season.

These conditions are also found in much of Central and South America, around the Mediterranean, in India, and in South Africa. The largest producers of corn, after the United States, are China and Brazil. Other large corn-producing countries are Mexico, India, Indonesia, South Africa, and the Philippines.

An Obscure Ancestry

Some botanists believe that members of the amaranth, or tassel flower, family may have been the wild ancestors of the corn plant. But even in the time of Columbus, corn could not fertilize itself, as do most wild plants or recent descendants of wild plants. The greatest weakness lay in the way corn produces its seed. The top of the stalk has a many-spiked tassel which grows pollen. The plant also has ears with filaments called silks which receive pollen. But the ears are completely wrapped with leaves, and the ends of the silks protrude only from the tips. Therefore the silks cannot get ample pollen unless the plants have many neighbours, as they do in a cultivated field. Botanists think that the plants could hardly survive in the wild state. Corn was apparently unknown in ancient times in the Old World. No evidence of it has ever been found in archaeological remains. There is no reference to it in the Bible or other ancient literature or in primitive art. The word corn in the Bible refers to wheat, not the American maize.

In the New World, however, all the principal types of corn that scientists recognize today were already in existence and under cultivation when the first explorers arrived. The wild ancestor of corn probably came from the Western Hemisphere.

Some botanists think the plant may be descended from teosinte, a grass that grows wild in Mexico and Guatemala. Another theory is that it originated in South America from a primitive pod corn which was also a popcorn. Pod corn kernels are enclosed in pods or chaffy shells. Such a wild corn has not been found.

Ancient Corn in New Mexico

In 1948, scientists of the Peabody Museum, Harvard University, discovered ancient corn in a cave in central New Mexico. The lowest levels of the cave floor contained primitive husks and kernels estimated to be 4,000 years old. This corn bore no relationship to teosinte, but it did have the characteristics of pod popcorn.

In upper and more recent deposits the scientists found corn that appeared to have been crossed with teosinte. Modern corn may therefore be a hybrid of teosinte and wild species which no longer exist, but the mystery is still unsolved.

The Corn Plant and Its Seed

The corn plant is a large member of the grass family (Gramineae). It has a fibrous, woody stalk that may grow to be from 6 to 20 feet high. At the top is its spiked tassel. This part produces the male flowers of the plant. Farther down, the stalk grows one or more spikes which develop into ears. Each one grows out from beneath the base of a leaf, and at first it is completely wrapped in leaves. The spikes bear threadlike filaments (silk) which are the female flowers. Each filament grows from a germ on the spike called an ovule.

The ovules are arranged in rows along the spikes. Each one will produce a seed, or kernel, if the filament of silk is fertilized by a pollen grain. To catch pollen, the green, tender tips of silk protrude from the top of the leafy wrapping around the spike.

All these parts appear after the stalk and leaves are well grown and the plant is receiving plenty of summer sunshine. When the flower parts develop, farmers say that the corn is tasselling out. Soon the tassels produce yellowish dust like grains of pollen. Each grain of pollen contains two sperms.

How Fertilization Takes Place

Now summer breezes gently shake the pollen-laden tassels, and billions of the tiny, sperm-bearing pollen grains jar loose. The wind carries them to the silk of neighbouring plants. Tiny receivers, called stigmas, at the ends of the silks, catch the pollen. Promptly the pollen grains send tubes growing down through the silks to the ovules. Then the sperm cells pass down the tubes and fertilize the ovules. Thereupon the spike grows into a large, pithy structure called a cob, while the ovules grow and ripen into seeds (kernels).

The growing seeds are made up of a soft yellow hull filled with milky liquid. Corn at this stage is in the milk. The milk has a sweet flavour, and field corn in the milk stage may be used as roasting ears. When field corn is ripe, the kernels are hard, firm, and starchy. Sweet-corn kernels do not get as hard.

Colours of Corn

When the first European settlers came to America, they found corn with different coloured kernels. The Indians liked particular colours for certain purposes and tried to grow them.

The pioneers preferred the yellow kind for field corn. About 1779 sweet corn was discovered in Pennsylvania. Gradually farmers began to save seed from desirable plants for planting the following year.

Posted 2012/03/04 by Stelios in Education

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INSECTS (Part 2 of 2).   Leave a comment

CLASSIFICATION

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.

THE IMPORTANCE OF INSECTS TO HUMANS

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.

HARMFUL 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.

INSECT

DISEASE CARRIED

RESULT

Tsetse fly

African sleeping sickness

Death

Mosquito

Yellow fever

Encephalitis

Malaria

Liver damage

Death

Chills

Fever

Rat flea

Bubonic plague

Death

Human louse

Typhus

Fever

Depression

Assassin bug

Chagas’ disease

Heart damage

Brain damage

Blindness

 

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.

METHODS OF INSECT CONTROL

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.

BENEFICIAL INSECTS

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.

BIBLIOGRAPHY FOR INSECT

Barbosa, Pedro and Jack Schultz. Insect Outbreaks (Academic Press, 1987).

Better Homes and Gardens Editors. Bugs, Bugs, Bugs (BH&G, 1989).

Blum, Murray. Fundamentals of Insect Physiology (Wiley, 1985).

Borror, Donald. An Introduction to the Study of Insects (Saunders College Publications, 1989).

Boy Scouts of America. Insect Study (BSA, 1985).

Burton, John. The Oxford Book of Insects (Oxford, 1982).

Gattis, L.S. Insects for Pathfinders (Cheetah Publications, 1987).

Goor, Ron and Nancy Goor. Insect Metamorphosis (Macmillan, 1990).

Higley, Leon. Manual of Entomology and Pest Management (Macmillan, 1989).

Horton, B.G. and others. Amazing Fact Book of Insects (Creative Editors, 1987).

Leahy, Christopher. Peterson Field Guide to Insects (Houghton, 1987).

Line, Les and Lorus Milne. The Audubon Society Book of Insects (Abrams, 1983).

Mayer, Daniel and Connie Mayer. Bugs: How to Raise Insects for Fun and Profit (And Books, 1983).

Seymour, Peter. Insects: A Close-Up Look (Macmillan, 1985).

Stiling, Peter. An Introduction to Insect Pests and Their Control (Macmillan, 1985).