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ANIMALS (Part 1 of 4)   Leave a comment


DEFINITION: 1 any living organism, excluding plants and bacteria: most animals can move about independently and have specialized sense organs that enable them to react quickly to stimuli: animals do not have cell walls, nor do they make food by photosynthesis 2 any such organism other than a human being, esp. a mammal or, often, any four-footed creature 3 a brutish, debased, or inhuman person 4 [Colloq.] a person, thing, concept, etc. thought of as a kind or type [today’s athlete is another animal altogether]

All living things are divided into two main kingdoms the animal and plant kingdoms and two or three other kingdoms that include bacteria, blue-green algae, and one-celled creatures with definite nuclei. What is the difference between a horse, for example, and grass? A horse moves about in the pasture eating grass. It trots toward you when you offer it a lump of sugar and shows pleasure when you stroke its head. The grass, however, is rooted to one place. It does not respond behaviourally to people or to the horse in any way.

Animals Move About and Sense Surroundings

Adult animals move freely from place to place during at least one phase of their life. Plants usually cannot move unless a force, such as the wind, causes them to move.

Most animals move freely from place to place and can sense their surroundings; that is, they can taste, smell, hear, see, and touch. Certain simple animals, such as the corals and barnacles, spend most of their lives fastened to one spot, but they are able to swim freely when they are young. Even these rooted animals have parts that move in order to capture food. Plants, however, cannot shift about at their own will. They react to heat, light, chemicals, and touch, but their responses are involuntary and automatic, quite different from those of animals.

All living things are made of cells. The walls of plant and animal cells are different.

Cellulose, complex carbohydrate consisting of 3,000 or more glucose units; basic structural component of plant cell walls; 90% of cotton and 50% of wood is cellulose; most abundant of all naturally occurring organic compounds; indigestible by humans; can be digested by herbivores, such as cows and horses, because they retain it long enough for digestion by micro organisms present in their digestive systems; also digestible by termites; processed to produce papers and fibres; chemically modified to yield plastics, photographic film, and rayon; other derivatives used as adhesives, explosives, thickening agents, and in moisture-proof coatings.

All living things are made up of cells of protoplasm. They may consist of a single cell, as does an amoeba, or billions of cells, as do trees and horses. The cell wall of a plant is composed of a woody material called cellulose. No true animal contains cellulose. Animal cells are bounded by a membrane composed chiefly of fat and protein.

Green plants make their own food. With the aid of the green substance called chlorophyll, they use the energy in sunlight to change carbon dioxide and water into carbohydrates and other food materials. No true animal contains chlorophyll.

Animals must eat, either directly or indirectly, the food manufactured by members of the plant kingdom. A horse cannot stand in the sun and wait for its body to make fat and proteins. It must move about the pasture in search of green grass. Even meat eaters for example, lions live on animals, such as zebras, which in turn subsist on plants.

The Variety of Animal Life

More than a million different kinds of animals inhabit the Earth. The exact number is not known, for new kinds are continually being discovered. They live in the seas, from the surface down to the black depths where no ray of light penetrates. On mountaintops and in deserts, in mud and in hot pools some form of animal life may be found.

Animals are infinitely varied in form, size, and habits. The smallest animals are bits of protoplasm that can be seen only with a microscope. The largest, the blue whales, may be more than 100 feet (30 meters) long and weigh 300,000 pounds (136,000 kilograms).

Some of the most familiar animals, such as dogs, birds, frogs, and fish, have a backbone and a central nervous system. They are called vertebrates, meaning animals with backbones. Animals without backbones are called invertebrates and include arthropods, worms, molluscs, and many other groups. Most of the vertebrates and many invertebrates have a head where sense organs are concentrated and have legs, wings, or fins for locomotion. Vertebrates and many invertebrates, such as the arthropods and worms, have bilateral, or two-sided, symmetry. This means that they have two mirror-image sides (a right side and a left side), distinct upper and lower surfaces of the body, and a distinct front and rear.

Some invertebrates, such as jellyfish, sea anemones, and starfish, display radial symmetry, in which the parts of the body are arranged around a central axis, similar to a wheel. Animals with radial symmetry live in marine or freshwater aquatic environments. Some drift with the currents, unable to swim in any definite direction. Others become attached to a solid object by one end and float with the mouth end upright. Tentacles arranged in a circle around the mouth sweep in food particles and ward off enemies.

One-celled animals called protozoans live in fresh and salt water. Many are shapeless creatures and cannot swim toward their food. They move along by squeezing out a finger like projection from the body. This is called a pseudopod, from the Greek meaning “false foot.” The pseudopod fastens to something solid, and the rest of the body flows into the fastened projection. The amoeba also moves in this manner. One-celled animals are very small. They are single blobs of liquid enclosed in a thin membrane and as such cannot attain a large size or a very definite shape.

Animals with Outside Skeletons and Feet

Molluscs have soft bodies that are not divided into specialized sections such as head, thorax, and abdomen. Many molluscs are enclosed in hard, hinged shells. Snails have a single large, fleshy foot located on the stomach side.

The heads of the octopus and the squid are surrounded by a circle of eight or ten tentacles that act as arms and feet. Oysters, clams, mussels, and scallops all have a single axe-shaped foot which they burrow into sand.

Most molluscs do not move around efficiently. Oysters fasten themselves to something solid and settle down for life, letting food drift to them. Scallops may move in zigzag leaps by clapping their shells together.

Joint-Legged Animals

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

Joint-legged animals, or arthropods, have bodies divided into segments that have specialized functions. These animals also have many jointed legs. Most arthropods are covered with a jointed skeleton made of a horny material. This outside skeleton is lighter than the shells of the molluscs The legs and muscles and many organs of the arthropod are attached to the outside skeleton.

The arthropods include insects, lobsters, crabs, centipedes, millipedes, scorpions, and spiders. They can run, jump, swim, and crawl. Some live mostly on land, while others live mostly in water. Many of the insects have wings and can fly. Arthropods inhabit most of the Earth’s environments, from the poles to the tropics, and are found in fresh and marine water and in terrestrial habitats.

How Back boned Animals Move

Vertebrates move through water and air and over the ground with great speed and skill. Birds, with their feathered wings, are the best fliers. Fish are the best swimmers. However, other vertebrates also can fly and swim. Bats fly on wings of membrane like skin. The flying squirrel glides on a broad membrane between its legs. The flying fish soars over the surface of the ocean by using its fins. Neither the fish nor the squirrel can soar great distances, however.

Some turtles swim with paddle like front legs. Some water birds can swim underwater with their wings. The mud skipper and walking catfish are fishes that walk on mud by pulling themselves along on their front fins.

Frogs, kangaroos, various cats, and some fishes are superior jumpers. Salmon leap up waterfalls when they travel from the sea to their home streams to lay their eggs. Tarpon, swordfish, and sailfish make great leaps out of the water when pursuing their prey or trying to escape an enemy.


Animals breathe in different ways:

  • Some animals, like amoeba and sponges, let oxygen move through their cell walls.

  • Fish and tadpoles breathe with their gills.

  • Insects bring air in through pores, or holes, called spiracles.

  • Mammals, birds, and reptiles breathe with lungs.

All animals must take in oxygen in order to change food into a form that the body can use. One-celled animals that live in water absorb oxygen directly through their membranes. The sponge is a very simple many-celled animal. The surface of a sponge is covered with millions of tiny pores. Water bearing dissolved oxygen and minute food particles flows through the pores and out of the opening at the top of the sponge.

Fish and tadpoles breathe by means of gills. Insects and caterpillars take air into the body through breathing pores called spiracles.

Mammals, birds, and reptiles obtain oxygen from the air. They take it into the lungs, and the oxygen passes through membranes in the lungs into particles called red blood cells. The bloodstream then carries the oxygen to all parts of the body. Amphibians have lungs, but they also have thin, moist skins that absorb oxygen directly.


Sea squirt, a tunicate or saclike marine animal, so called from its habit of ejecting water when touched; belongs to the phylum Chordata.

Hydra, primitive water animal of the class Hydrozoa and the genus Hydra.

All animals reproduce their own kind. One of the most primitive forms of reproduction is by fission, in which the individual organism divides to produce a replica of itself. Some animals, such as sea squirts, reproduce by budding: lumps appear along a branchlike organ and develop into young sea squirts. Sea squirts, sponges, corals, and other creatures that bud often remain together and form large colonies. The hydra also reproduces by budding, but in time the young bud separates and goes off to live alone.

Most animals reproduce by means of eggs from the female that are fertilized by sperm from the male. The eggs of some species are deposited in a nest or in some other manner before hatching. Most species of mammal and some species of reptile and fish bear their young alive, the fertilized eggs being retained within the body of the female.

The types of reproductive behaviour among animals are almost as varied as the kinds of animals themselves. Some species, such as most insects and turtles, deposit their eggs and give them no further attention. In colonies of the social insects, such as ants and bees, a single female lays all of the eggs, and workers provide care and nourishment for the developing young in the nest. The females of some reptiles, such as the king cobra and the blue-tailed skink, and amphibians, such as the marble salamander, stay with their clutch of eggs until they hatch but provide no protection or nourishment for the young. Some fish guard their young after they are born. Crocodilians protect the eggs before hatching and the young for several months afterwards. Many birds provide not only protection but also nourishment for the developing young. Mammals, which feed their young with milk produced by the mother, provide care for their young much longer than do other classes of animals.

Posted 2012/01/29 by Stelios in Education

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ANIMALS (Part 2 of 4)   Leave a comment


Many animals build temporary or permanent homes for themselves and their young. Birds occupy their nests only while they are incubating eggs and feeding the helpless nestling’s A few fish make temporary nests for their young.

No animal dwelling has excited more wonder and interest than the lodge built by the beaver. Almost as remarkable is the dome-shaped winter home of the muskrat. Underground burrows with sleeping rooms, food-storage rooms, connecting tunnels, and emergency exits are constructed by ground hogs, prairie dogs, European rabbits, gophers, kangaroo rats, and field mice. Chimpanzees and gorillas build temporary nests and sleeping platforms of sticks in trees. The living quarters made by the different kinds of ants can be intricate and complex. Certain tropical bats cut palm fronds in such a way that they droop to form a leafy shelter from the hot sun and torrential rains.


All animals have some means of defending themselves against enemies. A cat can usually outrun a dog and climb the nearest tree. If cornered, it will scratch and bite.

Moose, largest member of the deer family; called elk in Europe.

Many animals rely on speed, camouflage, teeth, claws, and even intimidation to escape other animals. The variety of means of protection is extensive. Porcupines and hedgehogs roll into a ball and raise their sharp quills. The quills come off and stick into the nose or paw of an unwary dog or some other enemy. Skunks spray a foul-smelling fluid from a gland when they are frightened. Deer, moose, and antelope fight with their antlers. An elephant’s trunk is a powerful weapon. It can be used to pick up another animal and smash it to the ground.

Squids shoot out a cloud of inky material and escape under its cover. Torpedo fish and several other kinds of fish have built-in electric storage cells by which they can deliver a paralysing shock. Some insects, snakes, and lizards protect themselves with their venom. Many amphibians produce poisonous skin secretions.

Many animals hide by means of protective colouration A baby deer is almost invisible in the forest because its spotted coat looks like patches of sunlight in the brown leaves. Many fishes, birds, insects, lizards, and snakes use nature’s camouflage to avoid being seen.

Feeding Behaviour

Vorticella (popularly called bell animalcules), genus of bell-shaped Protozoa.

Heliozoan, any protozoan of the order Heliozoa; often called sun animalcule; a single pseudopod may engulf food or several may work together.

Cilia (plural of cilium), hairlike, vibratory appendages found in some plants and animals.

Many one-celled animals (the vorticella and collar flagellate, for example) live in water. These very tiny animals and their feeding habits can be studied only under a microscope. They feed on even tinier organisms in the water. The vorticella is attached by its stalk to some solid object. At the upper end is a mouth surrounded by tiny hairs called cilia. The hairs sweep food particles into the mouth by setting up a whirlpool action in the water. The food is enclosed in a bubble called a food vacuole, where it is digested.

Flagellum (plural flagella), a whip like extension of certain protozoans.

The collar flagellate has a delicate, transparent collar. From the centre of it grows a whip like organ, the flagellum. The beating of the whip draws a current of water toward the cell. Food particles in the current pass through the wall of the cell into the food vacuoles.

The heliozoan, also called sun animal, moves about and captures food by means of pseudopodia. In this case the pseudopodia are stiff spines that radiate from the centre of the cell. The spines wrap around the food and enclose it in a vacuole.

Hydra, primitive water animal of the class Hydrozoa and the genus Hydra.

The hydra feeds most commonly on the larva of a kind of shellfish. It has a mouth surrounded with long tentacles. The tentacles sting and paralyse the prey and then shove it inside the mouth.

Common swallow (in North America, barn swallow), bird (Hirundo rustica).

Butterflies and moths have tube like mouth parts. With these they suck nectar from flowers. Grasshoppers and beetles have chewing, grasping, and tearing mouth parts

Birds and bats catch insects in flight. Woodpeckers hammer into the bark of trees for grubs, other birds comb the leaves with their bills for small insects, and hawks swoop down on rodents and on other birds.

The kangaroo rat is a harmless little animal that lives in the deserts of the south western United States. It lives on dry thistle and cactus leaves, seeds, and small juicy tubers that grow abundantly in the desert 1 to 2 inches (2.5 to 5 centimetres) below the surface. It collects seeds in its cheek pouches and stores them in underground chambers. Gophers and chipmunks also collect food in their cheek pouches and store it in underground pantries for future use.

Carnivores, Herbivores, Insectivores

Animals that eat other animals are called carnivores. The shark is a fierce carnivore. It lives on smaller fish, such as mackerel. Many mammals are carnivores. They all have special kinds of teeth for tearing their food into chunks and chewing it. Most of them have claws for catching and holding their prey. Among the carnivores are cats, dogs, raccoons, weasels, bears, hyenas, and civet cats. Some fish subsist on plant and animal life known as plankton. The baleen whale is an enormous animal, growing up to 100 feet long. It feeds upon shrimp like creatures only about 1 inch in length. When it finds a school of shrimp, it opens its mouth and gulps in several barrels of water. Horny strainers that hang from the roof of its mouth catch the shrimp and drain out the water.

A large group of animals are plant eaters (herbivores). Many herbivores are prey of the carnivores. Insects are the dominant herbivores in most parts of the world, although they may be less conspicuous than plant-eating mammals and birds. Herbivorous mammals include horses, cattle, sheep, goats, rabbits, rodents, elephants, deer and antelope, and monkeys and apes.

A few mammals live on insects moles, shrews, and hedgehogs, bats, armadillos, aardvarks, and anteaters. Many bird species are insect eaters, as are certain kinds of insects, such as ladybugs.

How Animals Sense Their Surroundings

The ability of animals to sense and respond to their surroundings is one way in which they differ from plants. Higher animals have sense organs to perceive light, sound, touch, taste, and smell.

Eyes are very important to most mammals. Animals that hunt and feed by night have very large eyes. Cats’ eyes have pupils that can open wide in the dark and narrow down to slits in the sunlight. Insects have compound eyes, made up of tiny units that break up the image into many small pictures. They also have two or three simple eyes that probably detect motion. The eyesight of some fish is especially keen.

Fennec fox, name of several species of small, fox like animals characterized by large pointed ears.

Ears are perhaps as important as eyes to some species. The fennec is a fox like animal that lives in the Sahara and hunts by night. Its large ears help it detect its prey in the darkness of a hot, dry climate, where food may be very scarce. The cat is also a night prowler, and it too has large, erect ears. The hearing organs of the field cricket and katydid are located on their forelegs. The organ is a thin membrane that vibrates in response to sound waves.

Many animals have sense organs unlike those of the mammals. The antennae of the moths, butterflies, and other insects seem to correspond to the organs of taste, touch, smell, and hearing.

Barbel, a soft, slender feeler around mouth of certain fishes, such as catfish, cod, drum fish, goat fish, sturgeon.

The barbels of the catfish and the whiskers of the flying squirrel and the cat are organs of touch. They are very useful for animals that explore in the dark. The lateral line of the fish is a rod of nerve cells running the length of the body. It probably helps the fish feel movements in the surrounding water.

The delicate forked tongue of the snake tastes the air. With it the snake can locate food and other snakes. The rattlesnake has sensory pits on the head through which it can detect a nearby warm-blooded animal. Even the simplest one-celled animals respond to touch. If a flatworm is touched, it may jerk away or curl up into a ball. It moves away from strong light or from water that is too hot or too cold.

In the warm, muddy rivers of Western Africa there are fish that send out small electric impulses and surround themselves with an electric field. Whenever another fish or other object approaches, the fish is made aware of it by the changes in the charged field. Thus a built-in electric system takes the place of eyesight in the dark waters and keeps the fish informed of its surroundings. Bats emit high-pitched squeaks and use the reflected sound waves to avoid objects and locate prey while flying. Dolphins and whales send out ultrasonic signals and are able to detect objects by reflections of the sound.

Migration and Hibernation

When winter comes to northern or high-mountain regions, animals must find some way to keep warm. Many birds and some mammals seek a mild climate by moving south or to lower elevations. They are said to migrate. Other kinds of mammals (bears and woodchucks, for example) store up fat in their bodies in the fall by eating all they can. Then they curl up in a cave or some other protected place and sleep during the cold period that is, they hibernate.

Most insects die in the wintertime. They leave well-protected eggs which hatch in the spring. Fishes, frogs, and aquatic arthropods and other water-dwelling animals may hibernate in mud or move to deeper water and become inactive.

Living Together in Colonies

Social insects, those living in communities and having differentiated forms or castes, as queens, workers, drones.

Some animals live with others of their own kind. Ants, honeybees and bumblebees, and wasps are called social insects because they live together in highly organized societies.

Some birds live in large colonies. Penguins, anis, and eider ducks are examples. Weaver finches work together to build huge community dwellings.

South American monkeys travel through the jungles in family groups. They scatter while they are searching for food but stay within sight or hearing of one another. Toward evening they regroup and spend the night together. Baboons live in large bands. They cooperate in getting food and post sentries to watch for danger when the group stops.

Posted 2012/01/29 by Stelios in Education

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ANIMALS (Part 3 of 4)   Leave a comment


Most animal activities that appear to indicate intelligence are simply instinctive. The most intelligent animals are the apes and monkeys. Dogs and elephants have been trained to serve humans in many ways. Horses, seals, porpoises, lions, and tigers are often taught to perform in circuses and aquariums. Talking birds, such as parrots, parakeets, and mynahs, learn to imitate sounds, but they do not have the capacity to think or to understand what they are saying.

Relationship to Human

Humans require the presence of other animals in a variety of ways. The domestication of animals has been important to the development of civilization. By pollinating flowers, bees help in the cultivation of orchard fruits, alfalfa, clover, and many vegetables. The earthworm, by churning up the soil, improves the growth of plants.

Birds eat insect pests, weed seeds, and rodents. Certain bats eat so many mosquitoes and other insects that some communities erect shelters for them to encourage their help. Hyenas, vultures, and carrion beetles keep country regions clean by devouring dead animals.

Countless animal products are used by humans: pearls (from the oyster), shellac and lacquer (from the lac insect), glue, and fertilizers are only a few examples. Important drugs are produced from the blood and glands of animals. Serums and antivenins for snakebite are made from the blood of horses. Experiments performed on such animals as rats, mice, guinea pigs, and monkeys have been responsible for great advances in medical knowledge and the conquest of human disease.

Dangerous animals include the parasites in the human body and in domesticated animals that cause serious diseases. Fleas, lice, rats, and mosquitoes are also carriers of such serious conditions as malaria and encephalitis. Insect pests cause billions of dollars’ worth of damage every year.


More than a million different kinds of animals inhabit the Earth. No one knows exactly how many kinds there are, for many new ones are discovered and named every year.

Beginnings of Animal Life

The first organisms in the history of the Earth must have been one-celled bits of protoplasm floating in shallow seas and ponds. Here they remained for millions of years. They developed from one cell to many cells, becoming more and more complex. In time some animals moved into fresh water. Others began to live on land. In these surroundings they changed still more, until today there is a bewildering variety of forms.

The creatures that developed a backbone and an internal skeleton are called vertebrates. They include all the familiar animals the mammals, reptiles, birds, fish, and amphibians. Animals without backbones are called invertebrates. They include insects, sponges, corals, jellyfish, clams, lobsters, and starfish.

The vertebrates make up only about 5 percent of all animal species. Invertebrates compose the remaining 95 percent. There are some 4,000 species of mammals. Insects number about one million species.

How Animals Are Classified

Phylum (from Greek, meaning tribe), a major division in biological classification.

To study the many forms of animal life in a systematic way, scientists have divided the animal kingdom into groups. These groups are based upon the structure of the animal’s body. The largest divisions are phyla (singular, phylum). The word phylum means “race” or “tribe.” The phyla are groups of animals with fundamentally different body plans.

Order, in biological classification, a group of related families.

Genus (plural, genera), a group of related species of plants or animals.

Each phylum is divided into classes, the classes into orders, and the orders into families. Families are subdivided into genera (singular, genus), and each genus is divided into species. All members of the same species are closely related. They are capable of interbreeding and producing fertile offspring. Animals of different species do not normally interbreed. Every animal has a scientific name, or binomial (having two names), consisting of the genus and species.

How Classification Shows Relationships

Classification shows relationships between animals in an increasingly specific order, from remotely related members of the same phylum to closely related species within a genus. House cats (Felis catus) and bobcats (Felis rufus) belong to the same genus (Felis) and family (Felidae) but to different species.

Dogs and cats do not appear to be related. Both, however, have backbones and are meat-eating mammals. Hence they belong to phylum Chordata (having a spinal cord), class Mammalia (mammals), and order Carnivora (flesh eaters); because of differences between them, however, they belong to separate families (dog, Canidae; cat, Felidae).

Whales and sharks both appear to be kinds of fish. Both are strong, streamlined swimmers of the sea. However, the whale is a mammal. It has lungs and is warm-blooded, gives birth to live young, and nurses its offspring with milk. Whales therefore belong to the class Mammalia. The shark, on the other hand, is a primitive kind of fish with a skeleton of cartilage instead of bone. Sharks, whales, and true fishes all have a backbone. Thus, they are placed in the same phylum (Chordata) and subphylum (Vertebrata). Sharks and fishes, however, are also in different classes, the sharks being in the Chondrichthyes and the true fishes in the Osteichthyes.

Classification also suggests which kinds of animals may have descended from other types. All multi-celled animals, for example, are supposed to be descendants of one-celled animals. This does not mean descent from one living kind of animal to another, however. All living animals are believed to have descended from common ancestors that were less specialized than they. These relationships may be shown on a treelike diagram called a phylogenetic tree. The word phylogenetic comes from two Greek words meaning “race history.”

Animals Without Backbones Invertebrates

The simplest animal-like organisms consist of a single cell a bit of protoplasm containing one nucleus. These organisms are called protozoans, which means “first animals” in Greek. These creatures are sometimes considered animals, but most classification schemes place them in a separate kingdom known as the Protista. Protozoans are very adaptable. They live in salt and fresh water, in moist earth, and as parasites in other animals.

Aside from the protozoans, all members of the kingdom Animalia have many cells and are referred to as metazoans. The simplest multi-celled animals make up the phylum Porifera (“pore bearers”). The most familiar kinds are the sponges. They are called pore bearers because they are covered with millions of tiny holes. Water flows through the holes, and from the water the sponges take in oxygen and the tiny water borne organisms that constitute their food and filter out wastes. Sponges have no mouth or digestive cavity, no nervous system, and no circulatory system. Several types of cells are present, but each generally functions as a unit without forming tissues, as in more complex metazoans. (Tissues are groups of similar cells bound together to perform a common function.)

Pouch like Animals

Coelenterata, phylum of animals including coral, hydra, jellyfish, and sea anemone.

The next, less primitive structural pattern in invertebrates is a hollow gut. The representative phylum is Coelenterata, a term stemming from the Greek words koilos (hollow) and enteron (intestine). Among the coelenterates are the corals, hydras, jellyfishes, and sea anemones. The body is composed of two tissue layers. The inner layer, or endoderm, lines the central digestive cavity. The outer layer, or ectoderm, protects the animal externally.

The coelenterates have a mouth like opening the only opening into the gut that takes in food and ejects waste material. Food-gathering organs such as tentacles and protective structures such as stinging cells surround the mouth. There is a primitive nervous system. (Coelenterates are also sometimes called cnidarians.)

Bilateral Animals with Heads

All the animals described above are headless creatures. They are either irregular masses or animals with shapes like a globe, a cylinder, a bowl, or a wheel. The latter are said to have spherical or radial symmetry; that is, they have similar body parts regularly arranged around a centre or a central axis, respectively. Some drift around in ocean currents, unable to swim efficiently in any particular direction. Some of them in their adult stages the corals and sponges, for example fasten themselves to fixed objects and do not move at all.

A flatworm called Dugesia, or planaria, is interesting because it shows two very important improvements in body structure. It belongs to the phylum Platyhelminthes (flatworms). It is the most primitive animal that has a definite head bearing sense organs. The mouth is on the underside of the triangular head. The body is differentiated into a front end and a rear end, a top and a bottom. It has bilateral symmetry: each half of the body is a mirror image of the other half. Most of the higher animals, including humans, are built on this pattern of body structure.

Platyhelminths are also the most primitive, living animals to have three cell layers. Between the ectoderm and the endoderm, which first appeared in the jellyfish and their relatives, is a middle layer: the mesoderm. Two-layered animals are small and fragile. The third layer gives solidity to the body and permits the animal to grow to a large size. Muscles and other complex organs develop from this layer.

Segmented Worms

Segmented worms have a more developed digestive system than Dugesia, which takes in food and ejects waste material through the same opening in the head. Segmented worms have a digestive tube with two openings a mouth and an anus through which wastes are expelled. The phylum Annelida (meaning ringed, or segmented) has a digestive system built on the same plan as the vertebrates. Earthworms and leeches are familiar annelids.

The Soft-Bodied Animals

The phylum Mollusca (from the Latin word for “soft”) includes the clam, oyster, chiton, snail, octopus, and squid. Molluscs have soft, fleshy bodies not divided into segments. The main part of the body is enclosed in a fold of tissue called the mantle. They have bilateral symmetry. Many of them are covered by a shell. They have a solid, protective structure outside the body, called an exoskeleton.

Posted 2012/01/29 by Stelios in Education

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ANIMALS (Part 4 of 4)   Leave a comment

The Largest Group of Animals

The phylum Arthropoda (“jointed foot”) has the largest number of species. In fact, about 90 percent of the million or more species living on the Earth today are arthropods. The insects total more than 800,000 species. Other arthropods include the centipedes and millipedes; the arachnids (spiders, scorpions, ticks, mites); and the crustaceans (barnacles, crabs, crayfish, lobsters, shrimp, water fleas). Obviously, the arthropod body plan has been highly successful. The members of this great phylum live on land, in fresh water, and in salt water. They can walk, fly, burrow, and swim. This is the only invertebrate group with jointed appendages (legs, feet, and antennae).

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

Arthropods, like molluscs, wear a supporting framework, or exoskeleton, on the outside of the body. Much more highly developed than the heavy, clumsy shell of the clams and snails, it is made of a substance called chitin. Rigid, waterproof plates of chitin are joined by thin, flexible membranes of chitin so that the animal can move freely and quickly. The muscles are attached to the inner surface of the armour Many important structures are connected to the outer surface. For example, the wings, legs, jaws, and antennae of the insects are all made of chitin and are attached to the outer skeleton. The body is divided into sections, or segments.

Spiny-Skinned Animals

One phylum with the characteristics of several others is the Echinodermata (“spiny-skinned”). All members of the group, which includes starfishes, sea urchins, holothurians (sea cucumbers), and crinoids (sea lilies), live in salt water some in the shallow shoreline waters, others in the ocean depths. The young, called larvae, have bilateral symmetry, but the adults have radial symmetry, like the coelenterates. These are the most primitive creatures having an endoskeleton, or skeleton that is embedded in the flesh. It consists of a mesh work of plates that are made of calcium. The plates are joined by connective tissue and muscles. Spines project from these plates.

Animals with Backbones

At the top of the animal kingdom is the phylum Chordata. This phylum consists of two groups of primitive chordates the tunicates and the cephalochordates and the main group of the vertebrates.

The major subdivisions, or classes, of the vertebrates are the fish, amphibians, reptiles, birds, and mammals. Members of this phylum possess the following structures at some period of their life, either as embryos or as adults:

Notochord. This is an internal supporting rod extending the length of the body. It is found in the embryos of all chordates, including human beings. Only the most primitive forms, such as the amphioxus, or lancelet, the lamprey, and the hag fish, retain it as adults. Remnants of the notochord are also present in sharks. In the higher chordates, such as amphibians, reptiles, birds, and mammals, the notochord is replaced during development of the embryo by a bony column of vertebrae, which gives the column and the animal flexibility.

Nerve tube. This lies in the mid line of the body on the top (dorsal side) of the notochord. In the annelid worms and the arthropods the main nerve is solid and lies on the underside (ventral side). In most chordates the forward end of the nerve tube forms a brain; the remainder is the spinal cord.

Pharyngeal gill slits or pouches. The lower chordates, such as the fish, breathe through openings in the side of the neck in the region of the pharynx. The embryos of the higher chordates have these slits, but they disappear in the adult.

Primitive Chordates

Amphioxus (or lancelet), a fish-shaped sea animal; about 2 in. (5 cm) long; pinkish white; several species known; classed in the phylum Chordata, subphylum Leptocardia.

The amphioxus is characteristic of the most primitive chordates. This animal is a laterally compressed, semitransparent sea dweller about 4 inches (10 centimetres) long. Scientists believe that it may be one of the ancestors of the vertebrates. It has a notochord and a tubular nerve cord along the back. It has no well-developed brain, however, and only traces of eyes and ears. Pigment spots along the body are sensitive to light. The pharyngeal gill slits strain food from the water. The tunicates, or sea squirts, and acorn worms are other primitive chordates.

Lampreys and hag fishes are the most primitive of the true vertebrates. They have a notochord. The skeleton is composed of cartilage. They lack jaws and paired limbs.


Mammals differ from other vertebrates in that they have bodies that are covered with hair at some period of their lives. They are warm-blooded, meaning that their body temperature is largely unaffected by the temperature of the air or water in which they live. The females have milk glands to feed their young. Whales, as noted earlier in this article, dolphins, and porpoises are the most unusual-looking mammals because they resemble fishes. The most primitive mammals are the egg-laying platypus and the echidna, or spiny anteater. The marsupials, which incubate their unborn offspring in a pouch for a time, are also considered to be somewhat primitive. The remaining mammals transmit nourishment to their unborn young through a placenta and give birth to fully developed offspring.

Assisted by J. Whitfield Gibbons, Senior Research Ecologist and Professor of Zoology, Savannah River Ecology Laboratory, University of Georgia.


Books for Children

Alday, Gretchen. Devoted Friends: Amazing True Stories About Animals Who Cared (Betterway, 1990).

Aylesworth, T.G. Animal Superstitions (McGraw, 1981).

Gabb, Michael. Creatures Great and Small (Lerner, 1980).

Hirschi, Ron. Who Lives In the Forest? (Dodd, 1987).

Hutchins, R.E. Nature Invented It First (Dodd, 1980).

Lauber, Patricia. What’s Hatching Out of That Egg? (Crown, 1979).

Lopshire, Robert. The Biggest, Smallest, Fastest, Tallest Things You’ve Ever Heard Of (Houghton, 1991).

Lurie, Alison. Fabulous Beasts (Farrar, 1981).

McCauley, J.R. Animals and Their Hiding Places (National Geographic, 1986).

McGrath, Susan. Saving Our Animal Friends (National Geographic, 1986).

Patent, D.H. Sizes and Shapes in Nature What They Mean (Holiday House, 1979).

Pope, Joyce. Do Animals Dream? (Viking, 1986).

Prince, J.H. How Animals Move (Elsevier/Nelson, 1981).

Pringle, L.P. Feral: Tame Animals Gone Wild (Macmillan, 1983).

Schulz, C.M. Charlie Brown’s Super Book of Questions and Answers About All Kinds of Animals (Random, 1976).

Sunden, Ulla, ed. Remarkable Animals (Guinness Books, 1987).

Sussman, Susan and James, Robert. Lies (People Believe) About Animals (Whitman, 1987).

Windsor, Merrill. Baby Farm Animals (National Geographic, 1984).

Books for Young Adults

Adamson, Joy. Born Free (Pantheon, 1987).

Adamson, Joy. Living Free (Harcourt, 1961).

Argent, Kerry. Animal Capers (Doubleday, 1990).

Baker, M.L. Whales, Dolphins, and Porpoises of the World (Doubleday, 1987).

Burton, Maurice. Cold-Blooded Animals (Facts on File, 1986).

Burton, Maurice. Warm-Blooded Animals (Facts on File, 1987).

Burton, Maurice and Burton, Jane. The Colorful World of Animals (Longmeadow, 1975).

Gibbons, Whit. Their Blood Runs Cold: Adventures with Reptiles and Amphibians (Univ. of Ala. Press, 1983).

Herriot, James. All Creatures Great and Small (St. Martin’s, 1972).

Herriot, James. All Things Bright and Beautiful (St. Martin’s, 1974).

Herriot, James and others. Animal Stories, Tame and Wild (Sterling, 1985).

Kohl, Judith and Kohl, Herbert. Pack, Band, and Colony: The World of Social Animals (Farrar, 1983).

Milne, Lorus and Milne, Margery. A Time to be Born (Sierra Club, 1982).

National Geographic Book Service. Wild Animals of North America, rev. ed. (National Geographic, 1987).

Nowak, R.M. and Paradiso, J.L. Walker’s Mammals of the World (Johns Hopkins Univ. Press, 1983).

Posted 2012/01/29 by Stelios in Education

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ANIMAL BEHAVIOUR (Part 3 of 3)   Leave a comment

The Prairie Dog Coterie – A Complex Social Group

The prairie dog is a rodent that maintains an elaborate social organization. Bond formation among prairie dogs depends on the exchange of auditory, visual, and chemical stimuli. The coterie the social unit of the prairie dog is maintained in a network of burrows occupying a fairly restricted area.

Prairie dog pups are altricial at birth that is, they are so undeveloped that they need adult aid for survival. When the pup is born, its mother is attracted to the helpless young organism. She licks the pup as it emerges from the birth canal, thus replenishing the salts she lost before and during birth. While licking the pup, she breaks the sac in which it developed as an embryo and thus stimulates its breathing response. The pup, still wet from birth, is attracted to the warmth of the mother’s body. Moments after birth, the mother and her offspring are exchanging highly attractive stimuli, quickly forming a social bond. As the pup nurses, it relieves the pressure in the mother’s milk gland. Again, the exchange of stimulation strengthens the bond between the mother and her offspring, thus helping to ensure the infant’s survival.

As the pup matures, other stimuli become attractive. When it is able to see and hear, the pup begins to recognize the relationship between stimuli that occur at the same time. Soon it leaves its burrow and encounters other adults that it stimulates. From birth, the prairie dog is constantly nuzzled and licked by its mother. When it emerges from its burrow, it is handled similarly by other prairie dogs.

Grazing land, non farm area where animals feed on grass or other plants.

These behavioural patterns maintain prairie dogs in a well-organized life space. There, the family unit reproduces, finds shelter, and feeds. Being grazers, prairie dogs check the growth of tall grasses that would prevent them from easily spotting predators. At the same time, their grazing habits encourage the dominance of fast-growing plants. Thus, the social organization of prairie dogs influences the ecological balances in their environment. Limited grazing space soon forces maturing prairie dogs to seek new areas. When they enter the burrows of another coterie their odour marks them as strangers, and they are rejected. Pairs of rebuffed animals band together to form new coteries.

The Chimpanzee Family

The chimpanzee is one of the great apes. It lives in a family unit even more complex than that of the prairie dog. The chimpanzee family moves as a group through familiar feeding and resting areas. It has also evolved effective ways of defending itself against predators or from belligerent chimpanzees attempting to mate with the family’s females.

When a chimpanzee has been attacked or has spotted a predator, it lets out an intense cry that raises the level of excitement of the other members of the family. They scream at the predator, throw rocks and other objects, and scamper off. As they flee, the females and the youngest chimpanzees are surrounded by the juveniles and the young males. The largest males guard the group. Thus, the action of a single chimpanzee serves as a signal that affects the behaviour of the rest of the family.

Animal Communication

Communication in the animal world takes many forms. These include chemical, visual, and audible signals. Attacked insects secrete a pheromone that so excites their con-specifics that they either attack or escape from the predator. Flocks of birds behave similarly, except that sounds rather than chemicals trigger the response. Vocalization also evokes social responses in the porpoise, an aquatic mammal. Porpoises communicate by means of whistles and other sounds. When a porpoise is born, females may be attracted by the mother’s whistles. They swim to the baby and nuzzle it. The mother does not attack other females at this time. Possibly, this experience with many adult porpoises in the earliest days of infancy helps form the tight social bond of porpoises.

Reciprocal stimulation affects the behaviour of any animal, whether briefly or for a long time. Each organism is the source of environmental changes that affect other organisms. For example, after an amoeba ingests a food particle, it excretes a metabolic by-product that changes the chemical characteristics of the environment. If another amoeba is nearby, it tends to approach the first, though it will not do so if the chemical concentration is too intense. A sexually mature male cricket stridulates rubs its legs together and produces a sound whether or not another cricket stimulates it. However, it is more likely to stridulate when it hears another cricket.

When one animal can prompt an anticipated response in another, it displays a more advanced type of communication. For example, in an experiment a chimpanzee was trained to obtain a banana by pulling on a rope attached to a weight. Then the experimenter increased the weight so that one chimpanzee could not raise it but two could. If the second chimpanzee had already been trained to pull the rope, the first was able to stimulate it to do so by gesture, vocalization, and shoving. The two would then pull together and get the banana. In this case, the consequence of the second chimpanzee’s behaviour was in some way anticipated by the first.

The directed activity of one animal toward another for the solution of a problem or the attainment of a planned goal is evident only in advanced species. Furthermore, man is the only species capable of transmitting ideas through a complex system of speech and writing.

The study of the evolution of language has given rise to a science called semiotics. This science attempts to understand the similarities and the differences among the many forms of communication.

Heredity and Behaviour

The evolutionary principle of selective adaptation holds that a species survives when it is able to adapt to environmental changes and when it is able to transmit to its offspring the genetic information that makes such adaptations possible. But how do genetic processes contribute to the development of behavioural patterns? Which behavioural patterns are hereditary? Which must be learned by each new generation?

In an effort to answer such questions, behavioural scientists have designed a number of experiments. In one type of experiment, closely related species with distinctly different behaviour patterns are hybridized. For example, two species of parakeets that practically share a natural habitat but do not interbreed were crossed in the laboratory. The parakeets of one species ordinarily tuck nesting material under their tail feathers. The others carry it in their beaks. The hybrid female offspring made inadequate tucking motions with the nesting material, and the twigs fell out from their feathers. However, all the hybrids carried the nesting material successfully in their beaks. Scientists felt that since all the hybrids performed some part of the tucking behaviour, it was probably the earlier form of behaviour in the evolution of these species.

The relationship between heredity and behaviour has fuelled an old but continuing controversy in the behavioural sciences. Some scientists believe that genetic processes underlie every kind of behaviour, while others think that the environment can modify genetically influenced behaviour In one type of experiment testing these views, animals with different genetic backgrounds are reared in the same environment. In another type, animals with the same genetic backgrounds are raised in different environments.

Chaffinch, European bird of the finch family, sought as a cage bird because of its beauty of voice and its ability in learning to sing tunes.

Cross-fostering is used to rear species with different genetic backgrounds in the same environment that is, the young of one species are raised by a female of another species. In one cross-fostering study, a female great tit reared a baby chaffinch with her own babies. Great tits and chaffinches are closely related birds that feed in different ways. The great tit holds food under its feet; the chaffinch does not. A chaffinch hatched by a great tit did not use its feet during feeding, while its nest mates did. Its feeding behaviour remained typical for its species, although it had no opportunity to observe other chaffinches.

However, when a great tit was reared in isolation, though it too demonstrated species-typical behaviour by holding its food down, it did so very clumsily. Only after repeated tries did its performance improve. This experiment showed that experience may be important even in genetically determined behavioural patterns.

Manipulation of the physical environment was used to study the subspecies of deer mice. One subspecies lives in the forest, is a climbing animal, and has a longer tail and larger ears than the other, a prairie subspecies that lives in grassy fields. The two subspecies were reared in the same laboratory and then released in a room containing artificial grass and wooden posts with flat tops. Although neither subspecies had experienced its species-typical environment, the forest deer mice organized their life space around the “trees,” and the prairie deer mice settled under the “grass.” However, when prairie deer mice were bred in a laboratory for more than a dozen generations, they no longer showed a preference for the field. The environment eventually so altered the genetic processes of these experimental animals as to change their species-typical behaviour

Meadowlark, bird of the genus Sturnella, family Icteridae; sharp-billed, 8 to 11 in. (20 to 28 cm) long; two North American species streaked brown above, with yellow breast crossed by black V; western meadowlark (S. neglecta) known for intricate fluting call.

Bird-song patterns are species-specific and have, therefore, been regarded as genetically determined. Studies of the development of species-typical song patterns have helped to clarify the relative roles of heredity and experience in the development of such patterns. For example, if a meadowlark is exposed to another bird’s song while it is learning to sing it will learn the other bird’s song; however, if the meadowlark is exposed to the songs of other meadowlarks along with those of another species, it will learn only its own species-typical song. The bird instinctively chooses its species-typical song when it is in a situation in which there is a choice.

The response patterns of birds are so varied that the contributions made by genetic processes and by the auditory and other experiences of a bird during singing are hard to separate. It may be that in the course of its development a bird produces certain sounds that are a function of its peculiar body make-up These sounds may be the fundamental vocalization of the bird’s species. Additional experience by the bird with hearing and producing its own song, as well as hearing those of other birds in a social setting, may yield the “dialect,” or song pattern, associated with the species. However, genes do not carry this pattern as such. Rather, they carry the code for the biochemical processes that develop certain body systems that, aided by experience, will affect the animal’s behaviour in its typical environment.

Assisted by Ethel Tobach, Curator, Department of Mammalogy, American Museum of Natural History, New York City.


Black, Hallie. Animal Cooperation: A Look at Sociobiology (Morrow, 1981).

Caras, Roger. The Private Lives of Animals (McGraw, 1987).

Fraser, A.F. Farm Animal Behaviour, 2nd ed. (Saunders, 1983).

Freedman, Russell and Morriss, J.E. The Brains of Animals and Man (Holiday, 1972).

Lydecker, Beatrice. What the Animals Tell Me (Harper, 1982).

McFarland, David, ed. The Oxford Companion to Animal Behaviour (Oxford, 1981).

National Geographic Society. How Animals Behave (National Geographic, 1984).

Pringle, L.P. Animals at Play (Harcourt, 1985).

Pringle, L.P. The Secret World of Animals (National Geographic, 1986).

Waller, E.J. Why Animals Behave the Way They Do (Scribner, 1981).

INSECTS (Part 2 of 2).   Leave a comment


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

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

Ancestors of the Modern Insect

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

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


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


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

Carriers of Disease


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




Tsetse fly

African sleeping sickness



Yellow fever



Liver damage




Rat flea

Bubonic plague


Human louse




Assassin bug

Chagas’ disease

Heart damage

Brain damage



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

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

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

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

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

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

Household Pests

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

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

Plant-Eating Pests

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

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

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

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

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

Insect Immigrants Upset Nature’s Balance

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

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

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


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

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

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

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

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

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

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

Four things that farmers can do to control insects are

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

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

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

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

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

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

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

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

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

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

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


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

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

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

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

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

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

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

The Principal Insect Orders

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

Subclass Apterygota

(wingless, no metamorphosis)

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

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

Subclass Pterygota

(winged, undergo metamorphosis)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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


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