Ross E. Hutchins
Insects are the largest single group of animals in the world. About 750,000 different kinds of insects are known, and more are being discovered every year.
Insects are found almost everywhere on earth. Some live deep in underground caves, and some on the tops of the world's highest mountains. There are insects that live on the surface of the ocean, far from land. Some live in dry deserts. Some live in hot springs whose water temperature is 120 degrees Fahrenheit. Many different insects have been found in cold barren Antarctica. One insect is able to live in vinegar. The young of another insect live in salty lakes. Insects can live in a great many different kinds of places. This is one of the reasons they have remained on earth for over 350 million years.
Many insects are very small. Only a few kinds grow to more than 1 or 2 inches. Some are so tiny they can hardly be seen. But the largest ones are bigger than some mice.
The small size of insects makes it possible for them to escape their enemies by crawling into cracks or other hiding places. And because they are so small, they need very little food and water to live. Thus their small size is a very great advantage.
What is an Insect?
All adult insects have six legs, three on each side of their bodies. This is the one sure way to tell an insect. Each insect's body has three main parts — a head, a thorax, and an abdomen. Insects also have one pair of feelers, or antennae, at the front of the head. Most insects have one or two pairs of wings.
Look carefully at a spider and count its legs. There are eight of them. A spider, therefore, is not an insect. The close relative of the spider, the scorpion, also has eight legs. It is not an insect either. Neither are the many-legged centipede and millipede.
The Insect's Outside Appearance
If you can do so, catch a grasshopper and examine it. The grasshopper is a typical insect. See if you can find the three sections of the body.
The antennae at the front of the head are easy to see. Although the head is small, you should be able to see the eyes. You will have to look closely to see the mouth unless the grasshopper is chewing.
The thorax is next to the head. The wings and the three pairs of legs are attached to it. There are powerful muscles in the thorax, which you cannot see without cutting the insect open. These muscles control the movements of the legs and wings.
The hind portion of the insect's body is the abdomen. It is the biggest part of the body. The abdomen is in sections, or segments, that look like rings attached together at the rims. This makes the insect's body flexible — that is, it can bend and twist easily.
The mating and egg-laying organs are at the tip of the abdomen. Some insects also have another set of feelers, called cerci, at the end of the abdomen. Silverfish and many water insects have large cerci.
The Outside Skeleton
A tough outer coat covers the grasshopper's body. Most insects have such a coat. This is the insect's skeleton. It is like a shell enclosing the insect. It covers the soft parts of the body and supports them. Did you ever see an insect collection? What you saw were the outside skeletons of the insects with the wings attached.
Most insects have tiny hairs, or spines, growing out from the skeleton. Such hairs are often also found on the legs, the antennae, and the mouthparts. The skeleton is firm but lightweight, and so does not hamper the insect's flight.
The skeleton is made of a waterproof substance. This is a great help to the insect because it prevents water from soaking into its body. It also keeps the body from drying out. This outside skeleton, therefore, makes it possible for insects to live through long periods of wet or dry weather.
The hard outer shell protects the insect in other ways as well. It helps keep out germs and harmful chemicals. The shell acts as a shock absorber and as a shield against too much heat or cold. It also protects the insect from some of its enemies.
How Insects Develop
Some insects give birth to live young; however, most insects produce eggs. Insect eggs are of many shapes. Some are long and oval; others are round like a ball. They often show beautiful surface markings and color patterns when seen through a magnifying glass.
Some insects lay only one egg at a time. Others, like termites, lay 10,000 or more in a day. They may lay this large number of eggs several times each year. Certain insects deposit large clusters of eggs in a protective case. The praying mantis and the cockroach are such insects.
Most insects go through several distinct forms as they grow from egg to adult. This method of growth is called metamorphosis. The word "metamorphosis" means "change in shape."
The Insect Larva
The young that hatch out of most insect eggs are called larvae (singular: larva). They look like worms and are sometimes incorrectly called worms. The "worm" in an apple or a tomato is really a larva.
You have probably seen caterpillars. They are the larvae of moths and butterflies. You may find it hard to think of a crawling caterpillar as the young of a butterfly. It does not look at all like the adult. The same thing is true of most insects. Their larvae sometimes have more than three pairs of legs. They have no wings. And their mouths and other body parts are different from those of the adult.
As soon as a larva hatches out of the egg, it begins to eat. It eats constantly, biting and chewing its food with powerful jaws, called mandibles. Its appetite never seems to be satisfied. It grows very rapidly, but its outer skin does not grow with it. Instead, the larva molts, or sheds its skin when it becomes too tight. Each time a new and bigger skin grows in place of the old one.
When a larva is full-grown, it stops eating. It buries itself under the ground, or it attaches itself to a leaf, the bark of a tree, or some other suitable place, and goes through a resting stage. This is called the pupal stage or pupa. A moth larva spins a cocoon around itself before becoming a pupa. A butterfly develops a shiny covering through which you can often see the body parts. The butterfly pupa is called a chrysalis. Other larvae develop a hard coat, or pupal case, as protection.
An insect may remain in the pupal stage for several weeks. Many kinds remain in this stage all winter. Meanwhile great changes take place within the insect's body. The parts of the body develop and fit it for life as an adult. When these changes are complete, the pupal case splits and the adult insect crawls forth. In a few hours its body dries out. If it is a winged insect, it is ready for flight.
Butterflies, moths, bees, ants, wasps, and beetles all develop in this way. All go through the egg, larval, pupal, and adult stages. At each stage, the insect changes greatly in appearance and behavior. Scientists say these insects go through a complete metamorphosis. But many insects skip some of these stages. Those that do are said to go through an incomplete, or gradual, metamorphosis.
Grasshoppers, crickets, cicadas, and dragonflies are among the insects that go through an incomplete metamorphosis. When the young hatches from the egg, it looks like the adult except that it is smaller and has no wings. It is called a nymph.
As the nymph grows, it molts when its skin becomes too tight. Tiny wings appear, which grow larger with each molt. With the last molt the wings become full size, and the insect is now an adult. Some insects with incomplete metamorphosis never develop wings. Lice and some crickets are examples of these.
How Long Do Insects Live?
There is no average length of life for adult insects as a group. The life of one kind of insect may be very short. Mayflies, for example, live for less than a day. During that time they do not eat. They mate, lay their eggs, and die. Most adult moths and butterflies live for only a few days or weeks.
Some insects live for one summer; some live for several years. Probably the insect with the longest life is the queen of one kind of termite that lives in the tropics. Some scientists think this queen termite may live for as long as fifty years.
The Insect's Senses
Insects have special organs for sensing the world around them, just as you have. Each sense organ usually responds to only one kind of sensation. The parts of an insect that receive sensation are called receptors. Insects' receptors are very different from our sense organs. For example, insects' "ears" are not found on their heads. Insects have no nostrils for smelling, yet their sense of smell is much stronger than that of a human being.
The antennae are remarkable sensation receptors. There are many different sizes and shapes of antennae among insects. All insect antennae are jointed, but some look like little knobs on the head of the insect. Others, such as those of the grasshopper, look like tiny strings of beads. The antennae of many moths are like waving feathers; butterfly antennae have clubbed tips.
Watch an insect as its antennae wave about in the air or feel the surface on which it is standing. The insect can feel, smell, and taste with its antennae. Some insects, such as the male mosquito, can hear with them. Insects use their antennae to feel whether a surface is wet or dry, smooth or rough, hot or cold. They also use them to sense the outside temperature and the humidity. Bloodsucking insects can sense the difference in temperature between an animal's body and the surrounding air with their antennae. This is how these insects find their victims.
The antennae contain most of the insect's smell receptors. Thousands of tiny cells on the antennae receive odors that inform the insect of a food supply. For example, flies smell the odors given off by decaying plants and animals. Cabbage butterflies smell the odor given off by cabbage leaves. Ants find their way to a source of food supply by scent trails left by other ants in their colony.
Smell helps insects find the right kind of plant or animal on which to lay their eggs. It also helps them spot a natural enemy. The antennae of male insects can detect the scent of a female of the same kind, sometimes at a distance of more than a mile. The female insect gives off the chemical scent, called a pheromone, to attract male insects. In some insects the feet, mouthparts, and hairs on the skeleton are also sensitive to smell.
How an Insect Sees
Adult insects have compound eyes, made of many individual lenses called facets. You can see these if you examine a grasshopper's eye with a magnifying glass. The eyes of some insects have as few as nine facets each; the houseflies' have 4,000; some dragonflies' eyes have 28,000.
Scientists have found a way of taking photographs through the lenses of the insect's compound eye. The photographs picture what an insect probably sees. They show that each facet sees a separate image of only a part of the object. Nerves going from each facet carry these separate images to the brain, where they are combined into one complete image. The more facets there are in the eye, the sharper is the complete image. But all insects are nearsighted. They cannot see an object clearly if it is farther than 2 or 3 feet away.
Each insect has two compound eyes. In most insects the eyes are on the sides of the head. This position of the eyes makes it possible for these insects to see in most directions. They are particularly able to detect moving objects.
Insects can see some colors. Plant-eating insects are especially sensitive to green. Butterflies that gather nectar from red and yellow flowers are more sensitive to these colors. Some insects, such as the honeybee, are able to see certain colors that the human eye cannot see. But the honeybee cannot see some colors that we can.
Most adult insects have another set of eyes. These are two or three simple eyes, each with one lens. They are called ocelli. The ocelli are located at the top of the insect's head. Unlike other animals, insects cannot turn, move, or focus either set of eyes. Although it is not known what insects actually see, scientists think that these simple eyes can only detect light and dark. The ocelli probably help make the insect more sensitive to light, so it can detect a broader range of light rays.
Taste and Feel
Tiny hairs on the mouthparts and the antennae contain most of the insect's taste receptors. Many insects taste with their feet as well — these insects can taste food by walking on it.
The hairs on the insect's skeleton, feet, and antennae are very sensitive feelers. Many insects have hairs on their abdomen that are also feel receptors. The receptors on the abdomen can sense vibrations in the earth when an insect is on the ground. They sense air currents when the insect is in flight.
Some insects have feel receptors on their wings. The receptors can sense that a solid object is approaching by the way the air is displaced as the object moves. This is how a fly can sense that you are trying to swat it and why it darts away.
The Insect's Ears
Some insects locate the opposite sex for mating by means of sound. The organs for hearing, or "ears," are never found on an insect's head. The head is too small to contain organs of hearing.
Ears are found on different parts of the body in many different insects. For example, a grasshopper's ears are on the sides of the abdomen, underneath the wings. The ears are small, round disks that pick up sound vibrations and send them to nerves inside the body. Some moths also have this type of ear. There are simple ears near the tips of the antennae of many other insects. In many insects some of the hairs on the body are also sensitive to sound.
Some butterflies and moths are deaf. The insects with the most developed sense of hearing are the insects that produce sound, such as the katydid. The katydid can hear sounds that the human ear cannot.
How Insects Make Sound
Insects make sounds that carry messages to other insects of the same kind. The ears of each kind of insect can pick out the sound intended for it. The sounds are usually mating calls made by the male to attract the female. But with mosquitoes the female makes the sound to attract the male. Sometimes males make sounds to drive off rival males or to frighten away enemies.
A beekeeper can tell by the way the bees buzz whether they are angry or contented. Some ants, also, make sounds, probably as danger warnings to other ants in the colony.
Insects do not have voices, as we do. Most insect calls, or "songs," are made by the rubbing of one part of the insect's body against another. The katydid is a good example of an insect the "sings" in this way. This insect's sound-making apparatus is located on the two front wings. A series of notches, called the file, is on the left wing. A part of the right wing is hardened to form a scraper.
When ready to sing, the katydid raises its front wings and moves them rapidly in and out. This causes the scraper to saw back and forth across the file, something like the way a violin bow scrapes across the strings. The sounds produced by the katydid's wings are carried through the air to the ears of any female katydid that may be nearby.
Some grasshoppers have little knobs on the inner surface of their hind legs. The insect makes sounds by rubbing these hard knobs against the hard edge of the front wings.
Cicadas have a different way of making sounds. Their songs are made with drums. The drum is in the abdomen of the male. Muscles attached to the drumhead pull it in and then let it snap back. This is done very quickly — sometimes as fast as 480 times a second. The shrill buzzing song of cicadas fills the air on a summer evening and can be heard a long way.
What Insects Eat
Insects eat many different kinds of material. Wood, paintbrushes, pepper, vinegar, wine bottle corks, wool, paper, flour, mushrooms, bits of meat, and decayed matter are only a few of them. Some bore into plants and suck their juice. Some bite off pieces of leaves and can strip a plant bare. Bees and most butterflies and moths sip nectar from flowers and eat pollen, without harming the plant.
Many insects hunt other insects for food. The little orange and black ladybird beetle is one of these. It eats tremendous numbers of aphids, which it finds on plants. The praying mantis is another. Almost any insect is food for the mantis. Often, after mating, the female even eats the male. And as the young hatch out of the case, some of them devour the others. But the praying mantis is harmless to humans. In fact, it is useful because it destroys many garden pests.
You may have seen dragonflies darting about over ponds, catching small gnats and mosquitoes. The giant waterbugs, which may be up to 4 inches long, kill tadpoles, minnows, and small frogs, as well as other insects, for food. Hunting wasps capture insects of various kinds and store them in their nests. These serve as food for the wasps' larvae.
There are insects, such as the female mosquito, that pierce the skin of other animals and suck drops of their blood for food. There are others, such as the cattle grub, that live inside another animal's body. A number of insects lay their eggs right on the body of other insects. When the eggs hatch, the larvae start eating the insect.
Some insects, like the cockroach, eat almost anything. But most insects will eat only one kind of food. The caterpillar of the monarch butterfly, for example, eats only milkweed leaves. It will starve to death if it cannot get these leaves. The caterpillar of a cabbage butterfly eats the leaves of plants in the cabbage family. It will also eat nasturtium leaves, because these contain an oil like that in the leaves of the cabbage family.
Termites are destructive wood-eaters. They get into damp wood through the ground. As they eat they bore tunnels inside the wood. They cannot live in daylight, so they never bore through to the surface.
How Insects Are Able to Get Their Food
The feeding habits of an insect depend upon its mouthparts. Over time, each kind of insect has developed specialized mouthparts that determine how it will feed. Insects that have powerful jaws bite and chew their food. Grasshoppers, beetles, cockroaches, and ants are examples of such insects. The mouthparts of some insects are developed into a hollow tube like a soda straw. These insects suck plant juices. Butterflies and moths have very long sucking tubes. When not in use the tubes are coiled beneath the insect's head.
Many insects have different mouthparts at different stages in their lives. For example, the caterpillar of the monarch butterfly has biting and chewing jaws, with which it eats milkweed leaves. During the pupal stage the mouthparts change to the sucking tube of the adult.
The hollow tubes of bloodsucking insects are like sharp, hollow needles. When a bloodsucking insect bites, saliva from glands in the mouthparts is injected under the victim's skin. The saliva prevents blood from clotting at the puncture until the insect has finished its meal. A chemical in the saliva causes the sting and the swelling of a mosquito bite.
Some kinds of flies and mosquitoes carry disease germs, such as viruses or parasites, in their saliva. When these insects bite, they inject the disease germs into a person's body. When an insect carries and spreads a disease but does not develop the disease itself, the insect is called a vector. Malaria and yellow fever are spread in this way.
The fact that the larva seldom eats the same food as the adult is a great help in insect survival. The adult does not eat the larva's food supply. The larva does not have to compete with the adult for food.
Digesting the Food
An insect's body cannot use food as it is eaten. The food must first be changed in order for the body to be able to absorb and use it. The process by which this is done is called digestion. Insects, like most animals, have a special digestive system for this purpose.
The insect's digestive system is a hollow tube that goes from the mouth to an opening in the tip of the abdomen. Near the head of the tube are little saclike projections; these are the salivary glands. The glands pour saliva into the mouth. Saliva contains a chemical, called an enzyme, that starts to digest the food.
The tube is widened out in several places to form digestive organs. One of these is the stomach. A number of glands pour digestive juices into the stomach. This is where most of the food is digested. The last part of the digestive tube is called the hind intestine. Digested food passes from the stomach into the hind intestine and is absorbed into the blood.
Slender tubes are attached to the hind intestine. These tubes remove waste material from the blood and empty it into the hind intestine. From there the waste is carried out of the insect's body along with other wastes from the digestive system.
The Insect's Nervous System
An insect has nerves and a brain to switch sensations it receives into action. But an insect's actions are automatic. For example, an insect cannot think, "Danger is approaching; I must fly away." Instead it feels a certain kind of air current on its body. Feel receptors send the sensation to nerve centers. The nerve centers automatically cause certain muscles to contract. This contraction makes the wings move, and the insect flies away.
A large nerve center in the head is the insect's brain. The more intelligent the insect is, the larger the brain. A honeybee, for example, has a larger brain than a beetle of the same size. Nerve branches go from the brain to the eyes and the antennae. When an insect smells suitable food with its antennae, nerves from the receptors send the message to the brain. The brain in turn sends a message through nerves to the legs or wings, and the insect moves toward the food.
The Insect's Blood System
An insect's blood is not red like ours. Our blood contains a red chemical, hemoglobin, that carriers oxygen throughout the body. The oxygen-hemoglobin combination gives our blood its bright red color. An insect's blood does not carry oxygen; therefore it does not need hemoglobin. Its blood is usually clear instead of red. Sometimes it is a yellowish or greenish color.
The insect's heart is part of a long tube running along the top of the body, right under the skin. The tube opens just under the brain. There are tiny openings with valves along this tubelike heart. Blood is sucked into the heart through these openings. The heart contracts and forces blood to flow toward the head.
In the head the blood pours out over the brain and then flows backward through the body. As it flows backward it bathes the body organs, muscles, and nervous system. It brings them digested food and takes away waste material.
You can see an insect's heart in some living specimens. If you look carefully at a cutworm, a mosquito larva, or some caterpillars, you can see the tubelike heart along the back. Watch it beat. You may be able to notice that the heart beats faster when the insect is warm than when it is cold. The changes occur because insects are cold-blooded; their body temperature changes along with the changes in the temperature of their environment. When the outside temperature drops, an insect's body temperature also drops and its body processes slow down.
How an Insect Breathes
Like all animals, insects must breathe. They need oxygen from the air to burn digested food. When the food burns, it gives the body energy. A waste product of this burning is a gas called carbon dioxide. The body breathes out the carbon dioxide together with the parts of the breathed-in air it did not use.
You breathe in and out through your mouth and the two nostrils in your nose. Insects have about 10 pairs of "nostrils" along the thorax and abdomen. Each segment of the insect's body has a pair of nostrils. They are little holes, called spiracles, which lead into a system of tubes that carry oxygen throughout the insect's body. If you look at a grasshopper through a magnifying glass, you can see the spiracles along the side of the abdomen.
Most insects breathe through their spiracles, but there are some exceptions. The water scorpion has a long breathing tube attached to the tip of its abdomen. It pushes the tip of this tube up through the surface of the water to get air. Many insects that live in water have gills instead of spiracles for breathing. The gills are special organs for taking in air that is dissolved in the water.
Insects were probably the first animals to fly. They developed wings many millions of years before there were birds or bats. Many insects have well-developed flying abilities; they steer accurately, hover, and are able to move sideways or backward in flight. The ability to fly has helped insects to survive. If conditions for life were not favorable, the insects could fly off to a different place.
Insect wings are very thin, like cellophane. They have many ribs, called veins. The veins help stiffen the wings. While all insect wings are alike in this way, they may be very different in other ways. The wings of butterflies and moths are covered with scales, which may be brightly colored. The forewings of beetles, called elytra, are hardened and shell-like. They fit like shields over the folded hind wings and protect them.
Some insects, such as flies and mosquitoes, have only one pair of wings. The hind wings have developed into stumps that help the insect balance itself when flying. These stumps are called halteres. Insects such as fleas, lice, bedbugs, and silverfish have no wings. Cockroaches have wings but do not often use them to fly. Among the ants the workers never have wings. Only the males and the queens have wings, at mating time. After the ants mate, the wings drop off and the insect remains wingless.
The wings of many insects move at great speeds. The wings of the housefly beat about 345 times a second. The wings of butterflies move much more slowly. They move only about 12 times a second.
Insects fly at different speeds. The housefly's speed is about 12 miles an hour. Hawkmoths have been clocked flying at 30 miles an hour, and some scientists say that dragonflies can fly even faster than that. These are average "cruising" speeds of the insects. They can fly faster if they are escaping from an enemy.
How Insects Move
Watch a fly as it walks. At each step it moves three legs forward at almost the same time. These are the front and hind legs on one side and the middle leg on the other. At the next step the other three legs move. In this way the insect is always resting solidly on three legs as it moves forward.
Most insect legs end in a pair of claws with a pad in between. In walking, the claws hook onto objects and help pull the insect along. This makes it possible for an insect to run very quickly. The pads between the claws have many tiny hairs with a sticky substance on them. They grip surfaces that are slippery, such as walls and ceilings, so that an insect can walk on them.
The legs of some insects are fitted for special purposes. For example, the hind legs of grasshoppers are long and have powerful muscles for jumping. This enables the grasshoppers to escape their enemies. The hind legs of some water beetles are very long and are set with stiff bristles. These legs work like oars in propelling the insect through the water. If you watch one of them swimming, you can see that it swims with a jerky motion, like a person rowing a boat.
An insect's legs, like the rest of its body, have skeletons on the outside. Muscles that control the leg movements are inside the skeleton. This protects the muscles and helps them work more efficiently.
Have you ever seen ants carrying sticks or pebbles? A scientist once saw an ant lifting a stone out of its nest entrance. He took both the ant and the stone back to his laboratory and weighted them. He found that the stone weighed 52 times as much as the ant. If a man of average weight were as strong as the ant is for its weight, he would be able to lift nearly 4 tons.
Other insects are even stronger than the ant. A bee can pick up things 300 times its own weight. Beetles are probably the strongest living things in relation to their size. If you had as much strength for your size as a beetle has for its size, you could easily lift almost 10,000 pounds.
Considering their size, insects are remarkably strong. One reason for their strength is the thickness of their muscles. The strength of a muscle depends on its thickness, not on its length. Another reason for the strength of insects is that they have more muscles than many animals. A person has about 800 muscles. A grasshopper has about 900, and some caterpillars have about 4,000. The muscles of insects also work better because of the way they are attached to the outside skeleton.
The strength of an animal does not depend entirely on its size. An animal may be 10 times larger than an insect, but it does not have 10 times the strength. This is because the ability of animals to lift things does not increase at the same rate as their size. The long muscle of a large animal, if it is no thicker than an insect's, is no stronger.
Some insects are able to jump great distances. Grasshoppers can jump 20 times the length of their bodies. At that rate a person would be able to jump one third the length of a football field. Fleas are probably the champion jumpers among insects. A tiny flea can make a jump 8 inches high and can go a distance of 13 inches. A person that could do as well could jump over a tall building.
The chief reason insects are such good jumpers is that they are so small. Large animals are heavier for their size than smaller animals are for theirs. For example, a 6-foot man may be about five times as long as the largest insect, which is about 15 inches long. But the man weighs hundreds of times more than the insect does. The insect can jump better because it carries less weight on its body for its size.
Considering the muscles, body structure, and weight of insects, scientists have come to this conclusion: If insects were to grow as large as humans, they would be little, if any, stronger.
How Insects Protect Themselves
Insects have many natural enemies. They are captured and eaten by birds, bats, moles, frogs, and other animals. They also prey on each other. Yet in spite of their enemies, insects remain the most abundant of all animals. Most insects lay a great many eggs at one time. Often the young that hatch from these eggs become adults in a few weeks and are able to lay eggs in turn. Their ability to reproduce rapidly in such large numbers is one reason they are so numerous.
Protection by Disguise
Insects protect themselves from their enemies in many ways. One of these is by disguising themselves so that they blend into their surroundings. This is called camouflage. Many caterpillars and walkingstick insects are camouflaged to look like twigs. Many insects are colored and marked with different patterns that make them look like the background upon which they rest, such as tree bark or leaves. When some beetles fold up their legs and fall to the ground, they look like clumps of dirt.
Some Harmless insects look ferocious and thus frighten off their enemies. For example, the hickory horned devil is a large caterpillar with vicious-looking spines. It scares its enemies away by its appearance.
Some moths and butterflies fool their enemies by looking like an insect that is bad-tasting or has a poisonous sting. This is called mimicry. For example, the viceroy butterfly looks almost like the monarch butterfly, which has a bad taste to animals that try to eat it. These animals leave both butterflies alone after having tasted a few monarch butterflies.
Another insect that protects itself by mimicry is the hornet fly. It has markings like those of a hornet but has no stinger. Insects, toads, and other small animals whose mouths have been stung by hornets do not try to eat hornets again. Neither do they try to catch the hornet fly.
Protection by Chemical Warfare
Many insects defend themselves by "chemical warfare." If you have ever been stung by a bee, you know how effective this kind of protection is. Poison stings are used by many bees, wasps, hornets, and some ants. The stinger is a modified egg-laying organ, so only females sting. Some caterpillars protect themselves in the same way.
Many stinging insects are brightly colored. This is how such an insect warns its enemies that it is poisonous and to stay away.
Some insects give off a bad-smelling chemical to drive enemies away. Stink bugs, broad-headed bugs, and lacewings are often called the "skunks" of the insect world. Perhaps the most unusual kind of chemical warfare is that used by the bombardier beetle. When disturbed, this beetle ejects a puff of gas from the rear of its body. The gas has an irritating effect on the enemy.
Most adult insects live by themselves in crevices, in soil, under rocks, or under loose bark. They are called solitary insects. Certain insects live together in colonies. These are called social insects. Ants, termites, most bees, hornets, and some wasps are examples of social insects.
Shelters of Social Insects Social insects build elaborate homes to shelter the colony. Bees build honeycombs and beehives. Hornets and wasps build paper nests. They make the paper by chewing bits of rotten wood and stems of plants. The material is mixed with saliva and becomes a pulpy mass. When it dries, the pulp stiffens into a gray, paperlike material.
Most ants build nests in the ground. The nests are honeycombed with tunnels. The activities of the colony are conducted in the tunnels. Termites nest in darkness. Their nests are made of chewed-up wood. Some termites that live in tropical regions build tall mounds in which the colony lives.
Shelters of Solitary Insects
Only a few solitary insects build shelters. One of these is the leaf roller, a type of moth. Each morning the leaf-roller larva rolls a leaf around itself. It seals the ends with silk thread from glands in its mouth. At night it breaks out of its shelter to hunt for food.
Some females build shelters for their eggs. Most common of these are the solitary bees and wasps. One kind of wasp digs a small tunnel in the ground. She stocks the tunnel with food for the larva that will hatch from the egg. She does this by paralyzing a caterpillar with her sting and dragging the still-alive insect into the hole. Then she lays an egg in the "nursery" and seals the opening.
Solitary wasps called mud daubers build mud nests, in which they lay their eggs. They lay in a stock of paralyzed spiders as food for the young and then deposit their eggs in the nest. The potter wasp builds a jar-shaped nest of mud on a twig. Another, the organ-pipe wasp, builds a nest in the shape of long tubes cemented together like an organ pipe. Each tube is a cell in which she lays an egg.
Shelters Built by Larvae
Some adult insects do not build nests for their young but make it possible for the larvae to build shelters. Certain wasps, aphids, and flies pierce leaves or stems of plants and lay eggs inside the plant tissue. When the larvae emerge, they cause that part of the plant to swell — that is, they produce a gall. Some galls have only one larva in them, and some have many. The soft plant tissue inside the gall supplies food for the larva, and the hard outside covering gives the insect shelter.
The larvae of some insects build shelters without any help from the adults. You may have seen the gray "tents" of tent caterpillars fastened in the forks of tree branches. The tent caterpillars build the tent from silk that they secrete.
Caddis-fly larvae also build shelters. Caddis flies are a group of insects that live near the shores of ponds and streams. The eggs are laid in the water. The larvae that hatch out are called caddisworms. The "worm" builds a case around itself of small pebbles or sticks, glued together or bound with silk. It moves by pushing the front part of its body and its legs out of its case and pulling the rest of its "house" along with it.
Protection for the Pupa
When it is in the pupal, or resting, stage, an insect is helpless. It needs complete protection. Cocoons spun by moth caterpillars give the pupae this protection. The cocoons are made of silk spun out from the salivary glands.
Cocoons are usually white, gray, or brown. They are easy to find in the winter, when trees are bare. You can see some kinds of cocoons hanging down from the branches, like tiny dull Christmas-tree decorations. If you try to tear one open, you will see why a cocoon is such a fine shelter for the pupa. The cocoon is so tough it will not tear easily.
Other insect larvae do not spin cocoons. Instead they develop a hard coat that serves as a shelter for the pupa.
The Importance of Insects
Many insects are very useful to humans. Honeybees give us honey and wax. Silk comes from the cocoon built by the silkworm, the larva of a moth. Shellac is made from the substance given off by the lac insect.
Insects carry pollen from flower to flower as they flit about to gather nectar. This transfer of pollen helps plants produce fruit and seeds. Insects also serve as food for many birds.
But insects are humanity's chief competitors for shelter, clothing, and food. Termites eat the wood of homes; carpet beetles eat carpets; and the larvae of clothes moths eat holes in woolen clothing. The cotton-boll weevil destroys cotton crops. These are only some of the destructive insects. Even more important, insects do tremendous damage to food crops. A swarm of locusts can eat a field bare in a few hours. The larvae of certain moths spoil apples, pears, and other fruits.
Some insects spread disease germs. The germs of encephalitis, or sleeping sickness, as well as those of yellow fever and malaria, are carried by insects. Flies live on filth and carry germs on their feet and antennae.
Controlling Harmful Insects
Humankind fights a constant battle against harmful insects. We do this in a number of ways.
One of these is by the use of insecticide sprays. (An insecticide is a chemical that kills insects.) For many years scientists have also been fighting harmful insects with other insects — a method called biological control. The scientists have done this by bringing in natural enemies of insect pests, such as viruses and bacteria.
Insects are being studied for two main reasons. Scientists want to find ways to encourage the spread of useful insects and new ways to control insect pests. To destroy the pest without harming useful insects is one of the chief aims of scientists. They study an insect in all its life stages, to find when it is easiest to destroy the insect. Some insects are easier to attack as larvae and some as adults.
Spraying insects with insect-disease viruses or germs is being studied. In one experiment low-flying planes sprayed viruses on areas infested with tent caterpillars. After being sprayed once a year for 2 years, tent caterpillars in those areas were wiped out.
Each time scientists discover a disease that affects an insect, they look for a way to spread that disease. One of these ways is to catch some of the insects in traps, infect them with the disease, and release them. The insects then spread the disease to others.
Modern entomologists — scientists who study insects — think that probably the best way to wipe out insect pests is to prevent the females from laying eggs. If no eggs are laid, in a few years that particular insect may be wiped out. To do this, research in methods of making insects sterile is being carried out. (Sterile means unable to reproduce.) The insects are treated with chemicals or are exposed to radiation.
Scientists are also studying certain special chemical sprays for plants. These chemicals are harmless to the plants. But they cause insects that eat them to become sterile.
Scientists have to be very careful when using this kind of technology that they do not cause a serious imbalance in nature. Eliminating one harmful insect may seriously alter the environment. If the mosquito population in one area is wiped out, the fish that feed on the mosquito may die out. If the fish die out, the birds that feed on the fish may also die out. Once this chain of events starts, it is very difficult to stop.
Baiting Traps for Males
It is often difficult to trap large numbers of insects in order to sterilize them. Entomologists have therefore experimented and found a way to trap the males.
Entomologists know that the females of many kinds of insects give off a scent to attract the male. The scent-producing chemicals of females are extracted and used to bait insect traps. Chemists have been able to make some scents artificially in the laboratory.
Males are attracted by the scent and flock to the traps in great numbers. Some kinds of male insects caught in this way are killed outright. Scientists have found it more effective to control other kinds by giving the males a disease or by sterilizing them. Then the males are released. Those infected with disease spread it to others. The sterile males mate with females, but the eggs never hatch out.
Entomologists hope that such techniques will lead to control of insect pests.