About Insects: About one million species of insects have been identified so far, which is about half of all the animals known to science. Insects live in almost every habitat on land. For example, distant relatives of crickets called rock crawlers survive in the peaks of the Himalayas by producing a kind of antifreeze that prevents their body fluids from freezing solid. At the other extreme are worker ants that forage for food in the Sahara Desert at temperatures above 47° C (116° F). Insects consume an enormous variety of food. In the wild, many eat leaves, wood, nectar, or other small animals, but indoors some survive on a diet of wool clothes, glue, and even soap. As a group, insects have only one important limitation: although many species live in fresh water—particularly when they are young—only a few can survive in the salty water of the oceans.
Insects are often regarded as pests because some bite, sting, spread diseases, or compete with humans for crop plants. Nevertheless, without insects to pollinate flowers, the human race would soon run out of food because many of the crop plants that we rely on would not be able to reproduce. Insects themselves are valued as food in most of the world, except among Western societies. They help to recycle organic matter by feeding on wastes and on dead plants and animals. In addition, insects are of aesthetic importance—some insects, such as dragonflies, beetles, and butterflies, are widely thought to be among the most beautiful of all animals.

Insects range in length from the feathery-winged dwarf beetle, which is barely visible to the naked eye at 0.25 mm (0.01 in), to the walkingstick of Southeast Asia, which measures up to 50 cm (20 in) with its legs stretched out.
The vast majority of insects fall into the size range of 6 to 25 mm (0.25 to 1 in). The heaviest member of the insect world is the African goliath beetle, which weighs about 85 g (3 oz)—more than the weight of some birds.
Regardless of their size, all adult insects have a similar body plan, which includes an exoskeleton, a head, a thorax, and an abdomen. The exoskeleton protects the insect, gives the body its form, and anchors its muscles. The head holds most of an insect's sensory organs, as well as its brain and mouth. The thorax, the body segment to which wings and legs are attached, is the insect's center of locomotion. An insect's large, elongated abdomen is where food is processed and where the reproductive organs are located.

A. Exoskeleton

Like other arthropods, an insect's external skeleton, or exoskeleton, is made of semirigid plates and tubes. In insects, these plates are made of a plasticlike material called chitin along with a tough protein. A waterproof wax covers the plates and prevents the insect's internal tissues from drying out.
Insect exoskeletons are highly effective as a body framework, but they have two drawbacks: they cannot grow once they have formed, and like a suit of armor, they become too heavy to move when they reach a certain size. Insects overcome the first problem by periodically molting their exoskeleton and growing a larger one in its place. Insects have not evolved ways to solve the problem of increasing weight, and this is one of the reasons why insects are relatively small.

B. Head

An insect obtains crucial information about its surroundings by means of its antennae, which extend from the front of the head, usually between and slightly above the insect's eyes. Although antennae are sometimes called feelers, their primary role is to provide insects with a sensitive sense of smell. Antennae are lined with numerous olfactory nerves, which insects rely on to smell food and detect the pheromones, or odor-carrying molecules, released by potential mates. For example, some insects, such as ants and honey bees, touch antennae to differentiate nest mates from intruders and to share information about food sources and danger. The antennae of mosquitoes can detect sounds as well as odors.

Antennae are composed of three segments, called the scape, pedicel, and flagellum. They may have a simple, threadlike structure, but they are often highly ornate. Some male giant silkworm moths, for example, have large, finely branched antennae that are capable of detecting pheromones given off by a female several miles away.
An insect's head is typically dominated by two bulging eyes, which are called compound eyes because they are divided into many six-sided compartments called ommatidia. All of an insect's ommatidia contribute to the formation of images in the brain. Insect eyes provide a less detailed view of the world than human eyes, but they are far more sensitive to movement. Insects with poor vision, such as some worker ants, often have just a few dozen ommatidia in each eye, but dragonflies, with more than 20,000 ommatidia, have very keen vision—an essential adaptation for insects that catch their prey in midair.
Most flying insects also have three much simpler eyes, called ocelli, arranged in a triangle on top of the head. The ocelli can perceive light, but they cannot form images. Clues provided by the ocelli about the intensity of light influence an insect's level of activity. For example, a house fly whose ocelli have been blackened will remain motionless, even in daylight.
The head also carries the mouthparts, which have evolved into a variety of shapes that correspond to an insect's diet. Grasshoppers and other plant-eating insects have sharp-edged jaws called mandibles that move from side to side rather than up and down. Most butterflies and moths, which feed mainly on liquid nectar from flowers, do not have jaws. Instead, they sip their food through a tubular tongue, or proboscis, which coils up when not in use. Female mosquitoes have a piercing mouthpart called a stylet. House flies have a spongy pad called a labellum that dribbles saliva onto their food. The saliva contains enzymes that break down the food, and once some of the food has dissolved, the fly sucks it up, stows away the pad, and moves on.

C. Thorax

The thorax, immediately behind the head, is the attachment site for an insect's legs and wings. Adult insects can have one or two pairs of wings—or none at all—but they almost always have six legs. In some insects, such as beetles, the legs are practically identical, but in other insects each pair is a slightly different shape. Still other insects have specialized leg structures. Examples are praying mantises, which have grasping and stabbing forelegs armed with lethal spines, and grasshoppers and fleas, which have large, muscular hind legs that catapult them into the air. Mole crickets' front legs are modified for digging, and backswimmers have hind legs designed for swimming.
Special adaptations of insect legs help small insects perch on flowers and leaves. House flies and many other insects have a pair of adhesive pads consisting of densely packed hairs at the tip of each leg. Glands in the pads release an oily secretion that helps these insects stick to any surface they land on. These adaptations permit house flies to walk upside down on the ceiling and climb up a smooth windowpane.
Insects are the only invertebrates that have wings. Unlike the wings of birds, insect wings are not specially adapted front limbs; instead, they are outgrowths of the exoskeleton. Insect wings consist of a double layer of extremely thin cuticle, which is interspersed with hollow veins filled with either air or blood. The wings of butterflies and moths are covered by tiny, overlapping scales, which provide protection and give wings their characteristic color. Some of these scales contain grains of yellow or red pigments. Other scales lack chemical pigments but are made up of microscopic ridges and grooves that alter the reflection of light. When the light strikes these scales at certain angles, they appear to be blue or green.
Unlike the legs, an insect's wings do not contain muscles. Instead, the thorax acts as their power plant, and muscles inside it lever the wings up and down. The speed of insect wing movements varies from a leisurely two beats per second in the case of large tropical butterflies to over 1,000 beats per second in some midges—so fast that the wings disappear into a blur. When an insect's wings are not in use, they are normally held flat, but for added protection, some species fold them up and pack them away. In earwigs, the folding is so intricate that the wings take many seconds to unpack, making take-off a slow and complicated business.
In addition to the legs and wings, the thorax contains part of an insect's digestive tract, which runs along the full length of an insect's body. The first section of the digestive tract is called the foregut. In many insects, the foregut contains structures called the crop and the gizzard. The crop stores food that has been partially broken down in the mouth, and the gizzard grinds tough food into fine particles.

Behind the thorax is the abdomen, a part of the body concerned chiefly with digestion and reproduction. The abdomen contains two sections of the digestive tract: the midgut, which includes the stomach, and the hindgut, or intestine. In all insects, a bundle of tubelike structures called the Malpighian tubules lies between the midgut and the hindgut. These tubules remove wastes from the blood and pass them into the intestine.
The abdomen holds the reproductive organs of both male and female insects. In males, these typically include a pair of organs called testes, which produce sperm, and an organ called the aedeagus, which deposits packets of sperm, called spermatophores, inside the female. Many male insects have appendages called claspers, which help them stay in position during mating.
Female insects typically have an opening in the abdomen called an ovipore, through which they receive spermatophores. In most females, this genital chamber is connected to an organ called the spermatheca, where sperm can be stored for a year or longer. Females also have a pair of ovaries, which produce eggs, and many female insects have an ovipositor, which can have a variety of forms and is used to lay fertilized eggs. Among some females, such as infertile bees, the ovipositor functions as a stinger instead of as a reproductive organ.
The abdomen is divided into 10 or 11 similar segments, connected by flexible joints. These joints make the abdomen much more mobile than the head or thorax; it can stretch out like a concertina to lay eggs, or bend double to jab home its sting. In many insects, the last segment of the abdomen bears a single pair of appendages called cerci. Cerci are thought to be sensory receptors, much like antennae, although in some insects they may play a role in defense.

III. Body Functions

Like other animals, insects absorb nutrients from food, expel waste products via an excretory system, and take in oxygen from the air. Insect blood circulates nutrients and removes wastes from the body, but unlike most animals, insect blood plays little or no part in carrying oxygen through the body. Lacking the oxygen-carrying protein called hemoglobin that gives the blood of humans and many other animals its red color, insect blood is usually colorless or a watery green. For oxygen circulation, insects rely on a set of branching, air-filled tubes called tracheae. These airways connect with the outside through circular openings called spiracles, which are sometimes visible as tiny "portholes" along the abdomen. From the spiracles, the tracheae tubes reach deep inside the body, supplying oxygen to every cell. In small insects, the tracheal system works passively, with oxygen simply diffusing in. Larger insects, such as grasshoppers and wasps, have internal air sacs connected to their tracheae. These insects speed up their gas exchange by squeezing the sacs to make them suck air in from outside.
Instead of flowing through a complex network of blood vessels, an insect's blood travels through one main blood vessel, the aorta, which runs the length of the body. A simple tube-like heart pumps blood forward through the aorta, and the blood makes its return journey through the body spaces. Compared to blood vessels, these spaces have a relatively large volume, which means that insects have a lot of blood. In some species, blood makes up over 30 percent of their body weight, compared to only 8 percent in humans. The pumping rate of their hearts is widely variable because insects are cold-blooded—meaning that their body temperature is determined by the temperature of their environment. In warm weather, when insects are most active, an insect heart may pulse 140 times each minute. In contrast, during extremely cold weather, insect body functions slow down, and the heart may beat as slowly as a single pulse per hour.
In the digestive system of insects, the foregut stores food and sometimes breaks it down into small pieces. The midgut digests and absorbs food, and the hindgut, sometimes working together with the Malpighian tubules, manages water balance and excretion. This three-part digestive system has been adapted to accommodate highly specialized diets. For example, fluid-feeders such as butterflies have a pumplike tube in their throats called a pharynx that enables them to suck up their food. Most of these fluid-feeders also have an expandable crop acting as a temporary food store. Insects that eat solid food, such as beetles and grasshoppers, have a well-developed gizzard. Armed with small but hard teeth, the gizzard cuts up food before it is digested. At the other end of the digestive system, wood-eating termites have a specially modified hindgut, crammed with millions of microorganisms. These helpers break down the cellulose in wood, turning it into nutrients that termites can absorb. Since both the microorganisms and the termites benefit from this arrangement, it is considered an example of symbiosis.
Insects have a well-developed nervous system, based on a double cord of nerves that stretches the length of the body. An insect's brain collects information from its numerous sense organs, but unlike a human brain, it is not in sole charge of movement. This is controlled by a series of nerve bundles called ganglia, one for each body segment, connected by the nerve cord. Even if the brain is out of action, these ganglia continue to work.

IV. Reproduction and Metamorphosis

A small number of insects give birth to live young, but for most insects, life starts inside an egg. Insect eggs are protected by hard shells, and although they are tiny and inconspicuous, they are often laid in vast numbers. A female house fly, for example, may lay more than 1,000 eggs in a two-week period. As with all insects, only a small proportion of her young are likely to survive, but when conditions are unusually favorable, the proportion of survivors shoots up, and insect numbers can explode. In the 1870s, one of these population explosions produced the biggest mass of insects ever recorded: a swarm of locusts in Nebraska estimated to be over 10 trillion strong.
In all but the most primitive insects, such as bristletails, the animal that emerges from the egg looks different from its parents. It lacks wings and functioning reproductive organs, and in some cases, it may not even have legs. As they mature, young insects undergo a change of shape—a process known as metamorphosis.
Most insects undergo one of two varieties of metamorphosis: incomplete or complete. Dragonflies, grasshoppers, and crickets are among the insects that experience incomplete metamorphosis. In these insects, the differences between the adults and the young are the least marked. The young, which are known as nymphs (or naiads in the case of dragonflies), gradually develop the adult body shape by changing each time they molt, or shed their exoskeleton. A nymph's wings form in buds outside its body, and they become fully functional once the final molt is complete.
Insects that undergo complete metamorphosis include butterflies, moths, beetles, bees, and flies. Among these species the young, which are called larvae, look completely different from their parents, and they usually eat different food and live in different environments. After the larvae grow to their full size, they enter a stage called the pupa, in which they undergo a drastic change in shape. The body of a pupating insect is confined within a protective structure. In butterflies, this structure is called a chrysalis, and in some other insects the structure is called a chamber or a cocoon. The larva's body is broken down, and an adult one is assembled in its place. The adult then breaks out of the protective structure, pumps blood into its newly formed wings, and flies away.
Once an insect has become an adult, it stops growing, and all its energy goes into reproduction. Insects are most noticeable at the adult stage, but paradoxically, it is often the briefest part of their life cycles. Wood-boring beetles, for example, may spend over a decade as larvae and just a few months as adults, while adult mayflies live for just one day.
For most adult insects, the first priority is to find a partner of the opposite sex. Potential partners attract each other in a variety of ways, using sounds, scent, touch, and even flashing lights, as in the case of fireflies. For animals that are relatively small, some insects have a remarkable ability to produce loud sounds. The calls of some cicadas and crickets, for example, can be heard more than 1.6 km (1 mi) away. As with other methods of communication, each species has its own call sign, or mating call, ensuring that individuals locate suitable mates.
In some species, females seek out males, but in others the roles are reversed. Male dragonflies and butterflies often establish territories, fending off rival males and flying out to court any female that enters their airspace. Like most land animals, most insects have internal fertilization, which means the egg and sperm join inside the body of the female. This process differs from external fertilization, in which a male fertilizes eggs that have already been laid by the female, typically in water. Some species achieve fertilization without direct contact between mating partners. For example, among insects called firebrats, males deposit spermatophores on the ground, and females find the spermatophores and insert them into their receptacles, or gonopores. But among most insects, males and females have to physically pair up in order to mate. In some carnivorous species, in which the males tend to be smaller than females, males run the risk of being eaten during the mating process. Male empid flies protect against this fate by presenting their mating partners with a gift of a smaller insect, which the female eats during copulation. By contrast, male praying mantises approach their mates empty-handed, and while mating is taking place, a female will sometimes eat her partner, beginning with his head.
Egg-laying behavior varies widely among different insect groups. Female walkingsticks simply scatter their eggs as they move about, but most female insects make sure that their eggs are close to a source of food. In some species, females insert their eggs into the stems of plants, and a few species, such as the American burying beetle, deposit their eggs in the tissue of dead animals. An unusual egg-laying behavior is shown by some giant water bugs, in which females glue their eggs to the backs of males after mating. Among some insects, such as cockroaches and grasshoppers, eggs are enclosed in a spongy substance called an ootheca, or egg-mass.
A few insect species have developed parthenogenesis—a form of reproduction that side-steps the need for fertilization. In one form of parthenogenesis, the half-set of chromosomes within an unfertilized egg is duplicated, and the egg then develops as if it had been fertilized. Parthenogenetic females do not have to mate, so they can breed the moment environmental conditions are right. This method of reproduction is common in aphids and other small insects that feed on plant sap. Most use it to boost their numbers in spring, when food is easy to find. In late summer, when their food supply begins to dwindle, they switch back to sexual reproduction.