Insects are members of the phylum Arthropoda (see \Tarthropod\t), class Insecta, the largest and most diverse class in the animal kingdom. The number of described insect species is estimated to be 750,000 and the actual number of living species is perhaps 3 million. Even at 750,000 species, however, insects outnumber all other plant and animal groups. Insects have three body divisions--head, thorax, and abdomen--and six legs borne on the thorax as an adult. Many insects possess wings as adults. Because of their small size, ability to fly, rapid reproductive rate, and external skeleton (exoskeleton), insects are highly successful animals that have thoroughly exploited every habitat in the biosphere except the polar ice caps. The exoskeleton is coated with a waxy layer that helps insects conserve body moisture. The small size of insects aids in their dispersal to more favorable habitats. Thrips and aphids are carried by frontal systems for long distances. Small insects have been collected by airplane at 3,000 m (10,000 ft). Migratory butterflies and locusts may fly hundreds of kilometers in a few weeks. Their rapid rate of development allows insects to respond fairly precisely, genetically speaking, to changes in the environment. That is, most insects mature in a year or less, and many insects have several generations each year. Because most individual insects are either male or female, recombination of genetic material takes place with each generation and allows \Tnatural selection\t to shape the genetic course of a species. Insects display almost every color, from drab, dull browns, and black to iridescent blues, reds and purples, or the metallic greens, blues, yellows, and reds of some \Lwasp\ls and \Lbee\ls. Size varies from tiny mymarid wasps less than 1 mm (.04 in) long to huge beetles or slender walking sticks up to 30 cm (12 in) long. Shape is equally variable. For example, legs may be adapted for walking, jumping, running, clinging to hairs, swimming, spinning silk, carrying pollen, hearing, or smelling, or they may be absent. Almost any aspect of insects is usually predictable within related groups but is highly variable among the orders. STRUCTURE AND FUNCTION Insects usually have a clearly defined head, thorax, and abdomen. The head bears antennae, eyes, and mouthparts; the thorax bears legs and wings; and the abdomen has various styli, cerci (antennalike sensory organs), and the genital apparatus. Most insects have a pair of relatively large compound eyes on the head as well as two or three simple eyes (ocelli) on top of the head. Food is chewed by the mandibles while it is held, sensed, and manipulated by the maxillae. Salivary glands generally open on or near the middle of the hypopharynx. The masticated food is directed through the preoral cavity, surrounded by the labrum, mandibles, maxillae, and hypopharynx, into the mouth. In the Hemiptera--fleas, flies, lice, and thrips--the mouthparts are modified into piercing stylets. Host plants or animals are pierced by the stylets, and juices are sucked up by the insect. The thorax is the middle body region of an insect from which the legs and wings originate. The thorax is joined to the head by the neck and to the abdomen. The thorax is divided into the prothorax, mesothorax, and metathorax. Each segment typically bears one pair of legs and the last two segments bear the wings. The legs of insects are divided by joints into six segments: the coxa (next to the body), trochanter, femur, tibia, pretarsus, and tarsus or foot. The legs of insects often have secondary functions which may affect leg size and shape dramatically. A common secondary function of legs in males is that of holding the female during mating. Except when thickened and tough, the wings of insects often have veins, or hardened tubes that strengthen the wing. Wing venation is important to the classification of insects. The abdomen is composed of 11 or fewer segments. The last few abdominal segments are often associated with the external portions of the reproductive organs and with them are called the terminalia. The terminalia in female insects are often modified into an elaborate egg placer (ovipositor) which in the bees and wasps has been modified into a stinger. The integument, or skin, of insects is particularly important because of its flexibility, its waterproof quality, and its hardness. The integument is formed by three layers: the basement membrane (connective tissue, which forms the innermost layer), the epidermis (a cellular layer just above the basement membrane, which secretes the cuticle), and the cuticle (the exoskeleton of the insect, which also lines the fore and hind gut). The cuticle consists largely of protein and of chitin (a substance closely related to the cellulose of plants). Cuticle is not extensible; growth is possible only until the wrinkles are smoothed out of the cuticulin. Then, an insect must molt (shed) to resume growth. In the abdomen, thorax, and head, invaginations of the integument into the body cavity give insects the equivalent of an internal skeleton in these areas of high mechanical stress. Circulatory System and Breathing The heart is a series of pumping chambers with an upper tube having openings along the side; the tube allows a colorless blood to be moved forward by a wave of contraction. Blood bathes the internal organs and is only partially enclosed by an upper diaphragm. Because insects are cold blooded, pulse rate may range from 140 per minute in an active insect to 1 per hour in chilled insects. Insects usually breathe by taking in air through a series of holes (spiracles) along the thorax and abdomen. Leading from the spiracles are tubes (tracheae) which interconnect to form a tracheal system. Feeding and Digestion Insects obtain food by a variety of methods, including biting, lapping, and sucking. Butterflies have a coiled proboscis that can straighten out and be thrust into a flower to suck out nectar. Honeybees lap nectar with their tongues. Bumblebees are equipped for both lapping and biting and are able to chew their way into a flower to reach nectar. Beetles, wasps, and ants have powerful biting mouthpieces. Some beetles are capable of chewing through lead or zinc. All insect jaws work at right angles; they are hinged at the sides of the head and bite inward, from side to side. Carnivorous insects such as the praying mantis catch and chew their prey. Some wasps, by contrast, paralyze their prey with venomous stings, lay their eggs in the bodies, and provide a living food supply for their young. The ichneumon fly deposits its eggs in the tissues of newly hatched insects and produces tiny, ravenous larvae that begin to devour their host. Termites are able to digest and receive nourishment from wood because tiny protozoans within their digestive system "predigest" the cellulose in the wood. The digestive system in insects is essentially a tube that begins with the mouth and is divided into a pharynx; esophagus; crop, stomach, or midgut, and intestine, or hind gut; colon; and rectum. The midgut has glandular outgrowths called the gastric ceca, which secrete digestive juices. The Malpighian tubes remove nitrogenous waste from the blood. Each blind tube empties into the hind gut at its junction with the stomach. The principal end product is nonsoluble uric acid, which is passed out with the feces. SENSE ORGANS AND NERVOUS SYSTEM Insects orient to the environment by means of touch and stretch receptors in the integument. Auditory receptors are usually stretch-type receptors associated with an "eardrum" (tympanum). Unlike other animals, insects have light receptors, or eyes, that can neither be turned nor moved and are usually fitted together into a pair of compound eyes. A compound eye may contain a honeycomb of as many as 28,000 lenses, like that found in the large dragonfly. Each lens serves as a basic unit, or ommatidium. The images from each ommatidium are somehow interpreted together in the insect's brain, but it is not known what the insect sees. In addition, many insects have two or three simple eyes, called ocelli, and these probably can only distinguish light from dark. Most insects detect odors through olfactory receptors often found on the antennae and in the mouthparts and feet. Beetles that feed on carrion can scent a carcass for many kilometers, but if their antennae are damaged they cannot detect the carrion from a few meters away. Ants leave trails of odorous chemicals (pheromones) that can be recognized by others in the same colony, even when the scent leads through a maze of crossing trails left by ants of other colonies. The antennae are also sensitive to sound waves, or vibrations of air molecules, by means of special structures called Johnston's organs situated on the antennae. Katydids and crickets have ears in the form of sensitive hairs on the front legs. The nervous system in insects primarily consists of paired nerve masses (ganglia) resembling a ladder lying on the lower inner surface of the body. In the head there is a ganglion (cluster of nerve cells). Nerves connect the cerebral ganglion with the nerve masses in the body, which control muscles and direct the endocrine system to release hormones. Growth and development in insects is largely regulated by hormones secreted by the brain. Hormones may be liberated in the blood to keep the insect actively growing or may be directed to specific endocrine glands via the connective nerve to control molting, heart beat, sugar content of the blood, juvenile characteristics, and egg production. LIFE CYCLE The reproduction system of insects consists of a pair of gonads (testes or ovaries) connected to a median duct, which opens at a gonopore. During copulation the male introduces sperm into a copulatory aperture, where it is stored in the female's spermatheca. As an egg matures, it slides into the median oviduct, where it is fertilized by the sperm. When the egg is laid, it may be glued to the substrate or covered with a protective secretion that originated from the accessory glands. Almost 85 percent of insect species develop by a process of complete \Tmetamorphosis\t, which is a series of successive stages that do not resemble one another. The remainder, except for less than 1 percent, undergo incomplete metamorphosis, where changes are not extreme. The life cycle of an insect usually starts with the placement of an egg on a substrate, the development of an immature form through successive molts, and then the emergence of the adult stage at the last molt. Adult insects usually mate, feed, and then lay eggs. Some insects give birth to live young (an aphid may produce more than 175 young). Some female termites lay 2,000 to 3,000 eggs per day and may live 15 to 50 years. Eggs may be variously shaped and covered by scales or secretions. Immature insects are called nymphs, naiads, or larvae. Nymphs develop gradually and resemble their adult stage. Naiads develop gradually but are aquatic. Larvae generally do not resemble the adult stage, and they develop through a pupal stage, usually nonmotile, often in a cocoon. Larvae often live in a habitat different from that of the adult and feed on different resources. Larvae have been named on the basis of form and function. Caterpillars, grubs, and maggots are immature stages of moths, beetles, and flies, respectively. Pupae have also been categorized by form (see \Tpupa\t). Obtect pupae have all the limbs and wings seemingly "glued down" to the body. Exarate pupae have the wings and legs standing out from the body. Coarctate pupae are exarate fly pupae that are formed inside the last larval integument (puparium). Certain cicadas develop for seventeen years. Care of the eggs and the young by insects ranges from the practice of walking sticks, in which eggs are scattered indiscriminately, to that of earwigs, lace bugs, and certain sawflies, in which the mother broods over the eggs and young nymphs or larvae. Ant, wasp, and bee queens rear the first brood of workers by themselves. Probably the most fascinating life history in the class Insecta is that of a beetle, Micromalthus debilis. One of the immature stages is capable of laying an egg or giving birth to live larvae. If the egg hatches, the resulting larva devours its mother larva and develops into a male beetle. Larvae that are born live can either develop into female beetles or reproduce as larvae. The offspring of adult Micromalthus beetles are unknown. The adult beetles are thought to be sterile. BEHAVIOR Insect behavior, for the most part, is hereditary, and responses to stimuli are mostly automatic or instinctive. Direct responses (moving toward or away from the stimuli) may be made in reaction to light, temperature, water, contact, gravity, or currents of air or water. Often a response to a stimulus can be modified by other stimuli, as well as by the insect's physiology, food, and state of development. Some behavior involves a series of different acts. Such complex behavior includes nest building and mating. While this behavior may appear intelligent, it is usually found to be instinctive. However, since insects can be taught to modify their behavior, it is believed that they have a limited capacity to learn. Examples include the ant that can learn a maze and the honey bee that finds its way home by recognizing landmarks. Social Behavior Many insects occur in groups, each group differing in the factors that bring the individuals together. Often the aggregation results from a mutual attraction to the same stimulus such as food supply. Ants, termites, and some bees live in more integrated groups, called societies. The insect society works as a unit despite the large number of individuals. One distinct feature is the division of labor, with coordination of the activities of each individual with great efficiency. Auditory Behavior Sound plays an important role in insect behavior. Only a few sounds produced by insects are heard by humans, because these sounds are either too low or too high pitched. Sound is produced in several ways. Rubbing one body part against another, called stridulation, may involve almost any part of the body, in various species. Some insects vibrate special membranes called tymbals, as in the leafhoppers. A few insects will strike a part of their body on the substrate, for example, some grasshoppers use their feet. Many insects produce sound on a continuous basis during certain periods; they are said to sing. The rhythmic features of insect songs are affected by such factors as temperature, chirp rate, and pulse rate. The principal role played by sound is in mating. Because songs of singing insects are different and because females will respond only to songs of their species, song plays a role in species isolation and evolution. Defenses Most insects try to escape when threatened and some insects "play dead," for example, some beetles fall to the ground after folding up their legs, giving the appearance of a clump of dirt. Many insects use shelters ranging from burrows in the ground to elaborate shelters constructed of various materials. Insects also employ camouflage. (See also \Tmimicry\t.) Many are so colored that they blend into their background, such as moths colored like the bark of trees. Some insects bear a close resemblance to objects in the environment, such as inchworms, which resemble twigs. Other insects will cover themselves with debris or excrement. Chemical defenses often involve distasteful body secretions, repellent secretions, or poisonous injection into an attacker. The use of the sting is probably the most effective and often a severe method. The only stinging insects are Hymenoptera (bees, wasps, and some ants). Flight Although most insects fly, their weight-to-wing ratio in theory is not advantageous to flight. They build up enough energy so that speed of the wing-beat makes up for the theoretical lack of lifting power. The vibrating insect wing actually follows a figure 8 or ellipsoid path. The effect is the same as a propeller going round and round, and the insect is drawn forward by a stream of air directed downward and backward. Insects may vibrate their wings at speeds of up to 600 to 1,000 complete beats per second, which is about 20 times faster than the wing beats of any bird. Insects move their wings by a combination of forces: the action of the flight muscles, which attach the wings to the thorax, and the buildup of forces in the thorax itself, which acts as if it contained a spring. Each contraction of the wing muscles compresses the thoracic box, which then springs back and thus helps in powering the next wing stroke. EVOLUTION AND CLASSIFICATION Insects are thought to have evolved from a centipedelike class of terrestrial arthropods (class Symphyla) at least as far back as the Devonian Period, more than 350 million years ago. Others believe that both the insects and the symphylans evolved from a common ancestral group called the Protosymphyla. All available evidence indicates that these first insects were wingless and not very different from those now included in the order Thysanura: the bristletails and silverfish common in houses. During the next 100 million years a great diversity of insects occurred over the land and winged insects appeared, representing the first animals to fly. The known living species of insects may be divided into 23 to 32 orders, depending on the authority cited. The most convenient classification, however, may be according to evolutionary grades, each representing adaptations to a wide range of environments. At the lowest level, therefore, are the primitive wingless insects such as silverfish and springtails (Collembola). The Collembola are the earliest fossil insects known and are now among the most abundant of all insects. The higher orders of insects have wings and are divided into two major groups: Palaeoptera, or "ancient wings," and Neoptera, which are the greater number of present-day insects. Palaeoptera are the most primitive winged species and cannot fold the wings over the abdomen when at rest. They contain two surviving orders: dragonflies (Odonata) and may-flies (Ephemeroptera). They are excellent flyers but need large open spaces to maneuver in with their spread wings. Neoptera do possess a wing-flexing, or folding, mechanism and can crawl into small spaces. Many orders of Neoptera have this ability, including Orthoptera (\Lgrasshopper\ls and \Lcricket\ls) and Isoptera (\Ltermite\ls). The highest evolutionary grade includes those insects that undergo complete metamorphosis as well as fold their wings. Insects that undergo complete metamorphosis emerge from the egg as often wormlike larvae that change first into a pupa, often encased in a cocoon, before emerging as an adult. This group includes the most diverse and successful of all insects: beetles (Coleoptera); butterflies and moths (Lepidoptera); flies (Diptera); fleas (Siphonaptera, which have become wingless); and the bees, wasps, and ants (Hymenoptera). Some bees and wasps and all ants have developed a highly organized social life, with differentiation of members of the community into castes with different duties. Evolution has also resulted in elaborate means of communication by pheromones or gestures (see \Tanimal communication\t) and highly engineered nest structures that even contain means for controlling temperature and humidity. INSECTS AND HUMANS The science concerned with insects is called entomology, which involves the description, classification, and evolution of insects, and research studies such as insect metabolism, behavior, genetics, and ecology. The fruit fly has long been a preferred experimental animal for research in basic genetics and has permitted discoveries that also apply to humans, other animals, and plants. Entomological research is also concerned with improving knowledge of those insects that are helpful or harmful to humans. Pollination by insects is essential for many crops, especially fruit. Bees are the most important pollinators and also make honey, one of the oldest crops developed by humans. Other products derived from insects include beeswax, used in polishes, and silk from the cocoons of the silkworm moth. Other insects, such as locusts and boll weevils, may destroy crops and stored products. Millions of locusts periodically swarm across parts of Africa and eat everything green in their path. The boll weevil, which is a tiny brown beetle, destroys millions of dollars worth of cotton annually in the United States. The gypsy moths defoliate a wide variety of orchard and forest trees. Much entomological research is directed at devising methods for controlling these pests. Many insects carry diseases fatal to humans and livestock. The science concerned with preventing such diseases is called medical entomology. Some of the diseases include sleeping sickness, bubonic plague, malaria, typhoid fever, dysentery, and cholera. Thus, although crops, houses, clothes, and even health may be damaged by insects, some benefits may be found in pollination and in the fascination of observing the fantastic variation in form, color, and size of insects. James R. Baker Bibliography: Borror, D. J., et al., An Introduction to the Study of Insects, 4th ed. (1976); Elzinga, Richard, Fundamentals of Entomology (1978); Klots, Alexander and Elsie, Living Insects of the World (1975); Rainey, R. C., ed., Insect Flight (1976); Saunders, D. S., Insect Clocks (1976); Swann, Ralph B., The Common Insects of North America (1972); Tweedie, Michael, An Atlas of Insects (1974) and Insect Life (1977); Wigglesworth, Vincent B., The Life of Insects (1964); Wilson, Edward O., The Insect Societies (1971).