Insects: A Comprehensive Introduction to the World of Arthropods

Insects

Introduction to Insects

Insect, an invertebrate animal related to spiders, ticks, scorpions, and centipedes. Insects make up approximately 80 percent of the total number of animals on earth. Familiar insects include bees, bugs, beetles, flies, ants, grasshoppers, and butterflies.

The goliath beetle has white stripes and grows up to four inches long.

An adult insect has six legs and a pair of feelers, or antennae. It has a jointed body divided into three segments: a head, a thorax, and an abdomen. Most insects have two pairs of membranous wings; some have one pair of wings; still others are wingless.

Insects are very adaptable. They live in a variety of habitats, and are able to withstand extreme climates and harsh environments. Insects are found in the Arctic, in the tropics, on mountains, in deserts, in water, in the soil, and in or on plants and animals. Some are harmful agricultural pests while others carry diseases. Some insects are beneficial, pollinating flowers and crops.

The grasshopper lives in a variety of habitats.

More than one million species of insects have been classified. Estimates of the total number of species vary from 2,500,000 to 10,000,000. There are approximately 100,000 species in North America.

The fossil record shows that insects existed 390,000,000 years ago. Some of the earliest insects, such as the cockroach, have changed very little. Others, such as the dragonfly, have changed considerably; fossils of dragonflies with 30-inch (76-cm) wing-spans have been found. Besides giving clues to the changes in structure and development of insects, the fossil record shows that they were as abundant 150,000,000 years ago as they are today.

The Structure of Insects

An insect does not have a spine or internal skeleton. Instead, it has an exoskeleton, a tough outer covering made up of several layers. The most rigid of these layers is composed of chitin, a horny, waterproof substance. Like a suit of armor, the exoskeleton is constructed of jointed plates and segments. Membranes connecting these segments give flexibility to the insect's body. The internal organs and muscles are attached to and supported by the exoskeleton. The surface of the exoskeleton is often vividly colored, and it contains sensory organs in the form of hairs, pits, and plates.

Head

The head is boxlike in shape and is composed of six fused segments. Located here are the brain, the eyes, the antennae, and the mouth parts.

Brain. The brain is discussed later in this article

Eyes. Most insects have two types of eyes—a pair of compound eyes and two or three simple eyes. The compound eyes are composed of hexagonal or circular facets, which record multiple images. The number of facets per eye varies, depending on the type of insect. For instance, houseflies have some 4,000 facets per eye; dragonflies as many as 30,000. The facets provide the insect with mosaic vision, which allows it to see quick movements.

The simple eyes, or ocelli, are located on the head between the compound eyes. The ocelli are not used for vision, but to detect changes in the intensity of light.

Antennae. Between and either below or above the compound eyes are a pair of antennae. The antennae are organs of smell, touch, and, in some insects, hearing. Loss of antennae leaves an insect almost helpless. There is much variation in the structure of the antennae among the different kinds of insects, and they are often larger in the male.

Mouthparts. Insect mouthparts typically consist of a labrum, or upper lip; a pair of mandibles, or jaws; a pair of maxillae, jaw-like appendages; a labium, or lower lip; and a hypopharynx, a tongue-like structure. These parts vary in shape and function from one kind of insect to another. The types of mouthparts that an insect has determine what kind of food it can eat and how it eats.

Generally, insects have mouthparts suited for either chewing or sucking. Some insects have strong mandibles with cutting edges for biting off solid food and chewing it. Other insects have mouthparts that form a proboscis, a tube that has a spongy end for soaking up liquids. In still other insects, the mouth-parts form a hollow beak used for piercing plant or animal tissues.

Thorax

The thorax is composed of three segments—the prothorax, the mesothorax, and the metathorax—each of which bears a pair of jointed legs. In insects with two pairs of wings, one pair is located on the mesothorax, the other on the metathorax. In insects with one pair of wings, the wings are located on the mesothorax. Within the thorax are the muscles that move the wings and legs.

Legs. Most insects use their three pairs of legs primarily for walking. However, the legs may be adapted for such other activities as digging, crawling, jumping, swimming, or grasping. The segment closest to the body is the coxa; then come the trochanter, femur, and tibia, and finally the tarsus. The tarsus, or foot, has one to five segments, and usually has one or two claws at the tip. The tarsus may be padded on the underside, as in some kinds of flies. The three pairs of legs may differ from each other in use and appearance. The legs are usually covered with hairs, and tarsi often have sensory organs of taste, smell, and vibration.

Wings. Wings used for flying are thin and membranous, and are strengthened by veins, tubular structures formed by supportive tissue. In most insects, both pairs of wings are for flying. In others, the front wings are thick, and either hard or leathery. These wings cover the membranous ones and protect them when the insect is not flying.

Insects of one large group, which includes the housefly and the mosquito, have no hind wings. Instead, they have a pair of halteres, knobbed organs used for maintaining balance during flight. Among the wingless insects are fleas and lice. There are many different variations in the wings of insects, and usually the order to which an insect belongs can be determined by its wings.

Abdomen

An insect's abdomen usually has 10 or 11 ringlike segments, but some of these are difficult to distinguish because of fusion or modification. Appendages are found on the last three segments. The last segment may have two to three cerci, taillike sensory appendages. In the female insect, the eighth and ninth segments are modified for mating and may be formed into an egg-placing organ, the ovipositor. In some insects, notably bees, wasps, and certain ants, the ovipositor is modified into a stinger, a needle-like shaft that contains either poison or a nonpoisonous substance that is irritating. In the male, structures called claspers, which are located on the ninth segment, are used for holding the female during mating.

Internal Systems

The internal systems of the insect perform such functions as digestion of food, excretion of body wastes, respiration, and circulation.

Digestion. The digestive system has three sections—the foregut, midgut, and hindgut. The foregut typically includes the mouth, the esophagus, and two sacs—the crop and the proventriculus. The crop stores undigested food. In insects that eat solid food, the proventriculus contains toothlike projections that grind food; in others, it serves mainly as a valve. The midgut consists of the stomach and gastric caeca, cone-shaped pouches that secrete digestive enzymes. The hindgut consists of the intestines, rectum, and anus.

Excretion. The main excretory organs are the Malpighian tubules, narrow tubules with a function similar to that of kidneys in vertebrates. The tubules empty into the intestines.

Respiration. Although insects breathe air, they do not have lungs. Instead, a system of branching air tubes, or tracheae, reaches all parts of the body. Air enters through openings called spiracles. Most adult insects have two thoracic and eight abdominal spiracles on each side of the body.

Circulation. Insects are cold-blooded animals with a simple open system for circulating blood. A tube, called the dorsal vessel, extends along the dorsal (upper) side of the body, under the body wall. It has two parts: the heart, in the posterior (hind) part; and the aorta, in the anterior (fore) part. The blood is drawn into the heart through several pairs of openings called ostia. It is pumped forward through the aorta into the head. From the head it seeps back between the internal organs to the abdomen, and is again drawn into the heart. The blood of insects is usually green, yellow, or colorless.

Nervous System. The nervous system responds to the sense organs and coordinates movement. It consists of the brain and pairs of ganglia, nerve centers connected by cords of tissue running along the lower surface of the body. The brain is composed of three pairs of fused ganglia, known collectively as the supraesophageal ganglion. The brain controls the movements of the eyes, antennae, and labrum. The subesophageal ganglion, a structure also composed of three pairs of fused ganglia, controls the movements of certain muscles behind the head and near the mouth. It is connected by nerve cords to thoracic ganglia, which are connected to abdominal ganglia. The thoracic ganglia and abdominal ganglia control movements of the body and legs. If the brain is injured, the ganglia take over its functions.

Life Cycles

Most insects come from fertilized eggs that are laid by the adult female. There are, however, exceptions. The eggs of some insects, for example, hatch within the female's body and the young are born alive; this is called viviparous birth. Some female insects produce young without mating, a process called parthenogenesis. And some insects are able to reproduce while immature.

The development followed by most insects is called metamorphosis. There are two types: incomplete and complete.

Incomplete, or Direct, Metamorphosis

This type of metamorphosis takes place in three stages—egg, nymph, and adult. Nymphs hatched from the eggs look like miniature adults, except they lack wings and are sexually immature. As a nymph grows and matures, the exoskeleton is shed, or molted, several times. Insects that undergo incomplete metamorphosis include grasshoppers and dragonflies. The nymphs of dragonflies and certain other insects are aquatic and are called naiads.

Incomplete metamorphosis takes place in three stages: egg, nymph, and adult.Complete, or Indirect, Metamorphosis

This type of metamorphosis takes place in four stages—egg, larva, pupa, and adult. It is the most complex form of development among insects and is found in such insects as bees, flies, and butterflies.

A larva hatches from the egg. It is worm-like, and does not resemble the adult insect it will become. Examples of larvae are caterpillars, grubs, maggots, army worms, and wrigglers. Some of the greatest destruction by insects occurs when they are in the larval stage. During this period they grow rapidly and require large amounts of food. The larva molts several times.

During the pupal stage, the insect is usually enclosed in a case. The pupa appears to be inactive, but major structural changes take place during this period. At the end of the pupal stage, the fully grown, mature insect emerges.

Insect Behavior

Most insects are solitary. However, some insects, such as ants, bees, and termites, live in large colonies. The colony is divided into castes, or classes. Most colonies are headed by an adult female, the queen. Typically, one caste is responsible for building and repairing the nest, while another is responsible for feeding and caring for the young. Other castes take care of the queen, locate food, and defend the nest against invaders.

Insects communicate with others of their species in a variety of ways. Some insects communicate by making vibrating noises with their wings or with membranes on their abdomen. Some insects make noises by expelling air from the pharynx or by beating a leg or wing on a hard surface. Some insects secrete various kinds of chemical substances called pheromones. Each kind of pheromone has a specific function, such as to attract members of the opposite sex, to identify an individual's caste, or to mark a trail.

Insect Study

The scientific study of insects is called entomology. Entomologists classify insects and study their anatomy and physiology, life cycles, and habits. They help develop insecticides and insect-resistant crops. The U.S. Department of Agriculture conducts research on means of controlling insect pests. Some state universities are also active in insect research. Through genetic engineering techniques, researchers are developing insect-resistant fruit trees and crops.

Useful and Harmful Insects

Although humans generally regard insects as harmful, most insects are actually harmless or helpful. In fact, less than 2 per cent of all species are harmful; but among them, they can cause major crop damage and spread serious diseases on a large scale.

Useful Insects

Insects constitute more than half of the diet of fishes, birds, amphibians, reptiles, and small mammals. In some parts of the world insects are eaten by humans.

Insects pollinate many different types of plants. They improve the soil by bringing nutrient-rich soil from deeper layers to the surface. Many insect pests are destroyed by predatory or parasitic insects.

Several commercial products are obtained from insects. Among these are honey, beeswax, natural silk, and shellac. The tree galls made by certain insects contain substances that are used in medicines and dyes, and in tanning leather.

Harmful Insects

Insect pests are responsible for causing billions of dollars' worth of damage each year. More than 600 species of insects in North America are considered harmful pests. Insects not only harm or kill plants and animals, but they also spread diseases, infest stored foods and grains, and cause damage to homes and other buildings and to clothing and furniture.

Most insects that spread disease do so by biting their victims, passing the disease organisms—such as bacteria, fungi, and protozoans—in their saliva. Plants and animals, including humans, are subject to various viral diseases carried by insects. Insect-borne diseases transmitted to humans include malaria and encephalitis.

Insects are controlled by a variety of methods. Common backyard insects are killed by insecticides or by “bug zappers,” fluorescent tubes covered by wires carrying an electric current. The tubes emit ultraviolet light, attracting insects; the insects are killed when they come in contact with the wires. Agricultural pests are killed by insecticides or by such organisms as parasitic bacteria or roundworms, which are released into infested areas.

Classification of Insects

Insects form the class Insecta of the phylum Arthropoda. Insects are divided into orders based on wing structure, mouth parts, and metamorphosis. Entomologists do not agree on the number of orders or their names. Most, however, divide insects into the 31 orders listed below. The orders are usually arranged with the most primitive insects first. Orders 1 through 4 do not undergo metamorphosis; 5 through 21 undergo incomplete metamorphosis; and 22 through 30 undergo complete metamorphosis.

1. Protura. Tiny; wingless; no eyes or antennae. An example is the proturan.

2. Diplura. Wingless; two long tail-like cerci. An example is the japygid.

3. Thysanura. Wingless; three long tail-like cerci.

4. Collembola. Tiny; wingless; tail-like appendage permits them to spring suddenly. An example is the springtail.

5. Grylloblattodea. Wingless; small eyes (or none); chewing mouthparts. Called cricket-cockroaches.

6. Orthoptera. Leathery forewings; large membranous hindwings (or none); chewing mouthparts. Most have long, muscular hindlegs for jumping.

7. Phasmatodea. Some wingless; others have large, colorful wings; chewing mouthparts. Bodies resemble twigs or leaves.

8. Dictyoptera. Leathery forewings; large membranous hindwings (or none); chewing mouthparts; triangular, downward-facing head.

9. Mantophasmatodea. Wingless; chewing mouthparts; long, straight body; downward-facing head; hook-like feet.

10. Dermaptera. Small leathery forewings; large membranous hindwings (or none). Chewing mouth-parts.

11. Plecoptera. Large, membranous wings; chewing mouthparts; soft body. The nymphs are aquatic.

12. Ephemeroptera. Membranous wings; no mouth-parts; soft body; two or three long tail-like projections. Aquatic nymphs.

13. Odonata. Large, glassy, membranous wings; chewing mouthparts. Aquatic nymphs.

14. Isoptera. Social insects living in large nests. Physical structure varies according to the role played by the individual in the caste system. Chewing mouthparts; soft body.

15. Embioptera. Males have narrow, membranous, brown wings; chewing mouthparts. Spin silken webs from glands on the forelegs. An example is the web-spinner.

16. Psocoptera. Typically four narrow, membranous wings held over the abdomen when at rest; chewing mouthparts. An example is the book louse.

17. Zoraptera. Typically four narrow, membranous wings shed at maturity; chewing mouthparts. Inhabit rotten wood and live in colonies. An example is the zorapteran.

18. Mallophaga. Wingless; chewing mouthparts; small, flat body, eyes reduced. Parasites of birds and mammals.

19. Anoplura. Similar to order Mallophaga, except with sucking mouthparts. Feed only on the blood of mammals.

20. Thysanoptera. Long hair fringes on narrow wings; sucking mouthparts. Spread viral plant diseases. An example is the gladiolus thrips.

21. Hemiptera. Forewings thick at the base and membranous at the tip. Sucking mouthparts. Many are plant pests that transmit disease; some are bloodsuckers.

22. Homoptera. Thickened, membranous wings; piercing-sucking mouthparts. All are plant eaters.

23. Neuroptera. Net-veined wings; chewing mouthparts; soft body. Both aquatic and terrestrial.

24. Coleoptera. Forewings hard, meeting in a straight line down the back; hindwings membranous; chewing mouthparts. This is the largest order of insects.

25. Strepsiptera. Females are saclike parasites within other insects. Males have tiny forewings; large membranous hindwings; chewing mouthparts. An example is the stylops.

26. Mecoptera. Long, narrow wings; chewing mouthparts; head lengthened to form a beak. In males, the tip of the abdomen may be turned up. An example is the scorpion fly.

27. Trichoptera. Long, hairy wings; chewing mouthparts. Aquatic larvae build stone cases.

28. Lepidoptera. Wings covered with scales, often colored; sucking mouthparts.

29. Diptera. Typically two membranous wings and two knoblike halteres. Sucking mouthparts.

30. Siphonaptera. Small; sides of body flattened; sucking mouthparts. All adults feed on the blood of mammals.

31. Hymenoptera. Smaller hindwings; chewing or sucking mouthparts; many with constricted waist.

Includes the social insects, except the termites.