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.

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.

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.

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.

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.