Insects are invertebrate animals that belong in the Phylum Arthropoda and Class Insecta. The class Insecta is divided into 2 subclasses: Apterygota, or wingless insects, and Pterygota, or winged insects. Subclass Pterygota is further divided on basis of metamorphosis. Insects that have undergone incomplete metamorphosis are the Exopterygota. Insects that undergo complete metamorphosis are the Endopterygota. Scientists have described and named about 800,000 insects and believe there may be from 1 million to 10 million kinds still undiscovered. Insects live almost everywhere on earth - from steamy tropical jungles to cold polar regions. They live high on snow-capped
, and in deserts below sea level. They can be found in caves deep in the earth, or flying many thousands of feet high in the sky. Most insects are small - less than a fourth of an inch long. They have an amazing variety of shapes and special structures. Insects have an outer bilateral exoskeleton to which the muscles are attached and which provides protection for internal organs. The body is divided into 3 main parts: the head, which include mouth parts, eyes, and antennae; thorax, which operates the jointed legs and/or wings; and abdomen, which has organs for digested food, reproducing, and getting rid of waste products. The major systems in insects are the circulatory, respiratory, nervous, muscular, digestive, and reproductive systems. In the circulatory system, blood is pumped by the heart in a tube to the aorta, the head, and to other organs then enters the ostia openings along the sides of the tube back to the heart. An insect's blood, like ours, carries food and waste products to and from the cells of the body. But unlike our blood, it has little to do with bringing oxygen to the cells. Insect blood is greenish, yellowish, or colorless. Few insects have red blood. The respiratory systems carries oxygen to cells and takes away carbon dioxide by means of a system of tubes. An insect breathes by means of tiny holes, called spiracles, along the sides of its body. The nervous system consists of a brain receiving information from the eyes and antennae. It controls the insect's body activities as a whole. Another nerve center in the head is connected to the brain and controls the insect's mouth parts. Each of the two nerve cords contains a cluster of nerve cells, called a ganglion, in each segment of the thorax and abdomen. The two ganglia in each segment are fused and form a sort of little brain that controls the activities of that segment. The ganglia often can work without the brain. The muscular system is made up of a few thousand small but strong muscles allowing the insect to carry objects heavier than it. Many insects can lift or pull an object 20 or more times heavier than the weight of their bodies. The digestive system is basically a long tube that extends from the mouth to the anus. The food enters the mouth and enters the forgut. The food moves along the tube until it reaches an enlarged area called the crop, where it is temporarily stored and partly digested. The the food passes into the gizzard, which has thick muscular walls that contract and grind the food into small bits. The food then passes into the midgut, where most digestion takes place. Nourishing parts of the food are absorbed into the blood, and wastes and undigested parts move into the hindgut. All wastes and extra water that enter the hindgut leave the body through the anus. Most insects reproduce sexually. A new individual is produced sexually when the female eggs produced in the ovaries has united with male sperm produced in the testes. During mating, the female receives the sperm and stores them in her abdomen. Later, when she lays her eggs, the sperm enter the eggs as they leave her body. Insect eggs have a variety of shapes and color patterns, but most are oval or round and are white or cream colored. The number of eggs laid varies greatly from one species to another. A female insect probably lays an average of 100 to 200 eggs during her lifetime. Both man and insect live almost everywhere, eat all kinds of food, and use all kinds of materials to build homes so they constantly live in conflict. Some insects seriously affect man's health and are parasitic on man and other animals. Insects that feed on human or animal blood can carry disease in their salivary juices and spread the disease to other animals. Many insects irritate us without disturbing our health. Some bite and sting, and some people are allergic to them. Other insects are injurious to our agricultural crops, food products, clothing, and wooden buildings. So far human beings have had only partial success in defending themselves against insects. Beneficial insects include bees, wasps, flies, butterflies, moths, and others that pollinate plants. Many fruits, including oranges, apples, strawberries and grapes depend on insect pollinators for the production of seeds. Some insects provide humans with products such as honey, bees wax, shellac, and silk. Some help keep the landscape clean by feeding on animal wastes and dead animals, or the remains of dead plants. Insects that live in the ground enrich the soil with their waste products and dead bodies. Many insects are helpful because they are predators and feed on harmful insects that destroy crops. . All insects form part of the great web of life that includes human beings and all other living things. They feed on plants and animals, but they also are food for plants and animals.