Insects are numerous invertebrate animals that belong to the Phylum Arthropoda and Class Insecta.
The class Insecta is divided into two subclasses: Apterygota, or wingless insects, and Pterygota, or winged insects. Subclass Pterygota is further divided based on metamorphosis. Insects that have undergone incomplete metamorphosis are the Exopterygota, while insects that undergo complete metamorphosis are the Endopterygota. Insects have an outer bilateral exoskeleton to which the muscles are attached and which provides protection for internal organs. The body is divided into three main parts: the head, which includes mouthparts, eyes, and antennae; the thorax, which operates the jointed legs and/or wings; and the abdomen, which has organs for digesting 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, it enters the ostia openings along the sides of the tube and flows back to the heart. The respiratory system carries O2 to cells and removes CO2 from cells by branching out to all cells of the body. The nervous system consists of a brain that receives information from the eyes and antennae, and controls the whole body. There are also 2 nerve cords containing ganglia fused together to control activities of the segment without the help of the brain.
The insect muscular system is made up of a few thousand small but strong muscles, allowing the insect to carry objects heavier than itself. The digestive system is basically a long tube where food enters the mouth, goes to the crop where it is stored, then to the gizzard where it is ground up, and then to the stomach where it is digested. The undigested parts and wastes are moved to the intestine and colon, and then released at the anus. In the reproductive system, a new individual is produced sexually when the female eggs produced in the ovaries unite with male sperm produced in the testes. Both humans and insects 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 human health and are parasitic on humans and other animals. Insects that feed on human or animal blood can carry diseases in their salivary juices and spread them to other animals. Many insects irritate us without disturbing our health. Some bite and sting, and some people are allergic to them. Additionally, some insects are injurious to our agricultural crops, food products, clothing, and wooden buildings.
So far, man has only had partial success in defending against insects. However, some insect species are beneficial to man. For example, the honey bee supplies us with honey, and the silkworm supplies us with silk. So bugs really aren’t that bad.