The head consists of a mouth, eyes, antennae, and ocelli. Antennae are little feelers that the bug uses to learn more about the smell of its outside environment, and ocelli are tiny organs that, depending on light intensity, stimulate an insect to become more or less active. Insects also mostly have four wings and six legs attached to the thorax. The abdomen at the very back contains the digestive and reproductive organs. This structure provides protection and is replaced as the insect grows.
Insects also have both a heart and a respiratory, or breathing, system.
Except for a dorsal vessel, these creatures lack a network of veins and arteries to carry their blood-like body fluids throughout their bodies. However, the dorsal vessel, which is located just under the insect's back, receives the body fluids and, through muscular contractions, pumps these fluids back throughout the insect's body. In addition, insects take in oxygen through tiny openings called spiracles, and this oxygen circulates through the small, vessel-like tracheoles. The insect's body tissues can then exchange their carbon dioxide waste gas for this oxygen. Collectively, insects have developed a variety of useful characteristics that allow them to survive in their varied environments.