Insects don't have veins or arteries, but they do have circulatory systems. When blood is moved without the aid of vessels, the organism has an open circulatory system. Insect blood, properly called hemolymph, flows freely through the body cavity and makes direct contact with organs and tissues.
A single blood vessel runs along the dorsal side of the insect, from the head to the abdomen. In the abdomen, the vessel divides into chambers and functions as the insect heart (14). Perforations in the heart wall, called ostia, allow hemolymph to enter the chambers from the body cavity. Muscle contractions push the hemolymph from one chamber to the next, moving it forward toward the thorax and head.In the thorax, the blood vessel is not chambered. Like an aorta (7), the vessel simply directs the flow of hemolymph to the head.
Insect blood is only about 10% hemocytes (blood cells); most of the hemolymph is watery plasma. The insect circulation system does not carry oxygen, so the blood does not contain red blood cells as ours does. Hemolymph is usually green or yellow in color.