You may know about the human circulatory system but what do you know about insects? Insects have a very different circulatory system and its blood is also very different. Insects have one major blood vessel the runs the length of the body. The blood vessel is open at both ends and thickens in parts to form hearts (insects have more than one heart). The heart pumps the blood out of the ends of the vein allowing it to flow around the body. The heart has small holes in it allowing the blood to reenter the heart, so the vein always have blood in it. Insect blood is also colorless or a slight yellow/green. If you have every squished an insect an seen red it was blood from an animal/person the insect had bitten.