Insect Circulatory System: Unlike humans, insects have an open circulatory system, where blood, known as hemolymph flows freely throughout the body, instead of flowing through veins and arteries. A tube called a dorsal vessel runs along the insect's thorax and abdomen. The heart us the dorsal vessel within the abdomen. As it beats, it moves the hemolymph around the body. Hemolymph lacks the protein hemoglobin, so the color of hemolymph, instead of being red, is often clear of greenish.