The circulatory system of insects is different from that of vertebrates in two important ways.

Insect blood (generally called hemolymph) has no respiratory pigment or hemoglobin. Respiration is carried out by the tracheal system. Rather than oxygen, the blood in insects distributes metabolites, hormones, water and waste to the appropriate organs. The circulatory system is extremely simple, so that the hemolymph is pumped forward through a single element, the dorsal vessel. This vessel leads to an aorta along which there are thoracic bulbs. The dorsal vessel’s simplicity belies its importance for it is the means by which hemolymph is circulated … it is the insect’s “heart”. Each chamber along the vessel begins and ends with a valve (ostium) through which the blood is pumped. The valve is structured in such a way that it pinches closed automatically with each contraction (see inset A), ensuring the hemolymph moves forward from the back to the head. 

Along the way, blood emerges at the wings, in the dorsal bulbs and at the frontal sinus (space between organs without delimitation). It then flows freely, bathing muscles and organs.  To better place the blood at certain times, auxiliary hearts are found on the thorax, at the wings and legs. In the wings, they help after molting to deploy the new, tightly packaged wings before the cuticle hardens. An insect’s every move can help to circulate the hemolymph in the insect’s body. Even respiration, accompanied by muscular contractions which effect the total volume of the animal help to guarantee the circulation of hemolymph throughout its body.