3.4. The circulatory system
Hemolymph, the insect body fluid (with properties and functions as described in section 3.4.1), circulates freely around the internal organs. The pattern of flow is regular between compartments and appendages, assisted by muscular contractions of parts of the body, especially the peristaltic contractions of a longitudinal dorsal vessel, part of which is sometimes called the heart. Hemolymph does not directly contact the cells because the internal organs and the epidermis are covered in a basement membrane, which may regulate the exchange of materials. This open circulatory system has only a few vessels and compartments to direct hemolymph movement, in contrast to the closed network of blood-conducting vessels seen in vertebrates.