The cells of all animals, including insects, are bathed in an extracellular fluid (ECF); and most cells of the animal exchange solutes with the ECF, not the external environment. In most animals, including insects, solute exchange between cells and ECF is facilitated by the bulk flow of ECF, powered by one-to-many pumps (including hearts). The physiological system mediating bulk flow is known as the circulatory system, the topic of this chapter. Most insects have one major pump, the dorsal vessel, and multiple accessory pumps, with the circulating portion of the ECF known as the blood or hemolymph. The insect circulatory system is open, meaning that the hemolymph flows freely around insect organs, in contrast to the closed circulatory system of vertebrates in which blood is retained within vessels. The structural organization of the insect circulatory system and the determinants of blood flow are described in Sections 5.1 and 5.2, respectively, and the composition of hemolymph is considered in Section 5.3.
Hemolymph has four key functions. It is the vehicle for transport of hormones and nutrients between tissues, and a site for storage of some nutrients and water; these two functions are considered in Section 5.3. It is also a crucially important component of the insect immune system. The humoral (non-cellular) immune function is addressed in Section 5.3 and the cellular function in Section 5.4. Other insect organs also contribute to insect immunity, notably the gut (Chapter 3), fat body (Chapter 6) and Malpighian tubules (Chapter 18).