The volume of the hemolymph may be substantial (20–40% of body weight) in soft-bodied larvae, which use the body fluid as a hydrostatic skeleton, but is less than 20% of body weight in most nymphs and adults. Hemolymph is a watery fluid containing ions, molecules, and cells. It is often clear and colorless but may be variously pigmented yellow, green, or blue, or rarely, in the immature stages of a few aquatic and endoparasitic flies, red owing to the presence of hemoglobin. All chemical exchanges between insect tissues are mediated via the hemolymph — hormones are transported, nutrients are distributed from the gut, and wastes are removed to the excretory organs.
However, insect hemolymph only rarely contains respiratory pigments and hence has a very low oxygen-carrying capacity. Local changes in hemolymph pressure are important in ventilation of the tracheal system (section 3.5.1), in thermoregulation (section 4.2.2), and at molting to aid splitting of the old and expansion of the new cuticle. The hemolymph serves also as a water reserve, as its main constituent, plasma, is an aqueous solution of inorganic ions, lipids, sugars (mainly trehalose), amino acids, proteins, organic acids, and other compounds. High concentrations of amino acids and organic phosphates characterize insect hemolymph, which also is the site of deposition of molecules associated with cold protection (section 6.6.1). Hemolymph proteins include those that act in storage (hexamerins) and those that transport lipids (lipophorin) or complex with iron (ferritin) or juvenile hormone (JH-binding protein).
The blood cells, or hemocytes (haemocytes), are of several types (mainly plasmatocytes, granulocytes, and prohemocytes) and all are nucleate. They have four basic functions:
- phagocytosis — the ingestion of small particles and substances such as metabolites;
- encapsulation of parasites and other large foreign materials;
- hemolymph coagulation;
- storage and distribution of nutrients.
The hemocoel contains two additional types of cells. Nephrocytes (sometimes called pericardial cells) generally occur near the dorsal vessel and appear to function as ductless glands by sieving the hemolymph of certain substances and metabolizing them for use or excretion elsewhere. Oenocytes may occur in the hemocoel, fat body, or epidermis and, although their functions are unclear in most insects, they appear to have a role in cuticle lipid (hydrocarbon) synthesis and, in some chironomids, they produce hemoglobins.