Hemolymph Composition

The hemolymph is the major extracellular fluid in insects. It makes up from 15% to 75% of the volume of the insect, varying significantly with species and individual physiological state. The hemolymph is the major transport medium for the exchange of materials between cells, such as hormones, waste materials, and nutrients. Through its regulation of ionic and chemical composition, it maintains the proper internal environment for cells as an extracellular extension of intracellular fluids. In this role, it contributes to the ability of the insect to live at both high and low temperatures. It serves as a major compartment and storage reserve for water. The hemolymph is far from being a static reservoir for the storage of metabolites, however. It is a dynamic tissue that changes with the changing physiological state of the insect. Additionally, it maintains the hydrostatic pressure required to maintain body shape in soft-bodied insects and to facilitate the splitting of the cast skin at ecdysis. The volume of the hemolymph often increases toward the end of each stadium and contributes to the rupture of the old cuticle. The ptilinum of flies is inflated by hemolymph to allow the adult to exit from the hardened puparium (Figure 7.10). FIGURE 7.10. The ptilinum of dipterans that is inflated at adult emergence and used to escape from the puparium. From Jones (1977) . Reprinted with permission. Copyright © 1977

The hemolymph consists of the liquid plasma and the cellular hemocytes. Plasma composition is variable. It is usually clear but may be colored green or yellow in some insects, reflecting the pigments that are present. Its pH can also be variable; it is generally slightly acidic but may be alkaline in some species. A variety of soluble components contribute to its total osmotic pressure. In most other animals, the major inorganic components of the body fluid are sodium and chloride, but in insect blood the composition can be quite different. In primitive apterygotes, sodium and chloride do indeed appear to be the most important osmotic effectors. Primitive exopterygotes, including Ephemeroptera, Odonata, and Dictyoptera, also largely use sodium and chloride, but with contributions from magnesium, potassium, and calcium. In endopterygotes, such as Diptera, Mecoptera, and Neuroptera, sodium is again an important cation, but chloride is replaced by higher concentrations of amino acids and other organic components (Figure 7.11). In the endopterygotes Hymenoptera and Lepidoptera, amino acids and other organic molecules play a major role along with potassium but with a reduced involvement of sodium. These ionic differences were once attributed to either phytophagous or carnivorous diets, because plant-feeding insects contained higher levels of potassium and insects feeding on other diets had higher levels of sodium. However, this generalization has numerous contradictions, and the relationship between the ionic composition of the hemolymph and basic diet of the insect is not entirely clear. FIGURE 7.11. Components of the hemolymph in representatives of six insect orders. Adapted from Sutcliffe (1963) . Copyright © 1963

The concentration of free amino acids in the insect hemolymph can be as high as 50 to 100 times that of mammalian plasma. For example, the total hemolymph amino acids reach 150 mmol/L in tsetse compared to about 2 mmol/L in humans. As discussed previously, these amino acids become more prominent in evolutionarily advanced groups. Exopterygotes are characterized by lower levels of amino acids than in endopterygotes and show uniform concentrations of the amino acids that are present. In contrast, endopterygotes contain some amino acids in much higher concentrations than others. For example, glutamic acid and proline are often found in significantly higher concentrations in the amino acid pool depending on the physiological state of the particular insect. Levels of free amino acids tend to be higher in females than in males, especially during egg maturation. In several insects, proline is used as a substrate for flight and declines during activity as alanine concentrations increase. Proline is also the predominant amino acid in honey bee queens, workers, and drones. The concentrations of methionine, glutamic acid, and aspartic acid are correlated with the activity of the silk glands during the development of the silkworm moth. Leucine and isoleucine tend to be present in lower concentrations in most insects.

Other organic components of the hemolymph include carbohydrates, various Krebs cycle intermediates, uric acid, and soluble proteins. As discussed in Chapter 6, trehalose, an α-1,1 disaccharide of two glucose residues, serves as the major circulating energy source in most insects (see Figure 6.22). It also is used as a cryoprotectant that stabilizes cell membranes to prevent damage from freezing and plays a role in the regulation of feeding. It is present at concentrations of 5 to 50 times higher than glucose, the circulating energy source in vertebrates. This higher level in insects may be one of the compromises necessary to make up for the inefficiency of the circulatory system in distributing these materials by diffusion. The blood sugar in insects must exist in a higher concentration to ensure that it reaches remote areas of the hemocele in sufficient concentration. If glucose were to serve this function as it does in vertebrates, the higher concentration required in the blood would interfere with the uptake of glucose by diffusion through the digestive tract. By using the disaccharide trehalose in the blood, which is usually rare in the diet, high levels of blood sugar can be maintained without interfering with the uptake of glucose. Levels in the hemolymph do not appear to be regulated homeostatically, however, as they can vary significantly depending on physiological state. FIGURE 7.22. A. Longitudinal section of the bumblebee, showing the path of the dorsal vessel through the narrow petiole and the flight muscles. B. The countercurrent heat exchange occurs with the passage of cool hemolymph from the abdomen within the heart at the same time that warm hemolymph flows backward in the hemocele. The warm hemolymph from the thorax prewarms the hemolymph from the abdomen before its heat can be dissipated when it flows into the abdomen. From Heinrich (1976) . Reprinted with permission. Copyright © 1976

Soluble proteins in the hemolymph include the vitellogenins, which are yolk proteins produced by the female fat body and taken up by the oocytes. Several enzymes, including esterases, chitinases, and proteases, appear in the hemolymph depending on the developmental stage of the insect.

The hemolymph may be a physical deterrent to predation. Autohemorrhaging, or reflexive bleeding, occurs in some insects when they are attacked. Hemolymph that is fortified with defensive terpenoids such as cantharadin may be released outside the body through intersegmental membranes to discourage ants and other insect predators. The loss of hemolymph may be substantial; in chrysomelid beetles, as much as 13% of the wet weight of the larvae can be lost through autohemorrhage. The hemolymph clots immediately and can bind ants in the coagulum that is formed. In many cases, the hemolymph is returned to the body cavity and the wound heals rapidly.