The Circulatory System

FIGURE 17.7. Proposed scheme for hemolymph clotting in insects.

hemocyte coagulogen, the cross-linking enzyme, and microparticles. These are bits of coagulocyte plasma membrane that have turned inside out, exposing negatively charged phospholipids. It is thought that the phospholipids may be important in the elimination of the dead or damaged cells, perhaps by attracting phagocytic hemocytes to the site of the wound. The hemocyte coagulogen swells on entering the plasma because of hydration and forms a gel, the basic framework of the clot. The clot is then stabilized and strengthened as the plasmacoagulogen becomes cross-linked to the gel. This step is enzyme-controlled and also requires calcium. It is possible that the cross-linking enzyme is part of the phenoloxidase enzyme complex, promoting tanning of the proteins, as occurs in the formation of exocuticle (Chapter 11, Section 3.3). This speculation is based on the observation by Li et al. (2002) that the insect coagulation pathway has components that also occur in the prophenoloxidase cascade (see Section 5.2).

Metabolic and Homeostatic Functions. Hemocytes have been implicated in a variety of metabolic and homeostatic functions, though for most of these convincing evidence is not available. Largely on the basis of electron microscopy and histochemical studies, in which the cells have been shown to contain, for example, glycogen, mucopolysaccharide, lipid, and protein, it has been suggested that hemocytes are important in storage of nutrients and their distribution to growing tissues, formation of connective tissue, synthesis of chitin, and maintenance of hemolymph sugar level. Crossley (1975) noted that certain amino acids, including glutamate, can be actively accumulated by hemocytes, suggesting that the cells might be important in hemolymph amino acid homeostasis. The ability to accumulate and store glutamate (thereby effectively removing it from the hemolymph) may be of special significance in view of this substance's role as a transmitter at the neuromuscular junction (Chapter 13, Section 2.3). Certain hemocytes (spherule cells in Diptera, oenocytoids in other insects) contain enzymes for metabolism of tyrosine, derivatives of which are important in tanning and/or darkening of cuticle (Chapter 11, Section 3.3) and have a bacteriostatic effect.

530 Various authors also have suggested that the hemocytes may have a significant role in chapter 17 detoxicating poisons, though again convincing evidence is lacking.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

Get My Free Ebook

Post a comment