534 bidirectional, the heartbeat is typically seen as a forwardly moving wave of peristalsis that begins near the posterior end of the dorsal vessel. However, reverse peristalsis is common, especially in pupae and adults, and may play a role in the tidal flow of hemolymph in association with the ventilation movements for gas exchange.

Hemolymph is composed of cellular components, hemocytes, and a non-cellular fraction, plasma. In primitive insects, inorganic ions, especially sodium and chloride, are the major osmotic effectors in hemolymph, but, through evolution, the tendency is for sodium and chloride to be replaced by magnesium, potassium, and organic anions, and for organic components to become the dominant contributors to osmotic pressure. However, diet may considerably influence the composition of plasma. Plasma, especially of larval endoptery-gotes, contains high and relatively constant concentrations of amino and carboxylic acids. Other organic constituents of plasma include enzymes and other proteins, trehalose and other carbohydrates, hormones, and lipids, but, with the exception of trehalose, their concentration is variable. Apart from the specific functions of its components, plasma is important as a transport system, a source of cell water, a hydrostatic system for transmitting pressure changes from one part of the body to another, and for some species in thermoregulation.

In many species virtually all of the hemocytes are in circulation; in some other species most hemocytes do not normally circulate through the body but remain loosely attached to tissue surfaces to be mobilized under specific conditions such as wounding, entry of foreign organisms, and ecdysis. Several types ofhemocytes occur of which three, prohemo-cytes, plasmatocytes, and granulocytes, are common to most insects. Prohemocytes are the stem cells from which other types differentiate. Plasmatocytes are the principle phagocytic hemocytes and, together with granulocytes, are nodule-forming and encapsulating agents. Granulocytes are probably also important in intermediary metabolism. In addition, in many species, cystocytes are present which participate in hemolymph coagulation.

Hemolymph plays an important role in insect immunity to disease. In insects innate immunity appears to be principally a hemocyte-based phenomenon and includes wound healing, coagulation, and destruction of invading organisms by phagocytosis, nodule formation and encapsulation for which the ability to distinguish between self and foreign (including altered self) is critical. Some innate immunity also results from agglutinating and lytic substances in the plasma. In keeping with their generally short life history and high fecundity, for most species acquired immunity is short-lived and non-specific. The immunogens are peptides and include cecropins, defensins, attacins, diptericins, and lysozymes. The first two substances alter permeability of the bacterial membrane, attacins and defensins interfere with cell membrane synthesis, and lysozymes degrade the murein in the cell wall.

Many parasitoids resist the immune response of their host, either by passive or active means. Passive resistance can result from the invader being in a location where it does not trigger an immune response or having a surface coat that is not recognized as foreign. Active resistance mechanisms include immobilization and destruction of the host's hemocytes and prevention of immunogen formation or action. In addition, the parasitoid disrupts the host's endocrine balance and modifies its metabolism, preventing normal host growth and channeling resources for the parasitoid's benefit.

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.

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