Metabolism

Substances absorbed through the gut wall (occasionally the integument; e.g., certain insecticides) seldom remain unchanged in the hemolymph for any length of time but are quickly converted into other compounds. Metabolism comprises all of the chemical reactions that occur in a living organism. It includes anabolism (reactions that result in the formation of more complex molecules and are, therefore, energy-requiring) and catabolism (reactions from which simpler molecules result and energy is released). Anabolic reactions include, for example, the formation of structural proteins or enzymes from amino acids, and the formation from simple sugars of polysaccharides that serve as an energy store. Many catabolic reactions have evolved for the specific purpose of producing the large quantities of energy required by the organism for performance of work.

The metabolism of insects generally resembles that of mammals, details of which can be found in standard biochemical texts. The present account, therefore, will be largely comparative in nature.

5.1. Sites of Metabolism

Chemical reactions are carried out by all living cells, though they are usually limited in number and, of course, are related to the specific function of the cell in which they occur. For example, in midgut epithelial cells, metabolism is directed largely toward synthesis of specific proteins, the enzymes used in digestion. Metabolism in muscle cells is specifically concerned with production oflarge amounts of energy, in the form of ATP, for the contraction process. In epidermal cells reactions leading to the production of chitin and certain proteins, the components of cuticle, are predominant. Certain tissues, however, are not so specialized

504 and in them a multitude of biochemical reactions, involving the three major raw materials

(sugars, amino acids, and lipids), are carried out. In vertebrates the liver performs these CHAPTER 16 *

multiple functions. The analogous tissue in insects is the fat body (Kilby, 1965; Keeley,

The fat body is derived during embryogenesis from the mesodermal walls of the coelomic cavities. In other words, it is initially a segmentally arranged tissue though this becomes obscured as the hemocoel develops. Nevertheless, and contrary to what a casual examination may suggest, the fat body does have a definite arrangement in the hemocoel characteristic of the species. Typically, there are subepidermal and perivisceral layers of fat body, plus sheets or cords of cells occur in other specific locations. Thus, the fat body presents a large surface area to the hemolymph, allowing the rapid exchange of metabolites (Dean et al., 1985). Contrary to what was thought originally, the fat body is not a single, uniform tissue (Haunerland and Shirk, 1995; Jensen and B0rgesen, 2000). Not only are there regional and species-specific differences, but also differences between larval and adult fat body, as well as in fat body cell types, have been reported. For example, Jensen and B0rgesen (2000) described 11 cell types in queens of the pharaoh ant, Monomorium pharaonis, based on their position, histochemistry, and ultrastructure. Groups of cells of each type are located in specific positions throughout the body. It should be stressed, however, that there is little evidence for the functions of these many histotypes.

The fat body is composed mainly of cells called trophocytes, though in some species urate cells (urocytes) and/or mycetocytes (Section 4.1.2) also can be seen scattered throughout the tissue. In embryos, early postembryonic stages, and starved insects the individual trophocytes are easily distinguishable, their nucleus is rounded, and their cytoplasm contains few inclusions. As such, they closely resemble hemocytes, with which they probably have a close phylogenetic relationship. In later larval stages and adults the trophocytes enlarge and become vacuolated. The vacuoles contain reserves of fat, protein, and glyco-gen. The trophocyte nuclei are proportionately large and frequently become elongate and much branched. During metamorphosis in endopterygotes, the reserves are liberated into the hemolymph. In some Diptera and Hymenoptera the majority of trophocytes also disintegrate at this time, and the fat body appears to be completely re-formed in the adult from the few cells that remain.

In a number of species uric acid accumulates in large quantities in specific cells, the urocytes, within the fat body. Cochran (1985) disputed the traditional view that this is a form of storage excretion (Chapter 18, Section 3.3) and proposed that such accumulation represents a mobile reserve of nitrogen, especially in species such as cockroaches and termites whose natural diet is deficient in this element. In cockroaches the urocytes surround mycetocytes (see below) and there is circumstantial evidence to suggest that the bacteria are intimately involved in the synthesis and utilization of the uric acid (Cochran, 1985).

5.1.2. Mycetocytes

Mycetocytes (bacteriocytes) are found in widely different groups of insects, though in common these have nutritionally poor or unbalanced diets such as wood (ants, ceramby-cid, and anobiid beetle larvae), phloem sap (aphids, planthoppers, mealybugs), and blood (bedbugs, tsetse flies, sucking lice). Mycetocytes are generally stated to be specialized fat

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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