The fat body

In many insects, especially the larvae of holometabol-ous groups, fat body tissue is a conspicuous component of the internal anatomy (Figs. 3.7 & 3.15). Typically, it forms a white or yellow tissue formed of loose sheets, ribbons, or lobes of cells lying in the hemocoel. The structure of this organ is ill-defined and taxonomically variable, but often caterpillars and other larvae have a peripheral layer of fat body beneath the cuticle and a central layer around the gut. The fat body is an organ of multiple metabolic functions, including: the metabolism of carbohydrates, lipids, and nitrogenous compounds; the storage of glycogen, fat, and protein; the synthesis and regulation of blood sugar; and the synthesis of major hemolymph proteins (such as hemoglobins, vitellogenins for yolk formation, and storage proteins). Fat body cells can switch their activities in response to nutritional and hormonal signals to supply the requirements of insect growth, metamorphosis, and reproduction. For example, specific storage proteins are synthesized by the fat body during the final larval instar of holometabolous insects and accumulate in the hemolymph to be used during metamorphosis as a source of amino acids for the synthesis of proteins during pupation. Calliphorin, a hemolymph storage protein synthesized in the fat body of larval blow flies (Diptera: Calliphoridae: Calliphora), may form about 75% of the hemolymph protein of a late-instar maggot, or about 7 mg; the amount of calliphorin falls to around 3 mg at the time of pupariation and to 0.03 mg after emergence of the adult fly. The production and deposition of proteins specifically for amino acid storage is a feature that insects share with seed plants but not with vertebrates. Humans, for example, excrete any dietary amino acids that are in excess of immediate needs.

The principal cell type found in the fat body is the trophocyte (or adipocyte), which is responsible for most of the above metabolic and storage functions. Visible differences in the extent of the fat body in different individuals of the same insect species reflect the amount of material stored in the trophocytes; little body fat indicates either active tissue construction or starvation. Two other cell types - urocytes and myceto-cytes (also called bacteriocytes) - may occur in the fat body of some insect groups. Urocytes temporarily store spherules of urates, including uric acid, one of the nitrogenous wastes of insects. Amongst studied cockroaches, rather than being permanent stores of excreted waste uric acid (storage excretion), urocytes recycle urate nitrogen, perhaps with assistance of mycetocyte bacteria. Mycetocytes (bacteriocytes) contain symbiotic microorganisms and are scattered through the fat body of cockroaches or contained within special organs, sometimes surrounded by fat body. These bacteria-like symbionts appear important in insect nutrition.

3.6.5 Nutrition and microorganisms

Broadly defined, nutrition concerns the nature and processing of foods needed to meet the requirements for growth and development, involving feeding behavior (Chapter 2) and digestion. Insects often have unusual or restricted diets. Sometimes, although only one or a few foods are eaten, the diet provides a complete range of the chemicals essential to metabolism. In these cases, monophagy is a specialization without nutritional limitations. In others, a restricted diet may require utilization of microorganisms in digesting or supplementing the directly available nutrients. In particular, insects cannot synthesize sterols (required for molting hormone) and carotenoids (used in visual pigments), which must come from the diet or microorganisms.

Insects may harbor extracellular or intracellular microorganisms, referred to as symbionts because they are dependent on their insect hosts. These microorganisms contribute to the nutrition of their hosts by functioning in sterol, vitamin, carbohydrate, or amino acid synthesis and/or metabolism. Symbiotic microorganisms may be bacteria or bacteroids, yeasts or other unicellular fungi, or protists. Studies on their function historically were hampered by difficulties in removing them (e.g. with antibiotics, to produce aposymbionts) without harming the host insect, and also in culturing the microorganisms outside the host. The diets of their hosts provided some clues as to the functions of these microorganisms. Insect hosts include many sap-sucking hemipterans (such as aphids, psyllids, whiteflies, scale insects, leafhoppers, and cicadas) and sap- and bloodsucking heteropterans (Hemiptera), lice (Phthiraptera), some wood-feeding insects (such as termites and some longicorn beetles and weevils), many seed- or grain-feeding insects (certain beetles), and some omnivorous insects (such as cockroaches, some termites, and some ants). Predatory insects never seem to contain such symbionts. That microorganisms are required by insects on suboptimal diets has been confirmed by modern studies showing, for example, that critical dietary shortfall in certain essential amino acids in aposym-biotic aphids is compensated for by production by Buchnera symbionts. An important role for bacteria is verified in acetogenesis and nitrogen fixation. Although insects were presumed to lack cellulases, they are present at least in termite guts, yet their role in cellulose digestion relative to that of symbionts is unclear.

Extracellular symbionts may be free in the gut lumen or housed in diverticula or pockets of the midgut or

The excretory system and waste disposal

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