18 Nitrogenous Excretion and Salt and Water Balance 1. Introduction Enzymatically controlled reactions occur at the optimum rate within a narrow range of physical conditions. Especially important are the pH and ionic content of the cell fluid, as these factors readily affect the active site on an enzyme. | 18 Nitrogenous Excretion and Salt and Water Balance 1. Introduction Enzymatically controlled reactions occur at the optimum rate within a narrow range of physical conditions. Especially important are the pH and ionic content of the cell fluid as these factors readily affect the active site on an enzyme. As the conditions existing within cells and tissues are necessarily dependent on the nature of the fluid that bathes them in insects the hemolymph it is the regulation of this fluid that is important. By regulation is meant the removal of unwanted materials and the retention of those that are useful to maintain as nearly as possible the best cellular environment. Regulation is a function of the excretory system and is of great importance in insects because they occupy such varied habitats and therefore have different regulatory requirements. Terrestrial insects lose water by evaporation through the integument and respiratory surfaces and in the process of nitrogenous waste removal. Brackish-water and saltwater forms also lose water as a result of osmosis across the integument in addition they gain salts from the external medium. Insects inhabiting fresh water gain water from and lose salts to the environment. The problem of osmoregulation is complicated by an insect s need to remove nitrogenous waste products of metabolism which in some instances are very toxic. This removal uses both salts and water one or both of which must be recovered later from the urine. 2. Excretory Systems . Malpighian Tubules Rectum The Malpighian tubules and rectum functioning as a unit form the major excretory system in most insects. Details of the rectum are given in Chapter 16 Section and only the structure of the tubules is described here. The blindly ending tubules which usually lie freely in the hemocoel open into the alimentary canal at the junction of the midgut and hindgut Figure . Typically they enter the gut individually but may fuse first to form a common sac or .
