Nitrogenous Excretion and Salt and Water Balance

Abstract

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, since these factors most easily affect the active site on an enzyme. Since 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 we mean 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 utilizes both salts and water, one or both of which must be recovered later from the urine.