Excretory Systems

Publisher Summary This chapter describes the excretory system in insects. Insects are able to occupy a wide variety of terrestrial niches and effectively deal with issues of water balance largely because of two important adaptations: their impermeable exoskeleton and an excretory system of considerable sophistication. The excretory system maintains the internal environment of the insect by separating and eliminating metabolic wastes and other toxic substances from body compartments. Because these wastes are often dissolved in water, excretory processes are also closely associated with osmoregulation and the maintenance of water balance. The Malpighian tubules are the primary excretory organs of insects and the driving force for excretion through them is the movement of ions across the epithelium. Superimposed on the excretion of primary urine by Malpighian tubules is a second system involving the rectum of the hindgut that recovers most of the ions and water, adjusting the excretory product so that it achieves the necessary osmoregulatory balance for the insect and excretes the secondary urine. In insects, the need for water conservation may have been the driving force for the incorporation of their nitrogen wastes into uric acid, which is an ideal excretory product for small terrestrial animals. Uric acid is highly insoluble in water and therefore fails to reach toxic levels in body fluids, so it requires about 50 times less water to dilute than does ammonia. Insects pay a high price for the benefits they derive from employing uric acid as a way to excrete nitrogen and still maintain a positive water balance. The synthesis of uric acid from protein results in the loss of several carbon atoms that could be used for other biosyntheses and requires a substantial amount of energy to build a larger, less toxic molecule.