3.7. The excretory system and waste disposal
Excretion — the removal from the body of waste products of metabolism, especially nitrogenous compounds — is essential. It differs from defecation in that excretory wastes have been metabolized in cells of the body rather than simply passing directly from the mouth to the anus (sometimes essentially unchanged chemically). Of course, insect feces, either in liquid form or packaged in pellets and known as frass, contain both undigested food and metabolic excretions. Aquatic insects excrete dilute wastes from their anus directly into water, and so their fecal material is flushed away. In comparison, terrestrial insects generally must conserve water. This requires efficient waste disposal in a concentrated or even dry form while simultaneously avoiding the potentially toxic effects of nitrogen.
Furthermore, both terrestrial and aquatic insects must conserve ions, such as sodium (Na+), potassium (K+), and chloride (Cl-), that may be limiting in their food or, in aquatic insects, lost into the water by diffusion. Production of insect urine or frass therefore results from two intimately related processes: excretion and osmoregulation — the maintenance of a favorable body fluid composition (osmotic and ionic homeostasis). The system responsible for excretion and osmoregulation is referred to loosely as the excretory system, and its activities are performed largely by the Malpighian tubules and hind- gut as outlined below. However, in freshwater insects, hemolymph composition must be regulated in response to constant loss of salts (as ions) to the surrounding water, and ionic regulation involves both the typical excretory system and special cells, called chloride cells, which usually are associated with the hindgut. Chloride cells are capable of absorbing inorganic ions from very dilute solutions and are best studied in larval dragonflies and damselflies.