Nitrogenous Excretion and Salt and Water Balance

Abstract

The removal of nitrogenous wastes and maintenance of a suitable hemolymph salt and water content are two closely linked processes. In most insects, the predominant nitrogenous waste isuricacid,whichis removedfrom the hemolymph viatheMalpighian tubules as the solublesodium or potassium salt. Precipitation of uricacid occurs usually inthe rectum as a resultof resorption ofions and water from, and acidification of,theurine. Allantoinand allantoic acid are excreted in quantity by some insects and may be the major nitrogenous waste. Urea is of little significance as a waste product, and ammonia is generally produced only in aquatic species. Insects are usually abletoregulate thesaltand water content of the hemolymph within narrow limits. In all insects, a primary excretory fluid, isosmoticwithhemolymphbut dif- fering in ionic composition, is produced in the Malpighian tubules. Production of tubule fluid is driven by active transport of potassium ions. When this fluid reaches the poste- rior rectum, it is modified according to an insect’s needs. In terrestrial insects selective resorption ofions and/or water occurs. Freshwater species osmoregulate by producing hy- poosmoticurine fromwhich useful materials have been resorbed.Inahyperosmoticmedium they become osmoconformers, their hemolymph osmotic pressure paralleling that of the medium. Brackish-water and saltwater insects have excellent ability to osmoregulate over a wide range of environmental conditions. In dilute media they behave much like freshwa- ter species, forming hypoosmoticurine and resorbing useful components. In mediawith osmotic pressures greater than that ofhemolymph,saltwater species drink excessivelyand produce hyperosmotic urine by secreting ions across the rectal wall; in contrast, brackish- water insects osmoconform by increasing the concentration of amino acids and trehalose in the hemolymph. Bothdiureticand antidiuretic hormones are known. The former stimulate Malpighian tubule fluid production and mayinhibit water resorption from the rectum; antidiuretic hormones mostly appear to act only by stimulating water resorption from the rectal lumen; however, in a few species the antidiuretic factor inhibits potassium ion transport (hence formation of the primary excretory fluid) in the Malpighian tubule.