SALT AND WATER BALANCE IN THE SPIDER, PORRHOTHELE ANTIPODIANA (MYGALOMORPHA: DIPLURIDAE): EFFECTS OF FEEDING UPON HYDRATED ANIMALS

urine via the anal excretory system (Malpighian tubules, midgut diverticula and stercoral pocket) at a mean rate of about 2-5-5//I g"1 day"1 and with a mean Na+/K+ ratio of about 1-0. Salts ingested from the prey were eliminated by two mechanisms. A K+-rich (Na+/K+ about 0-2) anal diuresis lasted about 3 days following a single meal and was maintained at more than SO^lg^'day"1 during feeding ad libitum. The second mechanism, interpreted as coxal secretion, func-tioned only during feeding itself and delivered Na+ into the prey at a constant rate of about 3 %h~ ' of total body Na+. This progressively raised the Na+/K+ ratio of the prey debris from 0-47 to 0-96 and, because of re-ingestion, recycled more Na+ than was originally present in the prey. Feeding was associated with large net increases in dry weight and ions, particularly K+, which were mainly stored in the diverticular tissue (midgut diverticula and Malpighian tubules embedded in adipose tissue). The stercoral fluid (final urine) was slightly hyposmotic to the haemolymph in starved and fed spiders. Only about half of its osmolarity was accounted for by Na+, K+ and Cl~. The volume of water gained from the meal was about equal to that lost in diuresis, and P. antipodiana drinks to maintain water balance because of relatively high transpirational and other losses. The primary function of the diuresis is probably elimination of ions from the meal, and not volume regulation