title: Regulatory responses of the coxal organs and the anal excretory system to dehydration and feeding in the spider Porrhothele antipodiana (Mygalomorpha: Dipluridae)
abstract: The roles of coxal and anal excretion in the regulation of haemolymph osmolality, [Na+] and [K+] were studied in the mygalomorph spider Porrhothele antipodiana (mass 0.7&shy;1.0 g) under differing conditions of feeding and hydration state. Dehydration of starved spiders by removal of drinking water caused progressive mass loss at a rate of about 2.5 % of initial body mass per day and was associated with increases in the whole-body [Na+] and [K+] and in the osmolality, [Na+] and [K+] of the haemolymph. On provision of prey, feeding partially restored this mass loss but further elevated body and haemolymph ion concentrations. Dehydration reduced fluid excretion by the anal excretory system and the four coxal organs in both starved and feeding spiders. Starved hydrated spiders initially produced anal urine at 5 &micro;l day-1 and this was progressively reduced to zero after 4 days of water deprivation. Spiders dehydrated to less than 12 % mass loss would nevertheless feed and this initiated a small post-prandial anal diuresis (<5 &micro;l day-1 compared with >30 &micro;l day-1 in fed hydrated spiders). Coxal fluid was produced by dehydrated spiders only during feeding and was delivered into the prey, the rate of production by single organs decreasing from about 19 &micro;l h-1 g-1 body mass in hydrated spiders to about 4 &micro;l h-1 g-1 body mass in spiders dehydrated to 11 % mass loss. There was an increase in urine [K+] and in the rate of anal K+ excretion associated with ad libitum feeding in dehydrated spiders. However, urine [Na+] and the rate of anal excretion of Na+ were not increased by feeding. This was associated with an increase in [K+] of the stercoral fluid above that observed in either fed or starved hydrated spiders, but no significant change in [Na+]. Conversely, [Na+] of the coxal fluid produced during feeding was increased by dehydration whereas [K+] was not. These observations are consistent with the previously postulated roles of the coxal organs (Na+) and anal system (K+) in the excretion of ions ingested with the prey. Full elimination of the prey ions was accomplished only after drinking water was resupplied, which initiated further anal and coxal diureses. Smaller anal and coxal diureses also occurred on rehydration of unfed spiders. The production of coxal fluid in the absence of prey is further evidence that the coxal organs have a true excretory function besides, presumably, assisting ingestion. During dehydration and feeding, P. antipodiana, unlike many insects, is unable simultaneously to conserve water and to eliminate ions by production of a highly concentrated excretory fluid. Both coxal fluid and anal urine were approximately iso-osmotic to the haemolymph and the urine was markedly hypo-ionic.
authors.initials: A
authors.lastname: Butt
authors.initials: H
authors.lastname: Taylor