Salt and water balance in the spider Porrhothele antipodiana (Mygalomorpha: Dipluridae).
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Total ion and water balance of the mygalomorph spider Porrhothele antipodiana under conditions of feeding and starvation, hydration and dehydration, and experimental salt loading was studied. Transpirational water loss was comparable to that of xeric arthropods, and respiratory water loss was reduced by the action of spiracular valves. Water loss associated with the silk was small, but elevated activity during web construction increased total water loss. Excretory water loss occurred in the urine and the coxal fluid. Urine was produced at a rate of 5-7 µl day-1 by hydrated starved animals, but increased markedly following feeding. Coxal excretion was normally limited to periods of feeding and fluid was produced at a rate of 72 µl hr-1gm animal-1. It is transported forward, via cuticular grooves on the ventral surface of the cephalothorax, to the mouth. Seventy percent of the fluid is reingested, implicating it in the feeding process. Dehydration results in a cessation of urine production by starved animals and a marked reduction in the volume excreted following feeding. The rate of coxal excretion during feeding is inversely related to the hydration state of the animals. The coxal fluid is generally somewhat hypo-osmotic to the haemolymph and is rich in Na ions. It thus represents an important vehicle for the elimination of Na. In contrast the anal system (Malpighian tubules, stercoral pocket etc.) produces a fluid of low Na/K ratio and is the major route for K elimination. It is demonstrated that for the animal to remain in salt and water balance a source of free water is required in addition to the prey. Reductions in rate of urine and coxal fluid production did enable the spiders to achieve a net gain of water from the prey, but the degree of concentration of these fluids was inadequate to excrete all of the ions gained during feeding.