Malpighian tubules of Aedes aegypti: Five tubules, one function

Urine formation in the yellow fever mosquito begins by secretion of Na, K, Cl and water in the distal, blind-ended portions of five Malpighian tubules. The tubules appear structurally similar. To answer the question whether the tubules are also functionally similar, the transport activities of all five distal Malpighian tubules in each of eight mosquitoes were evaluated in vitro by measuring the rates of transepithelial fluid secretion and the concentrations of Na, K and Cl in secreted fluid. Two-way analysis of variance revealed no differences between the functions of distal tubules in a single mosquito but highly significant differences between distal tubules from different mosquitoes. To confirm these results we repeated the experiment in mosquitoes placed on Na and K diets. Distal tubules isolated from mosquitoes on the Na and K diet respectively secreted significantly more Na and K than those of mosquitoes on the control diet. Two-way analysis of variance of the data showed again no significant differences between tubules in a single mosquito but significant differences between mosquitoes. We conclude that the five distal Malpighian tubules present in the yellow fever mosquito are functionally homogeneous. They display the properties of a functional unit that responds in a unified fashion to meet the salt and water problems of the animal.