Secretory activity of the segmented Malipighian tubules of Cenocorixa bifida (Hung.) (Hemiptera,Corixidae)

In vitro studies with whole Malpighian tubules of the water boatman Cenocorixa bifida show that the composition of the secreted fluid is similar to that reported in several other insects. All three distal segments of the tubule are secretory, but the rate of fluid, sodium, potassium and chloride contribution is greatest in the short, most distal segment (IV). The sodium, potassium, and chloride concentrations of fluid produced by segments II, III, and IV change sequentially in a gentle stepwise gradient; osmotic pressure does not change significantly. Segment II produces fluid with a high pH under cAMP stimulation thus raising the pH of fluid produced by the whole tubule. Segment III is the site of dye transport and the production of luminal secretory granules. Possible correlations between the morphology and transport properties of the different segments are discussed.