Physiology of secretion of the segmented malpighian tubules of Cenocorixa Bifida (Hemiptera-Insecta)

The secretion of the segmented Malpighian tubules of Cenocorixa bifida (Hungerford) was studied in vitro to determine if the Malpighian tubules of an aquatic predator function in a similiar manner to those of terrestrial insects, and to determine the importance of the different morphological segments in the ion transport of the whole tubule. Fluid secretion in the Malpighian tubules of C. bifida appears to be governed by the same in vitro factors found important in other insects. As in other insects, the secreted fluid is isosmotic to the bathing medium, while the potassium is hypertonic and sodium hypotonic over a wide range of bathing medium sodium-potassium ratios. At bathing medium potassium concentrations close to that of the insect's haemolymph, potassium and sodium are isotonic in the secreted fluid. The Malpighian tubules of C. bifida produce alkaline secretion when rate of secretion is increased by addition of cyclic AMP. Differences in ion and fluid transport between the segments of the Malpighian tubules of C. bifida are statistically significant, but only slight. This correlates with the lack of morphological differences in apical and basal infoldings between the segments. The major exceptions to this trend are (1) the high pH of Segment II fluid, (2) the transport of dyes by Segment III and (3) the production of 'secretory granules' by Segment III. Segment III is the most distinct ultrastructurally.Science, Faculty ofZoology, Department ofGraduat