| Summary: | This study primarily concerns the cellular failure in the organs associated with osmotic regulation in the water bug Cenocorixa bifida (Hung.), when this insect is placed in highly saline media. This species which normally lives in fresh to moderately saline waters dies in high salinity media. However, experiments show that it can survive for long periods in low saline waters and unfed adults live up to three weeks in unreplaced distilled water at 5°C.
In C. bifida the organs associated with osmotic regulation are the Malpighian tubules, the hindgut and the protocerebrum. It is shown that there are four Malpighian tubules, each of which has four morphologically distinct regions. The ileum has a distinct iliac pad, but no such structure is present in the rectum.
A light and electron microscope study of the Malpighian tubules and the iliac pad in insects in natural hypoosmotic medium show that these organs exhibit structural specialization associated
with directional movement of material across the walls.
The presence of numerous infoldings of the basal plasma membrane, the presence of mitochondria and a large number of vesicles on the basal (haemocoele) side of the distal three regions of the Malpighian tubules, plus the direction of movement of neutral red in experimental solutions, suggest that these three regions are concerned with active transport of material from the haemocoele to the lumen of the tubule. The presence of
mitochondria in the lumen border microvilli and the lumen border, the large pinocytic vesicles, and the path of neutral red in the first segment of the Malpighian tubules suggest that this proximal capillary-like region is concerned with the absorption of physiologically important solutes from the tubule fluid. The iliac pad shows infoldings of both the luminal and basal plasma membrane, numerous elongated mitochondria and a rich tracheole supply. The direction of passage of neutral red in this region, together with determinations made on the contents of the gut, suggests that this pad is concerned with solute uptake: the rectum is shown to be a simple storage chamber.
A study of haemolymph osmotic pressure changes when insects are in various media, together with simultaneous determinations of osmotic pressure of the urine, shows that C. bifida is able to hyperregulate its haemolymph over a range of media with a freezing-point depression of between 0 and -0.71°C. In media having a freezing-point depression of between -0.72 and - 1.10°C the insects tend toward conformity, while the haemolymph becomes hypoosmotic to the media in higher external concentrations. The insect produces a urine which is always hypoosmotic to the haemolymph, but the urine osmolarity increases with the increase in external salinity.
The electron microscope study of the Malpighian tubules and iliac pad showed no visible ultrastructural changes when insects were placed in various hypoosmotic media (including distilled water). However, in isomotic media, when there is a tendency for osmotic conformity, and in the hyperosmotic media, when it can be
shown that the insects hyperregulatory capacity fails, definite ultrastructural changes were found. In both isosmotic and hyperosmotic
media, the mitochondria of the Malpighian tubules and iliac pad showed structural breakdown. Further the plasma membrane infoldings towards the lumen of the iliac pad cells, became separate from the intima. These changes being consistent with failure of hyperregulation, indicate an inability of the tissue to function properly under isosmotic and hyperosmotic conditions.
The study of the protocerebrum showed that there are six to eight neurosecretory cells therein which can be classed as ‘A’ cells and which undergo changes when the insects are placed in different salinities. A relationship between osmotic regulation and the neurosecretion of these cells is indicated. === Science, Faculty of === Zoology, Department of === Graduate
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