| Summary: | The problems of, osmoregulation and oxygen consumption were Investigated In two species of Corixidae (Hemiptera), Cenocorixa bifida and C. expleta. Field collections support a distribution correlated with salinity. C. bifida and C. expleta acclimated to a range of different lake waters for three days showed a gain and loss of about five per cent body water in hypo and hyper-osmotic media when compared with their natural environment. Determinations of freezing point depression of blood showed that both species are able to maintain the osmotic pressure of their body fluids higher than that of the surrounding medium except in highly saline Gr₂ lake water. Both species became approximately iso-osmotic In Long lake water (concentration 435 mOs/1).
The results of this study Indicate that C. bifida and C. expleta can tolerate lake waters having as low as 5 and as high as 435 mOs/1 concentration under laboratory conditions and show no visible adverse effects. On transfer to media of lower concentration than the normal environment, C. expleta became swollen and sluggish. It appears that this species is unable to tolerate very low salinities. The results also Indicate that in Gr₂ water (concentration 753 mOs/1) the blood of both species becomes hypo-osmotic probably due to the breakdown of the hyperosmoregulatory mechanism, thus resulting in death.
The presence of filtered solution of amaranth and phenol red In different lake waters showed that these species drink water In hypo and hyper-osmotic media to obtain salts and water lost, in excretory products.
Sections of midgut showed swelling and vacuolation of epithelial cells in G. expleta when transferred to very low salinities like Spring House water. In highly concentrated Gr₂ lake water the midgut cells In both species were shrunken and detached from the basement membrane and in time they appeared to be completely broken down.
To maintain a difference of osmotic pressure between the body fluids and the surrounding medium both species do metabolic work. The measurement of respiratory rate by Warburg method and the periods of locomotory activity in nitrogen saturated media showed that both species consume more oxygen on transfer to lower and higher salinities than in their normal medium. These results suggest that both species are under osmotic stress and are doing increased metabolic work with the increasing osmotic gradient, but because of the difficulty in distinguishing this from activity, the latter may also be causing some increase in the respiratory rate.
The rate of oxygen consumption in the females of both species was found to be significantly higher than the male in all salinities. It is possible that this difference could be due to a greater expenditure of energy on egg maturation. === Science, Faculty of === Zoology, Department of === Graduate
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