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

The sensory organs in the transverse grooves of the dorsal labium of the water bug, Cenocorixa bifida, (Hungerford) (Hemiptera) were studied with the electron microscope. It was found that each sense organ is supplied by a single, bipolar neuron, which, together with its sheath cell, forms a sensory unit. The dendrite of the neuron is modified into various structures along its length; it has a root system, two basal bodies, and an axial filament complex. These structures are characteristic of many mechanoreceptors and chemoreceptors in insects. The sheath cell surrounding the dendrite possesses many characteristic fine structures, such as the desmosomes and the microtubules. According to their ultrastructure and their location near the mouth opening, it is most likely that these sensory organs are chemoreceptors. The significance of the presence of the ciliary regions in the dendrites of these organs is suggested to be related to the regeneration of the distal portion of the dendrite which may be torn off during the process of moulting. The axial filament complex may also serve as an internal support in the dendrite. / Science, Faculty of / Zoology, Department of / Graduate

In this study the consequences of water mite parasitism on water boatmen were investigated, concentrating on two host species of the genus Cenocorixa. It was established that mite parasitism severely restricted egg production in Cenocorixa bifida Hung.: whether this should be attributed to a nutritional drain or through hormonal intervention was considered. The possibility of mite interference in flight ability and post-imaginal flight muscle development was also investigated. It was found that mite parasitism of C. bifida in the field varied considerably between habitats, salinity of the lake water influencing both the mite species involved and the prevalence of mite parasitism. When tested both in the field and laboratory, there was no apparent difference in parasitism rates based on the sex, morph or teneral development of the host. It was concluded that individuals of a species were equally susceptible to attack. There was, however, a very definite difference in susceptibility between host species based on equivalent exposure under laboratory conditions. When C. bifida and Cenocorixa expleta Uhler in particular were compared, C. expleta was significantly preferred by the four main mite species infecting C. bifida. This was substantiated in field data. Considering the prevalence of mites on C. bifIda, and the susceptibility of C. expleta to parasitism, the probability of the latter being parasitized approaches 100% in lakes within the salinity tolerance range of mites. When parasitism of these two host species was further investigated, it became apparent that C. expleta cannot sustain mite parasitism and in most cases, died. Past workers have noted the limited coexistence of C. expleta and C. bifida. Despite both species being physiologically fresh water insects, they only cohabit lakes in the upper salinity range of C. bifIda. When the relative abundance of these two species was compared over the salinity range in which they coexist, C. expleta was rare until the upper salinity limit of mites was reached. There was a defined change in their relative abundance at this point, C. expleta being in the majority when salinity was "above this limit. It is evident that water mites severely reduce the reproductive success of C. expleta in low salinities. They are therefore instrumental in influencing the outcome of any biological interactions between C. expleta and C. bifida in these lakes. / Science, Faculty of / Zoology, Department of / Graduate