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

The epicuticular wax layer prevents excessive loss of body water in most insects. The innermost layer of the wax is an oriented monolayer, offering the greatest resistance to the movement of water molecules across the cuticle. Several workers have shown that this oriented monolayer undergoes phase transition at a certain temperature and that the loss of water increases rapidly above this temperature which is now known as transition temperature. Other researchers deny a sharply defined transition point and claim that loss of water increases exponentially with temperature. The controversy includes Corixids. This work was carried out in an attempt to study the water relations of Cenocorixa expleta (Hungerford) in dry air. An apparatus was built to pass dry air over a single insect at constant speed and temperature. Measurements were made of the evaporation rate and the temperature of the cuticle, using copper-constantan thermo-couples made from 47 s.w.g. wires. In one series of experiments, the insects were pre-treated by immersion in water or surfactant solutions of various temperatures before their evaporation rates were measured in dry air of 20 G. Adult C. expleta have a transition point which lies near 30 C but composite evaporation/temperature curves do not show it. The evaporation rates in dry air are slightly temperature-dependent below, and highly temperature-dependent above transition. When caused by high temperature air, transition appears to be reversible: the insects regain waterproofness at approximately 25 C. To prove or disprove the existence of a transition point it is necessary to measure the evaporation rates of an individual insect over the entire temperature range. Phase transition and loss of waterproofness can also be caused by water of 30 C to 35 C. In this case the effect is irreversible. Detergent solutions of sub-transition temperatures remove a small fraction of waterproofing agent, possibly the non-oriented portion of the wax layer. The presence of the transition phenomenon in G. expleta, i. e., the fact that these insects lose their waterproofing, suddenly at approximately 30 G, and the fact that they lose this waterproofing permanently if it is caused by high temperature water, may limit their successful survival in small water bodies and thus their distribution. / Land and Food Systems, Faculty of / Graduate