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Permeability of the insect cuticle to water and the transition phenomenon Oloffs, Peter Christian


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.

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