CEH Distribution in Europe and N orth America

Public Health Significance of Urban Pests 91 Second, Arlian 1977 also concluded that H DMs are unlikely to obtain significant amounts from the oxidization of carbohydrates and fats and, third, passive absorption through the outer surface of the body does occur, but only slowly Arlian Veselica, 1982. Finally, the active uptake of water is facilitated by a pair of glands, the supracoxal glands. These glands secrete a hygroscopic salt solution that contains sodium chloride and potassium chloride Wharton, Duke Epstein, 1979; the secretion flows down a duct, eventually reaching the HDM mouth. Providing the relative humidity exceeds the CEH, the salt solution experiences a net gain in water. Wharton, Duke Epstein 1979 found that the rapid uptake of water took place after three hours, when they moved dehy- drated D. farinae from a low relative humidity environment and placed them in a high relative humidity environment. Of the four ways HDMs lose water, first, when relative humidity is below the CEH, the salt solution dries out, eventually blocking the gland and thus preventing further water loss Wharton, Duke Epstein, 1979. Arlian Wharton 1974 observed that, in D. fari- nae it took 14 hours for transpiration from the supracoxal gland to become negligible. Second, mites and mite eggs are able to lose water through the outer surface of the body, but only slowly . Increasing temperature, however, increases the permeability of the cuti- cle and results in a more rapid water loss Arlian Veselica, 1982. Third, digestive secre- tions and reproductive fluids are only likely to account for a small proportion of the loss. Finally, at low relative humidity, HDMs generally feed less, and their reproductive rate is reduced Arlian, 1992. Also, their rate of excretion and oviposition is reduced.

3.1.3.2. CEH

CEH is very important in understanding how HDMs survive, especially in relation to their water balance. CEH is most commonly defined as the relative humidity below which mites are unable to maintain their water balance and lose water more rapidly than they can gain it. Arlian Veselica 1981a found that CEH for D. farinae varies with tempe- rature. CEH also appears to vary according to the state of hydration of the HDMs Arlian Veselica, 1981a. At 25ºC, Arlian Veselica found the CEH for D. farinae to be 58, which was significantly lower than the CEH of 70 found previously by Larson 1969. Arlian Veselica 1981b suggested that this was probably due to differences in experi- mental methods, their mites being dehydrated for significantly longer than Larson’s. A similar situation was reported for D. pteronyssinus. Arlian 1975 found the CEH at 25ºC to be 73, using mites that had not been pre-dehydrated. De Boer Kuller 1997 and then De Boer alone 1998, using pre-dehydrated mites, found the CEH at 16ºC to be between 56–58 relative humidity and at 20ºC to be between 58–60 relative humi- dity. When HDMs are exposed to brief daily spells of elevated humidity above CEH, they can survive in otherwise detrimental conditions, as Arlian, Neal Bacon 1998 found for D. farinae and de Boer, Kuller Kahl 1998 found for D. pteronyssinus. This has significant implications for their survival and for preventive environmental control measures.

3.1.3.3. Influence of temperature