RESPONSE TO HUMIDITY 157 



of the vapour pressure deficit to the saturation pressure at the existing 

 temperature. 



Absolute humidity in mass per volume is difficult to measure. Con- 

 ventionally one measures relative humidity. In an attempt to measure 

 the drying power of the air many investigators measure saturation 

 deficit (Anderson, 1936). Leighly (1937) and Thornthwaite (1940) 

 have pointed out that evaporation is a physical process dependent 

 upon a vapour pressure gradient between the evaporating surface and 

 the air and is not directly related to relative humidity or to saturation 

 deficit in the air. Hence, saturation deficit is of no value as an indicator 

 of moisture losses by evaporation or transpiration (see also Ramsay, 

 1935; WeUington, 1949; Edney, 1957). 



Responses to water vapour include: orientation from a distance to 

 high concentrations of water vapour, proboscis responses to water 

 vapour in the immediate vicinity, avoidance of regions of high and low 

 humidities, and aggregation in zones of preferred humidity. 



Just as the degree of hunger or satiation influences the response of 

 insects to food, so also does the state of water balance (and starvation) 

 influence the behavioural response to humidity. Desiccated flour 

 beetles {Tribolium castaneum) reverse their preference for humid areas 

 after water has been provided. The intensity of the reaction to dryness 

 also increases with increased starvation (WiUis and Roth, 1950). 

 Similarly, the cockroach Blatta orientalis, which prefers lower 

 humidities when its water balance is optimum, becomes hygropositive 

 when desiccated (Gunn and Cosway, 1938). Comparable reversals 

 have been demonstrated for the beetle Ptinus tectus (Bentley, 1944), 

 Drosophila (Perttunen and Erkkila, 1952), Aedes, and Tenebrio molitor 

 (Dodds and Ewer, 1952). It is clear from these and other studies not 

 only that a water deficit in the body elicits a moist reaction but also that 

 the presence of a maximum amountof water in the body elicits a dry 

 reaction (Perttunen, 1951; Syrjamaki, 1962). Other factors that 

 influence response to humidity are age (Perttunen and Ahonen, 1956; 

 Perttunen and Salmi, 1956; Syrjamaki, 1962), diurnal rhythms 

 (Perttunen, 1953), stage of the reproductive cycle (Perttunen, 1955 a), 

 stage of development (Hafez, 1950, 1953), sex (Roth and WiUis, 

 1951 c; Perttunen and Ahonen, 1956; Syrjamaki, 1962). 



Most of our speculation regarding the physiology of hygroreceptors 

 is based upon analyses of behavioural responses to humidity gradients. 

 The following species have been studied in some detail: Locusta 

 migratoria migratorioides (Kennedy, 1937), Blatta orientalis (Gunn 

 and Cosway, 1938), Culexfatigans (Thomson, 1938), Tenebrio molitor 



