CHAPTER VI 



Response to Humidity 



The behavioural responses of insects to water vapour mixed with the 

 permanent gases of the atmosphere indicate that these organisms 

 somehow perceive the water or differences resulting from changes in 

 the concentration of the vapour. A unique feature of water is its pres- 

 ence both in the organism and in the environment. The existence of 

 water on both sides of the integument poses problems that are not 

 encountered in the case of other stimuli. Conceivably water may act on 

 the organism independently of what is within (as do, for example, 

 odour molecules), or it may cause outward movement of internal water 

 so that the insect does not respond directly to a change in external 

 environment but rather to a change induced in its own internal 

 environment. For these reasons, and because all attempts to under- 

 stand the mechanism of hygroreception have been based upon experi- 

 ments which correlate the degree of activity or direction of movement 

 with the amount of water in the air, decisions must be reached as to 

 which relationship of water vapour to atmosphere gases is to be meas- 

 ured. If humidity receptors are conceived of as responding to the con- 

 centration of water molecules, then the absolute concentration of 

 water to which they are exposed should be measured. On the other 

 hand, if the humidity reactions are conceived of as operating through 

 the agency of evaporation or transpiration, then the drying power 

 of the air is the variable to be measured. 



Conventionally there are a number of humidity definitions (Humph- 

 reys, 1 940). The absolute humidity is either themass of water vapour per 

 unit volume or the gas pressure exerted by the water vapour per unit 

 area. Relative humidity is the ratio of the actual mass of water vapour 

 in a small volume to the maximum mass that can exist in the same 

 volume at the same temperature. It is also defined as the ratio of the 

 actual to the maximum pressure of water vapour per unit area that can 

 exist in the presence of a flat surface of pure water at the same temper- 

 ature. Saturation deficit may be defined as: (1) the amount of water 

 vapour in addition to that already present, per unit volume, necessary 

 to produce saturation at the existing temperature and pressure; 

 (2) the difference between actual and saturation pressure; (3) the ratio 



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