RESPONSE TO HUMIDITY 161 



permit evaporation of internal water, with a consequent change in 

 chemical composition or osmotic pressure in the milieu of the recept- 

 ors or by mechanical deformation ; they could respond to temperature 

 changes due to evaporation; they could act as hygrometers, i.e., 

 structures with special hygroscopic properties (Pielou, 1940). In some 

 instances (e.g., spruce budworm) responses may not be mediated by 

 special receptors at all but rather by means of general changes in the 

 body fluids occasioned by evaporation through non-sensory areas. 

 For a recent discussion of these matters, the paper of Syrjamaki (1 962) 

 should be consulted. 



