188 



ANALYSIS OF THE ENVIRONMENT 



evaporating surfaces, knowledge of humid- 

 ity relations of organisms will remain in an 

 unsatisfactory condition. This does not 

 mean that the humidity of the air is unim- 

 portant. Although humidity exerts a second- 

 ary, rather than a primary, influence on the 

 dynamics of evaporation, certain correla- 

 tions with ecological events are apparent in 

 temperature relations (p. 207), as a com- 

 ponent of the complex that is summarized 

 as the evaporating power of the air (p. 

 206). Even when humidity is considered 

 as relative humidity and as a separate en- 

 vironmental factor, it still has certain 

 ecological importance. 



the number of ahghting mosquitoes remains 

 practically constant; activity decUnes 

 sharply as the saturation point is ap- 

 proached more closely. The feeding of 

 Culex fatigans ceases when the daily mean 

 relative humidity is under 40 per cent; 50 

 per cent is favorable for feeding (Uvarov, 

 1931). Clothes moths can complete their 

 life history under experimental conditions at 

 the lowest relative humidity tested (20 per 

 cent) when their food contained 5.8 per 

 cent of moisture. Even so, with optimum 

 temperature, their hfe cycles were shorter 

 and the adults Uved longer at 75 per cent 

 relative humidity, when the food contained 



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15 



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30 



40 



VAPOR PRESSURE IN MM. 



Fig. 41. Velocity of development of the egg of the "cotton stainer" insect, Dysderciis howarcU. 



(Redrawn after MacGiU. ) 



Some of the observed relations are: Tree 

 frogs react to humidity gradients, even 

 though the vapor tension of their skin can- 

 not be measured. Many insects come to a 

 lighted screen in vastly greater numbers 

 during rain as compared with clear days or 

 nights. Although certain workers find no 

 correlation between the flight of moths and 

 humidity, others report that the capture of 

 night-flying noctuid moths is correlated 

 with the relative humidity (r.h.), with 

 maximum flight at an evening value of 54 

 per cent relative humidity (Cook, 1921). 

 The number of mosquitoes alighting on man 

 increases almost directly with an increase 

 in relative humidity up to 85 per cent; 

 from 85 to 95 per cent relative humidity 



12.2 per cent moisture, than at lower or 

 higher values (Griswold and Crowell, 

 1936). 



The subsocial, log-inhabiting beetle (Pas- 

 saliis cornutus) shows relatively low activity 

 when the relative humidity is high. As the 

 relative humidity decreases from near satu- 

 ration, there is a definite gradual increase in 

 activity down to 20 per cent relative humid- 

 ity, the lowest point measured. When the 

 log in which such beetles burrow is broken 

 open, the beetles are exposed to decreased 

 humidity, and the resulting increase in 

 activity has adaptive value in finding an- 

 other suitable habitat (O. Park, 1937). The 

 probability that arthropods react to humid- 

 ity is emphasized by the demonstration of 



