234 SOME ASPECTS OF ECOLOGY 



tions were found to be in good accord with those conducted in the 

 laboratory. Thus, at the end of the dry season, adults of Glossina 

 tachinoides are abundant and a high proportion of the females 

 are pregnant, and it was found that at this period the seasonal 

 climatic conditions were close to the optimum as defined in the 

 laboratory. For a detailed discussion of the relations of various 

 insects to humidity, reference should be made to the writings of 

 Uvarov (1931) and of Buxton (1932). 



Readers interested in the various methods of technique relative 

 to the measurement and control of humidity should consult 

 papers by Buxton (1931) and Buxton and Mellanby (1934), which 

 are mainly concerned with the maintenance of required humidities 

 within closed vessels such as desiccators. In many cases it is 

 preferable to pass air, conditioned to the required humidity, 

 through the containers within which the insects are lodged : 

 such a method, for example, is described by Gunn (1934). The 

 purely physical problems, which are fundamental in attempts to 

 establish a relation between the loss of w^ater from an animal 

 and the evaporating power of the air, are discussed by Ramsay 

 (1935a). 



It is customary in entomological experiments to express 

 humidity in relative terms — i.e., if the relative humidity, for 

 example, be 60 per cent, it means that the air holds that amount of 

 water vapour, as compared with the maximum, at the particular 

 temperature of the experiment. The loss of water from the insect 

 and the direct influence of humidity upon it are related to the 

 evaporating power of the air. The relative humidity may be the 

 same in two separate experiments under different temperatures, 

 yet the amount of water which the air would require to take up 

 before reaching the saturation point would be different in the two 

 cases. Relative humidity consequently does not exj^ress the 

 evaporating power of the air, and in cases where it is desirable to 

 measure this latter factor the saturation deficiency of the air is 

 the index required. The saturation deficiency, it may be added, 

 provides comparable figures for the evaporating capacity of the 

 air upon which loss of water depends. The rate of evaporation is 

 influenced by several factors, including temperature, air move- 



