ANIMALS— PHYSICAL ENVIRONMENT 71 



temperature is optimum for the development of a 

 certain insect only when the humidity is about a cer- 

 tain figure ; if the humidity is much below that figure 

 the temperature may be unfavourable or even fatal. 



The curves in Figs. 28 to 30 show clearly that the 

 cessation of active Hfe in summer is less than that 

 in winter. One is accustomed to associate the 

 summer of Mesopotamia with fierce heat, acute 

 discomfort and the sudden death of Europeans who 

 were unable to avoid active work in the middle of 

 the day : apparently to the adult insect the summer 

 is less unfavourable than the winter, for it will be 

 seen that January provides far fewer winged insects 

 than July or August. Our conclusion, then, is 

 this : that aestivation and hibernation are common 

 phenomena among desert insects ; until we have 

 accumulated a mass of detailed analysis we must 

 accept it that these phenomena are, in a general 

 way, due to the climate in summer and winter being 

 unfavourable. 



Ants, as Wheeler has shown, adapt themselves to 

 the desert seasons in several ways. In more ordinary 

 surroundings some ants are insect-eaters, but very 

 many Uve upon plant juices which they obtain either 

 directly or from Plant-lice (Aphidse) and Scale 

 insects (Coccidse) : in deserts they have to contend 

 against periods of drought. Some of them therefore 

 become extremely active predators, ranging far and 

 wide and running with great rapidity. Many others 

 collect seeds and pieces of dried plants, and depend 

 upon this harvest for food and moisture during the 

 summer. Others, again, collect plant juices, and 



