ANIMALS— PHYSICAL ENVIRONMENT 103 



with such fluctuation of temperature and humidity 

 as might prevent their colonizing houses in the 

 desert. 



Many of the small wild rodents close the mouths 

 of their burrows by day with a plug of earth, and 

 no doubt this tends still further to stabiHze the 

 cUmatic conditions which prevail in the burrow. 

 Detailed observations on this point are lacking for 

 many species, but among rodents which plug the 

 burrow one may mention Dipodillus dasyurus and 

 Jaculus loftusi and Nesohia huxtoni in Mesopotamia, 

 and the American genus Dipodops ; among those 

 which leave the burrow open are Jaculus (species ?) 

 in Palestine, and Dipodomys deserti (Fig. 38) in the 

 American deserts. That the conditions in a burrow 

 are widely different from those which prevail out- 

 side is sufficiently shown by Pitman's observation 

 on the Jerboa, Jaculus loftusi, in Lower Mesopotamia. 

 This species is absolutely nocturnal. " Their earths 

 go steeply into the ground for 2 or 3 feet and the 

 Httle creatures evidently lie up all day in the cool, 

 damp earth as I always noticed that captive speci- 

 mens, of which we tried to keep many from time 

 to time, suffered tremendously from the great heat 

 during the day, even when kept in the coolest of 

 places dug right into the ground, so much so that 

 we never managed to keep any alive for more than 

 a few days." 



Among the burrowing insects one must mention 

 two groups, the Crickets and the Ants. Species 

 of Cricket (GryUidse) are often found even in the 

 more barren parts of the Great Palaearctic Desert ; 



