94 ANIMAL LIFE IN DESERTS 



be interesting to study the effect of belts of desert 

 upon the distribution of leeches, planarians, and 

 other naked damp-skinned animals. 



Other animals are unable to survive the desicca- 

 tion, but become temporary colonists of water-holes 

 in deserts and when the water dries up are locally 

 exterminated. Among them are denizens of per- 

 manent waters, which are occasionally introduced 

 into a temporary pool by floods ; this is an accident 

 of common occurrence to small fish, and doubtless 

 to a large number of other forms of life. Winged 

 insects also, whose early stages are passed in waters, 

 frequently colonize these temporary pools. This 

 temporary colonization is of course most commonly 

 seen in parts of the desert close to permanent pools 

 or rivers : temporary pools in such places are 

 invaded by many forms of gnats and mosquitoes, 

 by gad-flies (Tabanidse) and by dragon-flies. It is 

 probable that the Tabanidae and dragon-flies seldom 

 pass successfully through their early stages, which 

 are of long duration ; but the gnats and mosquitoes 

 frequently pass rapidly through several generations. 

 As the pool shrinks in size and increases in saltness 

 it becomes a fit home for the larvae of small fhes of 

 the family Ephydridae, and these are sometimes 

 present in enormous numbers. 



But it is the permanent inhabitants of these 

 temporary pools which are most interesting, because 

 their methods of solving the problems which confront 

 them are so varied. The Phyllopod Crustacea 

 (e.g. Apus and Estheria) exist through the drought 

 as eggs, and can survive a period of drying which 



