164 Insects and Disease 



purely mechanical way without serving as a definite 

 necessary host for the parasite. 



The tsetse -fly is found only in tropical Africa 

 and is limited in its distribution there to certain 

 very definite, narrow, brushy areas along the water's 

 edge. If these places can be avoided there seems 

 to be little danger. Those who are fighting the dis- 

 ease have found that if the brush in the vicinity of 

 watering-places and ferry-landings is cleared away 

 such places become comparatively safe. These 

 flies do not lay eggs but produce full-grown larvae 

 which soon pupate in the ground. 



ELEPHANTIASIS 



In many tropical regions human blood as well as 

 that of other animals is the normal habitat of cer- 

 tain worm-like parasites (Nematodes). They are 

 not entirely confined to the tropics but may ex- 

 tend far up into the subtropical regions. Five or 

 six different species of these parasites are known, 

 only one of which, however, has been shown to be 

 of any pathological importance, as far as human 

 beings are concerned. 



This species, Filaria bancrofti, is not only very 

 widely distributed, but in regions such as some of 

 the South Sea Islands a very large per cent of the 

 natives have the filariae present in their blood. 



