328 A BOOK OF INSECTS 



jungle country bred continuous sickness. The Isthmus 

 is not yet a health resort. But in the immediate Canal 

 regions it is no longer a country dangerous to health. 

 The Americans have laid by the heels the mosquitoes 

 which carried the disease. All likely breeding grounds or 

 swamps are saturated with kerosene. You go for miles 

 and the air stinks with the black slimy stuff. Nearly 

 every ditch is smeared with it. Where pools accumulate 

 in the vicinity of the workings, niggers with copper cans 

 on their backs saunter about and spray freely." 



The tse-tse flies of the African continent are also 

 disease bearers. They carry about the minute, fish-like 

 Protozoans known as trypanosomes. One of these 

 parasites exists in the blood of large wild animals, such 

 as buffaloes and antelopes, which are unharmed by its 

 presence. They are known as " reservoir hosts." Presum- 

 ably their races have become immune through long con- 

 tinued inoculation. But when this particular trypano- 

 some is introduced by the fly (Glossina moi'sitans) into 

 the blood of domestic cattle, horses and dogs, it gives rise 

 to the fatal, wasting disease called "nagana." Another 

 trypanosome causes sleeping sickness, from which 200,000 

 natives died in Uganda in a recent period of five years. 

 This is known to be carried by a distinct tse-tse fly 

 (G. palpalis), possibly by other species. It is now certain 

 that the trypanosome retains its activity for many weeks 

 after being sucked up by the fly, and it seems probably 

 that the parasite undergoes changes and multiplication in 

 the insect's body. This is known to happen in the case of 

 another kind of trypanosome, a parasite of rats, which is 

 carried by the rat-flea and the rat-louse. It is also 

 thought that the sleeping sickness parasite, like that which 

 causes nagana, may exist in the blood of other vertebrates 

 besides man : the crocodile, among other animals, has 



