76 



THE PROTOZOA AS PARASITES OF MAN 



epidemics are impossible. In the tropics P. falciparum is endemic, 

 and all children become infected by the age of eighteen months, 

 but after ten years of age they develop a partial immunity. The 

 chief symptom shown by such patients is a considerable lethargy. 

 The insect which carries the parasite is of the genus Anopheles 

 (Fig. 46), different species being important in different parts of 



Anopheles 



CULEX 



Fig. 46. — A comparison of the various stages in the hfe-history of Anopheles (left) 



with those of Culex (right). — From Shipley. 



Note how the larvce and pupae hang from the surface film of the water (represented by a thin line). The 

 organs by which they are suspended contain air tubes, and if these be prevented by a film of paraffin 

 from functioning the insect is drowned. Note also that the eggs of Culex cling together as a raft. 



e., two views of an egg, magnified. 



the world. There is no justification for trying to restrict the 

 word mosquito, which is simply the Spanish for little fly, or gnat, 

 to this one genus, while using gnat only for the commoner Culex ; 

 there are many other genera of related flies, and not all species 

 of Anopheles are natural carriers of malaria. All gnats lay their 

 eggs in water, though there are great variations in their ecological 

 requirements, and the easiest way to reduce malaria is to prevent 

 the breeding of the insects (p. 257). For long the only direct attack 

 on the parasites was by quininC; a drug discovered empirically by 



