136 MOSQUITOES IN RELATION TO MALARIA [CH. 



these organs and may be seen filling the secretory cells and also 

 becoming free^n the lumen. The sporbzoites are now incapable 

 of further development in the mosquito, but if the latter feeds 

 on a human being, large numbers of them are introduced into 

 the wound together with the salivary secretion. They thus 

 enter the blood and there penetrate into the red blood corpuscles 

 and start the schizogonous cycle with which we commenced. 



The whole of this process, from the differentiation of the 

 macro- and microgametocytes to the formation of the sporo- 

 zoites, is known as the sexual or sporogonous cycle, and is 

 completed in from ten to twelve days. Thus a mosquito that 

 has ingested blood containing the gametocytes does not become 

 infective until this incubation period has elapsed. Then its 

 salivary glands contain the sporozoites, and the insect probably 

 remains infective during the remainder of its life. 



Bionomics of Mosquitoes in Relation to Malaria. 



The habits of the particular species of mosquito concerned 

 in the transmission of malaria in any particular locality are of 

 the highest importance, and therefore, at the risk of repetition, 

 some of the more important features may be briefly mentioned. 



The majority of Anophelines are more commonly associated 

 with village life, and are especially prevalent in more or less 

 uncultivated districts, and, unlike the Culicines, their breeding 

 habits are usually unsuitable for their persistence in towns. 



With respect to this point, however, there are exceptions, 

 and it will be necessary to refer to the connection between the 

 habits of various species in relation to the spread of malaria. 



At the time of the discovery of the mosquito cycle, only 

 some half-dozen species of Anopheles were known, most of 

 which are European species. There are now more than 100 

 well defined and accurately described species, as well as many 

 named varieties, and these taken collectively have a distribu- 

 tion which extends almost all over the globe. These species 

 differ not only in morphological details, but what is more 

 important from our present point of view, they vary greatly 

 in their habits and in their relation to malaria. 



