24 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I25 



tion builds up to a high level. These months coincide with the period 

 when the workers on the coffee plantations are most active and, there- 

 fore, most exposed to infection. This is discussed more fully in the 

 section entitled "The Inhabitants, Their Occupations and Customs." 

 Toward the end of the dry season, in March and April, the small 

 streams that serve as breeding places primarily for S. ochraceum (see 



Map 12. 



section entitled "Classification of Permanent Streams by Morphologi- 

 cal Age") dry up, partially or completely, and there is a definite re- 

 duction trend in the population of the species (see the section on 

 ecology) . It can be seen that in areas where there are extreme dry and 

 wet seasons, the breeding of simuliids, especially the anthropophilic 

 species that attach to floating vegetation rather than to rocks, may be 

 markedly affected. 



At plantations Acultzingo and Beliz (table 4), where all conditions 

 appear equal to those in the onchocerciasis zones, it is believed that the 



