306 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I25 



of 6'. ochraceum. Attention should be called to the fact that several 

 Other anthropophilic species are distributed throughout large regions 

 of the onchocerciasis zones as well as outside of them. 5. callidum, 

 although found well distributed in the disease zones (map 6), is al- 

 ways found in numbers so small that its importance in transmission 

 can be discounted. 



Although the distribution of S. ochraceum in relation to the distri- 

 bution of onchocerciasis cannot in itself completely incriminate ochra- 

 ceum as the principal vector or explain the limited and circumscribed 

 disease zones, the prevalence or paucity of this species in certain re- 

 gions may be a better index. It is true that the concentration of S. 

 ochraceum is greater in the endemic regions than in regions free of 

 the disease. As mentioned under "The Streams" in the section on 

 epidemiology, 6^. ochraceum abounds in the "infant"-type streams. 

 Such streams or rivulets are more prevalent on the Pacific slopes of 

 the Sierra Madre. Here, in contrast to the Atlantic versant, the in- 

 clination of the land is greater, erosion is rampant owing to this inclina- 

 tion and to the type of soil which is composed primarily of volcanic 

 sand and gravel, the slopes of the volcanoes are more extensive before 

 reaching the coastal plane, thereby supporting a larger number of 

 more rapid-flowing streams, the rainfall is greater, and the vegetation 

 is more lush. This region, which boasts of the heaviest population and 

 most extensive cultivation, is also that of greatest endemicity of oncho- 

 cerciasis. In such areas, 5". metallicum, callidum, exiguum, haemato- 

 potum, and veracruzanum may also be present, but because of their 

 stronger zoophilia tendencies, they probably would not play as im- 

 portant a role in transmission as would ochraceum. Thus, because of 

 its more anthropophilic tendencies, ochraceum would be more likely to 

 be an efficient vector, even when present in much lower concentrations, 

 than the more zoophilia species. As seen in table 6, the natural infec- 

 tion rate in metallicum is much greater than in ochraceum. This 

 might be explained by the zoophilia tendencies of metallicum, its 

 microfilariae possibly representing species infecting horses and cattle, 

 as well as species infecting humans. In regions of the onchocerciasis 

 zones where ochraceum does not exist or where it is present in only 

 greatly reduced populations, metallicum, callidum, exiguum, haemato- 

 potwm, and veracruzanum probably serve as more important inter- 

 mediate hosts. 



LIFE HISTORY 



In studying the life histories of the Guatemalan Simuliidae attention 

 was given primarily to the three principal anthropophilic species in 



