NO. I BLACK FLIES OF GUATEMALA — DALMAT II 



extreme local condition is brought about by the unique location of the 

 area, situated so that it receives the northeast tradewinds, the south- 

 west winds from the Pacific, and additional moisture-carrying air 

 currents from Mexico. Every variation within these extremes can be 

 found in Guatemala. The seasons, more properly perhaps, can be 

 divided into the "rainy season" (May to October) which almost coin- 

 cides with the "hot season," and the "dry season" (November to 

 April) which almost parallels the "cool season." 



Within any one region, and at great proximity, may be found 

 startling variations in climatic conditions due to the particular location 

 of the area, the direction of the slope, or the degree of exposure to 

 prevailing winds. Thus, en route to our laboratory, in passing from 

 the northeastern side of the Volcano Acatenango (exposed to the 

 tradewinds) to the southwestern side (exposed to the Pacific winds) 

 at the highest point reached by the road (over 8,500 feet), the same 

 climatic conditions are rarely encountered. It may be perfectly clear 

 on the northeastern exposure, with a blue sky and cumulus clouds, 

 while on the southwestern side, reached in a matter of seconds by 

 merely following a turn in the road, it will be drizzling or raining and 

 the clouds will so enshroud the road that f oglights become a necessity. 

 Within less than 13 miles on the same road, one also tends to put on 

 and remove warmer outer garments at least twice. 



THE ONCHOCERCIASIS ZONES 



The present study is primarily concerned with the principal oncho- 

 cerciasis zone, which is situated in the foothills region, extending as a 

 75-mile-long band (500 square miles) along the Pacific slope of the 

 Sierra Madre (maps 3-14 and pi. 2, fig. 2) from the Volcano San 

 Pedro in the west to Tecuamburro in the east. This zone, 1,500 feet 

 to 4,800 feet in altitude, experiences a pronounced dry and wet season, 

 as does the entire Pacific slope. The high volcanoes are the center 

 of local areas of precipitation, and these have their effect on the sur- 

 rounding country. With the western slopes of the volcanoes all facing 

 the Pacific, and the eastern slopes, at the higher altitudes, intercepting 

 the northeast tradewinds, there is almost daily precipitation somewhere 

 on their summits. This onchocerciasis zone was formerly considered 

 as two zones : one encompassing parts of the Departments of 

 Escuintla and Santa Rosa and the other including parts of Escuintla, 

 Chimaltenango, Solola, and Suchitepequez. The finca El Zapote and 

 its contiguous annexes lie, geographically, directly between the two 

 zones as previously constituted. A survey, by the author, of the black- 

 fly population in this intermediate region yielded all three species 



