NO. I BLACK FLIES OF GUATEMALA — DALMAT 29 



branches must be bent (agobeo) to stimulate formation of new shoots, 

 followed by selection of the most productive (in April or May and 

 again in August) ; the coffee fields must be weeded periodically (end 

 of harvest, May and September) and wild sprouts of coffee cut out; 

 in July and August, a few months before the harvest, shade is re- 

 moved (desombra) from those sections of the plantation where the 

 coffee is retarded so that ripening can be hastened and the harvest will 

 not be too extended ; then, finally, comes the harvesting and processing 

 (October- January) which requires an increased number of laborers. 



To reach his work in the field a peon often has to walk 3 to 4 miles 

 on narrow footpaths through dense rain forests. Flowing through 

 these woodlands are numerous small, shallow streams, a few inches 

 to a few feet in width, abounding in emergent and floating vegetation, 

 which serve as breeding places for the anthropophilic species of black 

 flies. Because of the usually long walk to and from the fields, the 

 worker is exposed to bites during almost the entire period of activity 

 of the flies. 



During the harvest, if the worker is employed in the mill (beneficio) 

 or on the drying patios, he is also well exposed to the simuliids. While 

 working at the fermenting tanks or while shoveling coffee beans on 

 the patios (pi. 4, fig. 2) the workers usually remove their shirts and 

 roll up their trousers. It is a common sight to see these men with 

 literally thousands of flies feeding on their backs, which are traversed 

 by vertical stripes of blood. This period of greatest finca activity, 

 when women and children as well as the men are in the fields, also 

 corresponds to the season of greatest black-fly population. Because of 

 the large concentration of persons, it is probable that infected flies can 

 more readily transmit their parasites. Since the incubation period of 

 onchocerciasis is not definitely known, it is not possible to draw 

 definite conclusions as to when maximum transmission occurs. How- 

 ever, studies of seasonal variation of natural infection in S. ochraceum, 

 S. metallicum, and 6". callidum show that the percent of infection in 

 the flies is approximately the same in the season of the harvest and 

 during the remainder of the year (table 5). Thus it would seem that 

 with the same rate of natural infection, with the tremendous increase 

 in the number of flies, and with the concentrated human population 

 during the period of the harvest, transmission should then reach its 

 peak. 



When his usual work in the fields is finished, the man engages in 

 various chores. He may gather firewood for his home, sharpen his 

 machete and azadon (giant hoe), which are his principal agricultural 

 tools, or, commonly in Yepocapa, weave sleeping mats (petates) (pi. i, 



