NO. I BLACK FLIES OF GUATEMALA — DALMAT 373 



determine which become infected in nature and to support the de- 

 velopment of the filarid larvae to the infective stage. Gibson (1951a) 

 showed experimentally that .S". ochraceum, metallicum, and callidum 

 were all capable of ingesting microfilariae from infected individuals 

 as well as of supporting their subsequent development. He also de- 

 termined the rates of natural infection with Onchocerca spp. to be 

 0.38 percent in S. ochraceum, 1.04 percent in metallicum, and 0.62 per- 

 cent in callidum. Gibson found that during a 2-year period the natural- 

 infection rate in all three species was zero from October through 

 December (1951b and 1952) when the finca workers are most exposed 

 to the bites of the flies. Since such findings appeared contrary to 

 what was anticipated, it was decided to repeat Gibson's studies, at 

 least through the period when natural infection was not found, dissect- 

 ing larger samples of flies. 



Approximately 200 flies were collected every month from each of 

 nine fincas in the Municipality of Yepocapa. These were brought to 

 the laboratory where they were dissected in physiological saline, the 

 head, thorax, and abdomen being examined separately. Since the 

 characters whereby the developing forms of Onchocerca volvulus in 

 the flies can be distinguished from those of other Onchocerca species 

 still have not been discovered, all Onchocerca larvae found in the 

 three species of Simulium dissected were considered to be 0. volvulus. 

 This investigation was carried on from August 1952 through April 

 1953, when the Onchocerciasis Laboratory was terminated. The re- 

 sults have been summarized in table 5 (p. 30). 



Naturally infected flies were caught during all months of the investi- 

 gation with the exception of December. The rate of natural infection 

 in October was higher than in any other month of this study and the 

 November rate was at least as high as the average. Since the general 

 Simulium population increased, on an average, almost sixfold during 

 the months of October through April, the infected fly population un- 

 doubtedly was substantially diluted. Thus the probability of collecting 

 naturally infected flies would be reduced, unless the fly samples dis- 

 sected were to be proportionately increased. This was achieved to a 

 small extent in January, February, and March, but not in December, 

 at the peak of fly abundance. Perhaps on this account, no naturally 

 infected flies were collected during December, while small numbers 

 were encountered from January through April. 



It can be seen in table 5 that the infection rate in flies fluctuates 

 markedly from month to month and from finca to finca. This may be 

 due to the small samples of flies that were dissected and/or to the 

 variability in the infection rate between the different localities involved. 



