374 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 12$ 



It is likely that Gibson's negative natural infection data for the be- 

 ginning of the dry season (October-December) result from the fact 

 that his fly population samples were collected from only two localities, 

 and that his average monthly total of flies dissected did not exceed 

 250 flies. Erroneous conclusions could easily be drawn under such 

 conditions. For example, if one were to consider the natural-infection 

 rates of flies on fincas Conchita, Buena Vista, Sibaja, and Santa 

 Teresa only (table 5), where the natural-infection rate is generally 

 low throughout the year, the dissection of only small samples of flies 

 would probably uncover natural infection during the rainy season, 

 when the dilution factor is reduced, but not during the dry season, 

 when the infected flies would be lost in the tremendously increased 

 total population. According to the data in table 5, these fincas show 

 low natural-infection rates in the flies for August, September, Octo- 

 ber, January, March, and April, but no infection for November, 

 December, or February. On the other hand, if the fincas Montevideo, 

 Recreo, and Recuerdo, where the naturally infected population is 

 higher throughout the year, were to be used for the natural-infection 

 studies, it would be more likely that infection would be found at any 

 time during the year, even if only small samples of flies were dissected. 

 Table 5 shows a relatively high infection rate during August, Septem- 

 ber, October, and November, a lower infection rate during January, 

 February, March, and April, and none during December. The prob- 

 able reason for zero infection in December has already been given. 

 However, it should be noted that the natural-infection rate of the flies 

 is much higher during the months of August, September, and October 

 on these fincas than at Conchita, Buena Vista, Sibaja, and Santa 

 Teresa ; also, more naturally infected flies were found at Recreo and 

 Recuerdo in January, February, and March than were found at the 

 Conchita-Buena Vista group of fincas. Thus, it can be seen how im- 

 portant it is to include in natural-infection studies regions that will 

 definitely yield infected flies, and to dissect sufficiently large numbers 

 of flies, especially during the period of great abundance when infected 

 ones are more likely to be encountered. 



Although Gibson (1951a) did find natural infection in S. callidtim, 

 the extremely low and sporadic population of this species in the oncho- 

 cerciasis zones and the absence of infection in it in the present study 

 indicate that it does not play a significant role in the transmission of 

 onchocerciasis. The relative importance of 6". ochraceum and 5". me- 

 tallicum will be discussed more fully in the following section. 



