NO. I BLACK FLIES OF GUATEMALA — DALMAT 317 



have found that flies fed on the thigh of an infected person will take 

 up at least as many microfilariae as those that were fed on the upper 

 torso. 



Since 65 percent of all the flies collected from human subjects were 

 metallicum (table 9), and since the natural infection with larval filariae 

 in this species was found to be greater than in ochraceiim (table 6, 

 p. 41), it would appear that metallicum might be the more important 

 vector of onchocerciasis. However, if it could be shown that the natu- 

 ral infection in metallicum is probably composed of larval filariae from 

 other animals, as well as from humans, the role of this species in the 

 transmission of human onchocerciasis might not be as important as it 

 appears superficially from examination of data contained in tables 6 

 and 9. This could only be determined circumstantially, since the de- 

 veloping larvae of Onchocerca species from man and other animals 

 still are not distinguishable in the flies. Thus it was decided to learn 

 if S. metallicum would feed alternately on human and other animals. 

 Specimens captured while they were feeding on human subjects were 

 fed subsequently on other animals ; likewise, ones taken from animals 

 other than man were later fed on human subjects. The species con- 

 stantly demonstrated indiscriminate feeding habits. Since metallicum 

 will alternately bite man and other animals, and since, of the total 

 population of this species, the great majority prefer hosts other than 

 man (table 10), it is quite probable that a large part of the larval 

 filariae found in wild-caught specimens represent Onchocerca species 

 of horses or cattle, mammals commonly infected in Guatemala (Gib- 

 son, 1951a). To substantiate this, it still remains necessary to dis- 

 cover characteristics by which one can distinguish, in their fly-inhabit- 

 ing stages, the species of Onchocerca that infect man from those that 

 infect other animals. 



Since S. ochraceiim, metallicum, and callidum are usually found in 

 the regions where the disease is prevalent, readily feeding on man 

 and becoming infected, and since they are easily collected in the region 

 of our field laboratory, most of our experimental work was done with 

 them. However, this does not preclude the possibility that other 

 species attacking man may be involved in the transmission of oncho- 

 cerciasis in at least some regions of the disease zones. This was dem- 

 onstrated experimentally for 6". exiguum, veracruzanum, and haemato- 

 potum (see "Animal Associations" in section on epidemiology). 



Feeding Time 



The transmission of onchocerciasis by means of the bite of Simulium 

 flies has been doubted by some workers because so few infective-stage 



