NO. I BLACK FLIES OF GUATEMALA — DALMAT 377 



In reviewing the biting habits and duration of blood meals (see 

 "Feeding Time" in the section on ecology), we learned that i'. ochra- 

 cemn usually feeds on its host longer than metallicum or callidum. 

 Also, while metallicum and callidum are "nervous" feeders, landing 

 several times, moving around on the skin before inserting the mouth- 

 parts, and easily disturbed while feeding, ochraceum will land directly 

 without hovering around the host, will pierce the skin without much 

 initial movement, and once feeding, it will not remove its mouthparts 

 or fly off, even when touched by the fingers. The behavior pattern 

 and feeding time of ochraceum indicate that this species probably is a 

 more efficient agent in the transmission of onchocerciasis. Its longer, 

 uninterrupted blood meals would provide a more suitable situation 

 for the migration of the infective stages to the mouthparts, as well as 

 for the ingestion of large numbers of microfilariae from infected 

 individuals. 



Of all the environmental conditions, relative humidity is the one 

 that most affects the activity and life of the adult flies. 5^. ochraceum 

 demonstrates a definite preference for a high relative humidity in a 

 sunny atmosphere, while metallicum prefers a lower relative humidity. 

 High relative humidity with bright skies prevails in the coffee-growing 

 regions along the Pacific slopes of the cordillera. 



Referring to the breeding places of the black flies, S. ochraceum 

 chooses small streams that are usually narrower than 5 feet, with a 

 depth ranging from i inch to 5 inches, and with a small volume flow. 

 These breeding places are usually covered by a dense canopy of vege- 

 tation — emergent, shrubs, low trees, and tall trees. Such infant and 

 young streams are typical of the 3,000- to 5,000-foot altitude range 

 in the coffee-growing regions of the Pacific slopes (see section on life 

 history). Although metallicum and callidum will also breed in these 

 streams, they prefer the larger watercourses, with greater volume 

 flow, in which zoophilic species abound (see section on life history, and 

 "Species Associations" in the section on ecology). Such streams are 

 at least as common outside of the onchocerciasis zones as they are 

 within it. Again, circumstances point to the greater probability of 

 ochraceum being the principal vector, since its preferred habitat coin- 

 cides with that of maximum onchocerciasis endemicity. 



Although we are willing to accept the hypothesis that vS". ochraceum 

 is the principal vector of human onchocerciasis, certain puzzling ques^ 

 tions concerning it can be raised : Why are the disease zones so much 

 more circumscribed than the distribution of 5". ochraceum f Why is 

 the natural-infection rate in ochraceum as low as it is, even in areas 

 where more than 50 percent of the residents are infected with oncho- 



