NO. I BLACK FLIES OF GUATEMALA — DALMAT 315 



be more zoophilic. Table lo summarizes the results of a second series 

 of experiments in which flies were collected from both a human and 

 other animal subject, these situated next to each other. It is clearly- 

 demonstrated that 5. ochraceum prefers man to any other animal host. 

 5". metallicufit prefers horses, mules, donkeys, and cattle, but selects 

 man before pigs, sheep, dogs, cats, or any avian host. 5". callidum also 

 shows the same host preferences as mctallicum with the exception that 

 it also attacks goats before man. It should be noted (table lo) that, 

 of the flies collected from any of 19 distinct animal hosts, 5'. metallicum 

 was always the dominant species taken. 



Body Regions Preferred 



It is sometimes stated that 5. ochraceum is a more effective trans- 

 mitter of onchocerciasis to man than either S. metallicum or callidum 

 because it attacks the upper regions of the body that are more generally 

 exposed and that contain high concentrations of microfilariae in the 

 subcutaneous tissues, while the latter two species bite only on the 

 lower regions where microfilariae are not found. Three series of ex- 

 periments were carried out, designed to determine the validity of this 

 belief. In the first, a human subject was exposed to the bites of flies, 

 all his clothes removed except his under shorts. Three individuals 

 collected flies from the subject, recording where the flies were biting. 

 Temperature, relative humidity, and light reflected from the subject 

 were determined every 30 minutes. Forty-nine such experiments were 

 carried out, each lasting from 6:30 a.m. until 5 p.m. In the second 

 series, the subject was exposed only from the waist up in half of the 

 experiments, and from the waist down in the other half. In the third 

 series of experiments several groups of flies were fed on various areas 

 of the upper body regions of an infected individual, while other groups 

 were fed on the lower limbs. It will be seen in table 12 that when a 

 person is fully exposed to the bites of the three principal anthropophilic 

 species, 6^. ochraceum shows definite preferences for the upper regions 

 of the body, while S. metallicum and 5^. callidum prefer the lower re- 

 gions. However, as shown in table 13, when the preferred region is 

 covered, any one of the three species will bite any part of the body that 

 is exposed. Actually, workers on the coffee plantations in the oncho- 

 cerciasis regions are exposed to all three species. The men usually roll 

 up their trousers and shirt sleeves, while the type of clothing worn by 

 the women permit biting on the head and neck, as well as on the feet 

 and legs (pi. 5, fig. i). To further invalidate the reasoning that 

 ochraceum is a better transmitting agent than metallicum or callidum 

 by virtue of its preferred biting regions, it should be noted that we 



