376 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 1 25 



infection with Onchocerca volvulus. In table 9 (p. 45) it will be no- 

 ticed that, of the species attacking man, only 5". metallicimi, ochraceum, 

 and callidum do so to a significant degree. Since only 4 percent of the 

 flies biting man were callidum in comparison to 65 percent for metalli- 

 cum and 30 percent for ochraceum, it again appears that callidum must 

 play only a minor role, if any, in the transmission of the disease. In 

 comparing mefallicum and ochraceum only, it would appear that metal- 

 licum is of much greater importance as a human biter than is ochra- 

 ceum. It is true that metallicum bites man freely and that wherever 

 it exists it far outnumbers ochraceum. However, if we compare the 

 host preferences of these two species (table 10, p. 46), when both hu- 

 man and other animal bait are available to the flies, side by side, we find 

 that ochraceum is strictly anthropophilic, while metallicum definitely 

 prefers hosts other than man. In nature, both species harbor larvae of 

 Onchocerca spp. (see "Animal Associations" in the section on epi- 

 demiology and ''Natural Infection of Adults with Onchocerca volvu- 

 lus" in the section on ecology) . Since we still cannot distinguish the 

 species of the developing Onchocerca larvae found in the flies, it is 

 possible that natural infections may be of either human or other animal 

 origin (see "Animal Associations" in the section on epidemiology). 

 Comparing the two species of black flies, however, since metallicum is 

 definitely more zoophilic in its host preferences, and since it will feed 

 alternately on human and other animal hosts (see "Body Regions Pre- 

 ferred" in the section on ecology), it is more likely that its infections 

 are of mixed origin. Thus 6*. ochraceum, with its marked anthro- 

 pophilic habits, emerges as the more probable vector. 



It has frequently been stated that 5". ochraceum is a more effective 

 transmitter of onchocerciasis to man than are metallicum or callidum 

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

 exposed, while the latter two species bite only on the lower regions 

 where microfilariae are not found. Although it is true that ochraceum 

 prefers the upper regions while metallicum and callidum usually select 

 the lower limbs (table 12, p. 316), microfilariae are present in both re- 

 gions (see "Body Regions Preferred" in the section on ecology). Also, 

 although the preferred body part may be covered, any one of the three 

 species of flies mentioned will bite on other less desirable areas 

 (table 13, p. 316). However, the microfilariae in the legs concentrate 

 in the thighs, which are generally less accessible to the flies than the 

 upper part of the body. Thus, under normal circumstances, the species 

 biting on the upper body regions would be more likely to ingest micro- 

 filariae with their blood meals. It appears probable, then, that ochra- 

 ceum is the most effective transmitter on the basis of its feeding 

 preference for these upper regions of the human body. 



