NO. I BLACK FLIES OF GUATEMALA — DALMAT 43 



with O. volvulus, and found that all three species were capable of sup- 

 porting the development of the parasite (table 8). 



During a 3-year period the present author made approximately 4,000 

 collections of larvae and pupae of Simiilium spp. in 510 streams 

 throughout Guatemala. Over 160,000 larvae and more than 115,000 

 pupae were represented in these collections; of the latter, 78,500 were 

 reared to adults. In addition approximately 70,000 adult simuliids 

 were collected while feeding on human subjects during 3,200 individ- 

 ual exposure periods. A study of these collections indicates that in 

 some areas of one of the nearly inaccessible onchocerciasis zones of 

 Huehuetenango (contiguous with the disease zone in Chiapas, Mex- 

 ico), 6^. ochraceum is completely absent and apparently is replaced by 

 S. veracrusanum as the principal anthropophilic species. In the lower 

 altitude range of the Yepocapa onchocerciasis zone (below 2,500 feet), 

 5'. exiguum is the dominant human biter, while in other regions in the 

 same zone 6^. haematopotum seems to replace S. ochraceum in im- 

 portance. These findings stimulated the study of Gibson and Dalmat, 

 just reported. The results indicate that S. veracriicanurn, haemato- 

 potum, and exiguum must be considered potentially capable of trans- 

 mitting human onchocerciasis in those parts of the endemic zones 

 where they replace or supplement 5". ochraceum, metallicum, and 

 callidum, the three species commonly accepted as the intermediate 

 hosts in Mexico and Guatemala. 



S. ochraceum is most frequently referred to as the principal inter- 

 mediate host of Onchocerca volvulus. Some grant it this distinction 

 because they claim ochraceum bites man alone while the other an- 

 thropophilic species also bite animals ; other authors favor ochraceum 

 because it prefers to attack principally the upper regions of the body, 

 where nodules and microfililariae are more numerous, while ^9. metal- 

 licum, callidum, and other biters of humans prefer the lower regions. 

 Neither of these reasons seems particularly valid. The findings of the 

 present author (see section on ecology) show that S. ochraceum at- 

 tacks man as well as other animals, and that ochraceum, metallicum,, 

 and callidum will bite any exposed part of the body should their pre- 

 ferred regions be covered. Our experience has been that the flies can 

 more easily be infected by feeding them on the thighs of an infected 

 subject than on his back or shoulders. Although biopsies are usually 

 taken from the upper regions for convenience, certainly those species 

 that prefer biting on the lower extremities will also become infected. 

 In support of this, Gibson (1951a) reported the rate of natural in- 

 fection in S. metallicum (which prefers the lower regions) as being 

 approximately three times that of 5". ochraceum (which prefers the 

 upper regions). 



