22 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I25 



mum temperatures throughout the year may, to some degree, affect 

 the rate and character of black-fly development. Only few Simulium 

 ochracenni were found in areas much above 5,000 feet. At the other 

 extreme, the relatively high mean temperature at San Jose (table i), 

 Tiquisate, and Bananera (table 4) may so affect the streams by lower- 

 ing the dissolved-oxygen content that the principal vectors of oncho- 



DISTRIBUTION OF SIf. 



l.P„COR,„S 

 C.PB.COR-IIS 



Map 10. 



cerciasis are not able to breed in them. Certainly Simulium ochraceum 

 has never been found in streams situated below 500 feet altitude. This 

 may be due to the lack in the true coastal area of the proper stream 

 type (see section on "Streams") for breeding of the principal an- 

 thropophilic species, as well as to the effect of the high mean 

 temperature. 



In the Pacific coastal belt between 1,500 and 5,000 feet, the fluctua- 

 tion from the mean daily temperature is relatively slight in any 24-hour 

 period, and the mean daily temperature is relatively constant through- 



