346 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I25 



cies, S. ochraceum, metallicum, and callidum, all seem to breed most 

 prolifically between 4,000 and 5,000 feet. However, while metallicum 

 and callidum will breed in good numbers from almost the coast to 

 altitudes over 6,000 feet, ochraceum confines its breeding rather 

 strictly to the zone between 3,000 and 5,000 feet. This region of maxi- 

 mum ochraceum breeding also corresponds to the zone of maximum 

 infection with onchocerciasis. 



5. veracruzanum, another strongly anthropophilic species within its 

 range, breeds principally in the altitude zone from 4,000 to 6,000 feet, 

 much preferring regions between 5,000 and 6,000 feet. Since the re- 

 gion above 5,000 feet is not important insofar as onchocerciasis is 

 concerned, and since veracruzanum is not very abundant in most areas 

 below 5,000 feet, its importance in the transmission of the disease 

 could be only slight. 



STREAM TEMPERATURES IN RELATION TO SPECIES BREEDING 



As would be expected, those species that prefer higher altitudes 

 (table 23) are also those that are found breeding in streams with lower 

 water temperatures (table 24), and conversely, those that prefer lower 

 altitudes are found in streams with higher temperatures. Thus, in 

 considering the principal anthropophilic species, S. metallicum and 

 callidum, which were found in abundance in lower-altitude zones than 

 S. ochraceum, prefer water temperatures between 16° C. and 21° C, 

 while 5. ochraceum, more restricted to higher altitudes, is found in 

 abundance in streams with 14° C. to 21° C. All three species have 

 optimums between 18° C. and 19° C. 



pH OF STREAMS IN RELATION TO SPECIES BREEDING 



Almost all species prefer breeding in streams with pH between 7.1 

 and 7.5 (table 25). Several species appear to be able to breed in 

 waters with a wide range of pH, but only one, Gigantodax aqua- 

 marensis, breeds exclusively in water with a pH lower than 6. As 

 stated in the general part of the section on life history, this species is 

 found breeding in streams with pH about 5. These streams flow in 

 a region with numerous sulfur springs and obviously are carrying 

 large amounts of sulfuric acid. No other insect or larger animal life 

 was found breeding in the same streams, and the larval stage of aqua- 

 marensis was prolonged up to 9 weeks, probably owing to the effect 

 of the acidity. 



Considering the principal anthropophilic species, the range of pH 

 in which .S". callidum breeds most frequently is extensive, that of 



