52 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I25 



as far as their contribution to black-fly breeding is concerned. These 

 last only for a period of hours to a few days, hardly long enough for 

 the completion of the development of the immature stages of black 

 flies. However, temporary streams, lasting from 2 to 5 weeks, have 

 been found to support the breeding of Simulium metallicum and 5". 

 downsi, while others, flowing from a month to the entire 6 months of 

 the rainy season (pi. 10, fig. i), have been found to serve as breeding 

 grounds for 5". mexicanmn, johhinsi, jacumhae, and veracrusanum, as 

 well as for the two species just mentioned. 



In the Yepocapa region, permanent streams originate in one of the 

 following manners : The water may merely percolate from the ex- 

 posed wet surface of an earthen wall or from faults in rocks (pis. 9, 

 fig. 2; 10, fig. 2; II, fig. i). This constant flow usually forms pools 

 beneath the wall, and the constant erosion will gradually produce a 

 stream course. If there is sufficient drip, the wall may be cut back to 

 form a cavern (pi. 11, fig. 2). At other times, especially in the forma- 

 tion of infant streams, the water may percolate from muddy soil over 

 a large area on gently sloping land, giving the appearance of a small 

 swamp (pi. 12, fig. i). The almost imperceptible trickles of water 

 gradually unite to form a resultant stream. The streams most com- 

 monly originate from an underground spring or from several such 

 springs that unite to form one stream of water which leaves the earth 

 at a point where the natural curvature of the land intersects a water 

 table, and where the earth is sufficiently porous (pis. 12, fig. 2; 13, 

 fig. i). Usually the flow at the point of emergence or origin of the 

 stream is slight, but at times, where the stream has been flowing under- 

 ground for a good distance, the flow may be very great. Thus, it can 

 be seen that along any one slope where the water table is intersected, 

 several streams might emerge at about the same level. Any one of 

 these streams may join with other similar streams to form a resultant 

 large stream, or it may form a tributary of an already established 

 stream. During the dry season of the year (November to April), the 

 volume of the permanent streams is practically constant ; during the 

 rains (May to October) , the volume is greatly increased with the flash 

 swells during and immediately following a heavy rain, but it subsides 

 again as soon as the surface runoffs have been channeled away. 

 Weaker or less-resistant larvae are loosened from their anchorage in 

 large numbers and killed by mechanical means as a result of these 

 sudden and violent swells. It is these permanent streams that serve 

 as the principal breeding places of both the anthropophilic and zoo- 

 philic species of Simulium. The larvae and pupae are found in all 

 parts of the stream from its source to its mouth, where it enters an- 

 other larger stream. 



