54 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I25 



Steep than the young ones, the falls and rapids usually are replaced 

 by a more graded river bed, the zigzag pattern gives way to meanders, 

 and the river assumes a dendritic pattern rather than a singular one. 

 The bed is approximately like that of the young stream, still support- 

 ing a large growth of emergent vegetation and being shaded by dense 

 growth. Both anthropophilic and zoophilic species of Simulimn breed 

 in these streams. 



(d) The more mature stream exhibits a broad U-shaped profile. 

 There is usually a complex network of streams with piracy as a com- 

 mon manifestation. Because of the extensive number of tributaries, a 

 large part of the adjoining region is brought more to the slope of the 

 river bed. Deposition may occur, forming narrow flood plains, sand- 

 bars, and beaches (pi. i8, fig. i). This type of stream favors the 

 breeding of zoophilic species of Simidium, although a number of 

 anthropophilic species will also be found in them. 



(e) The old stream flows through a terrain that exhibits a very 

 gentle rolling topography with a minimum of vertical slope. There is 

 a great reduction in the number of tributaries with the single stream 

 remaining dominant. In spite of the small grade, the flow is great 

 except toward the mouth of the river where it usually becomes slug- 

 gish. Wide beaches, often much wider than the main stream itself, 

 prevail along many sections of such a river. There is practically no 

 emergent vegetation and the shrub and tree line is far removed from 

 the water's edge (pi. i8, fig. 2). Small, smooth, somewhat rounded 

 boulders are often dispersed along the shore line with driftwood de- 

 posited among them. Almost without exception these streams only 

 support the development of zoophilic species. 



In the Municipality of Yepocapa there are about 10 rather large 

 adolescent streams (volumes from 5,000 gal./min. to 765,000 

 gal./min.), each with from 50 to several hundred tributaries entering 

 them just in that section of their course passing through Yepocapa 

 (approximately 5 miles). More than 85 percent of the tributaries are 

 less than 200 feet long, and morphologically are "infant" or "young" 

 streams. In the main body of these 10 adolescent streams are found 

 large populations of zoophilic species and very few anthropophilic 

 ones. Near the banks of these streams, where vegetation may be trail- 

 ing along the surface of the water, S. metallicum will be found. In the 

 hundreds of infant and young tributaries that cross the Yepocapa 

 terrain are found abundant breeding places oi S. ochraceum, metalli- 

 cum, and callidum, the principal anthropophilic species of that region. 

 It can be seen that, in a region abounding in streams of this type, the 

 chances for transmission of onchocerciasis are excellent. 



