56 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I25 



not join with other streams along their course. This phenomenon is 

 brought about by underground springs that feed the streams at various 

 levels, corresponding to different water tables. When the upper section 

 of small streams dries up during the dry season of the year, the streams 

 then originate at that level where a particular water table can still 

 supply a sustained flow. During the rainy season this same water 

 table would also be adding to the volume of water in the stream. Thus, 

 when a particular stream is to be treated in its entirety, it has been 

 found more accurate to determine its volume at the point where it 

 joins with another stream. 



THE STREAM BED 



The floor and walls of a stream, those structural characteristics that 

 form the watercourse, are considered as the stream bed. It is com- 

 posed primarily of earth, stones, rocks, sand, gravel, mosses, ferns, 

 and higher plants. The composition of the stream bed, to some extent, 

 determines what age classification a particular section of the stream 

 will have, and this, in turn, is a controlling factor in the breeding of 

 various species of Simulium. As evidence of this fact, where bare sand 

 or gravel alone forms the bed, no breeding of black flies will be found ; 

 the larvae of many zoophilic species prefer for their anchorage the flat 

 faces of the larger rocks, where they are exposed to the strongest flow 

 of the stream, and they are therefore not often found in the younger 

 streams which are lacking in such substrata; the larvae and pupae 

 of a few of the zoophilic species prefer mud to other substrata; in 

 general, the anthropophilic species prefer abundant marginal and emer- 

 gent vegetation and debris found in infant and young streams. 



The bed, vegetation, and morphological age of a particular portion 

 of a stream is affected to a considerable extent by the substratum 

 through which the stream passes. Harder soils will tend toward beach 

 formation ; softer, more porous soils will favor erosion and the forma- 

 tion of a deeper stream floor; certain soils will favor the growth of 

 various classes of vegetation while others will inhibit it. 



SUMMARY 



The larvae and pupae of different species of black flies are adaptable 

 to widely different stream conditions. They will be found from the 

 source of a stream to its mouth ; they will breed in extremely slow- 

 flowing streams, in almost stagnant water, in pools with only small 

 overflow, or in waterfalls; they may be completely encased in mud, 

 attached to vegetation, stones, rocks, debris, or to a variety of artifacts. 



