NO. I BLACK FLIES OF GUATEMALA DALMAT 53 



CLASSIFICATION OF PERMANENT STREAMS BY MORPHOLOGICAL AGE* 



The permanent streams referred to can be classified as : (a) infant, 

 (&) young, (c) adolescent, (d) mature, and (e) old. In discussing 

 any one of these stream "ages," it should be understood that the char- 

 acteristics of the stream are being given at only one locality. It is the 

 morphological "age" rather than the geological "age" of a particular 

 section of a stream that is referred to when the terms "infant," 

 "young," "adolescent," "mature," or "old" are applied. Thus, the 

 same stream may show different ages at intervals along its course. The 

 classification serves, therefore, more as a convenience in associating 

 certain stream characteristics with the Simulium fauna they support 

 or favor than as an index of the geological development of the stream, 

 (a) An infant stream is one formed by the convergence of several 

 minute trickles of water, generally originating as collections of under- 

 ground or cliff seepage. This stream may vary in width from one inch 

 to about one foot. It has no definite wall or cross section and the water 

 channel seems almost haphazard. The stream bed is hardly distinguish- 

 able from the contiguous dry areas (pis. 13, fig. 2; 14, fig. i). Vege- 

 tation, rather than being of a truly aquatic type, appears to be com- 

 posed of trailing parts of plants that grow along the sides of the water 

 course, as well as of debris and decaying leaves that also cover the 

 adjacent ground. Such streams, which may enter and emerge from 

 the ground several times as they pass along a slope, are often tribu- 

 taries of young or adolescent streams. Only anthropophilic species 

 are found breeding in them. 



(b) The young stream (pi. 14, fig. 2; pi. 15; pi. 16, fig. i) is rela- 

 tively narrow, with steep walls, and V-shaped cross section. It has 

 few, if any, tributaries, and these are very short. The stream presents 

 a zigzag, ungraded pattern, often with rapids and small falls, and 

 sometimes with pools. Characteristically, it has abundant emergent 

 and cover vegetation and small deposits of debris. The bed of the 

 young stream consists mainly of an arenaceous mixture topped with 

 small to large stones, and rarely with large rocks. In some streams 

 the sand accumulates around the large rock outcroppings to such a 

 height that the upper faces of the rocks themselves form the main 

 part of the stream bed, the spaces between them being filled with sand 

 and gravel. The walls of the stream may be composed of earth, vege- 

 tation, rocks, or any combination of these. These young streams serve 

 primarily as breeding places for the anthropophilic species. 



(c) The adolescent stream (pi. 16, fig. 2 ; pi. 17) has the walls less 



* Adapted, in part, from Elishewitz, 1944. 



