NO. I BLACK FLIES OF GUATEMALA — ^DALMAT 55 



One further type of waterway, present on every coffee finca in the 

 Yepocapa region, should be mentioned at this time. It is the toma 

 (pi. 8, fig. i), or man-made water channel used to lead water off from 

 natural streams to the area of greatest finca activity. During the period 

 of the coffee harvest, when the toma is constantly used to supply water 

 to the processing plant, its earthen banks are usually maintained with 

 straight, steeply sloped walls, the cover vegetation along its borders 

 is regularly trimmed, and the debris and emergent vegetation is kept 

 to a minimum. At such times, black-fly breeding is at a minimum in 

 the tomas. However, during the rest of the year, when large quantities 

 of water are not mandatory for the work of the finca, the tomas are 

 often neglected, becoming overgrown with marginal and emergent 

 vegetation. With their usual volume of 250 to 300 gal./min., they 

 assume the characteristics of a young stream, thereby offering excel- 

 lent breeding facilities for the anthropophilic species, especially 6'. 

 metallicum. In those regions at the bottom of the onchocerciasis zone 

 where only larger streams are found, the toma may be the waterway 

 most important for the development of the anthropophilic species. 

 Several fincas cement the walls of the tomas (pi. 19, fig. i), at least 

 along that part entering the processing plant. Unless debris gathers in 

 these sections of the toma, breeding will not usually be found. Al- 

 though experiments in which the flow of water in the toma was dis- 

 continued for a few days and then reestablished showed that larvae 

 and pupae were killed, this management of the tomas would be of little 

 significance in controlling the flies since other larvae migrate into the 

 toma from above as soon as the water flow is continued. 



Another type of waterway that should be mentioned is the waterfall 

 (pi. 19, fig. 2), which does not seem to fit in any of the categories 

 already discussed. The waterfall does support breeding of several 

 species of black flies in Guatemala, none of which are anthropophilic. 

 The larvae usually attach to pendent vegetation or to the upper faces 

 of rocks along the edges of the falls where the current is less severe, 

 but still extremely rapid. 



It has been noticed in the literature that many investigators, in calcu- 

 lating the amount of insecticide necessary for treating a particular 

 stream, determine the volume of the stream at its origin. This would 

 not be practical in the Yepocapa region. If the volume of a large 

 stream is taken at a particular point, it will be found greater than the 

 sum of the volume of the same stream taken at its origin plus the 

 volumes of all the tributaries entering it between the origin and the 

 point of calculation. The tributaries also increase in volume from their 

 origin to the point of entering the larger stream, although they may 



