NO. I BLACK FLIES OF GUATEMALA — DALMAT 35 1 



metallicum is somewhat more restricted, while that of ochraceum is 

 most limited. 



WIDTH AND DEPTH OF STREAMS IN RELATION TO SPECIES BREEDING 



Considering only the three principal anthropophilic species, it will 

 be seen in tables 26 and 27 that Simulium ochraceum is more restricted 

 in its stream habitats than either metallicum or callidum. While me- 

 tallicum and callidum breed freely in streams as deep as 3 feet and 

 as wide as 1 5 feet or more, ochraceum definitely prefers water courses 

 narrower than 5 feet, with a depth ranging from i inch to i foot, the 

 optimum being between i inch and 5 inches. Such oc/irac^wm-breeding 

 streams are commonly found along the slopes of the volcanoes in the 

 onchocerciasis regions (see "Classification of Permanent Streams by 

 Morphological Age" in the section on epidemiology). Here the 

 streams are very young, formed by the union of several underground 

 springs. They emerge where the underground water table is inter- 

 sected by the natural curvature of the land. Although metallicum and 

 callidum do breed in such streams along with ochraceum, they are not 

 as restricted to such habitats as is the latter species. 



Cnephia rohlesi and Cnephia pacheco-lunai breed almost entirely in 

 minute trickles of water that pass over a swampy area supporting 

 large quantities of vegetation. Such rivulets usually dried up during 

 the months of November through March, when neither larvae nor 

 pupae of these species could be collected. Although 5. larvispinosum 

 is found associated with waterfalls (table 30), it actually breeds at 

 the extreme sides of the current where there are narrow, shallow 

 branches from the main falls that have a greatly reduced volume. This 

 is indicated in tables 26 through 29. 



Simulium haematopotum, earlei, exiguum, pulverulentum, samboni, 

 yepocapense, and mexicanum, on the other hand, show definite prefer- 

 ences for streams over 15 feet wide. Of these species, haematopotum 

 and exiguum, both of which attack man in good numbers, are found 

 in the lower regions of the volcanic slopes, where the streams are 

 morphologically older and, therefore, wider. 



The width and depth of a stream are most important in determining 

 the species breeding therein insofar as they affect the volume (rate of 

 flow) of the particular water course. 



VELOCITY (CURRENT SPEED) AND VOLUME (rATE OF FLOW) OF 

 STREAMS IN RELATION TO SPECIES BREEDING 



In calculating the current speed, a very simple but effective method 

 was used. A floating cork was dropped onto the surface of the stream 



