362 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL, I25 



at a known point and was collected after a measured period of time, 

 the distance traveled being recorded. When, owing to numerous 

 rocks, floating debris, or other obstacles, the cork could not continue 

 in its path for the standard time interval, the cork was collected at 

 that point where it was halted by the natural barriers, and the time 

 was then measured. Reducing the figures to inches traveled by the 

 cork per second, we obtained the velocity of flow at the surface of 

 the stream. The mean velocity of the entire stream (average of veloci- 

 ties taken at an infinite number of depths) was calculated by multi- 

 plying the surface velocity by the known factor, four-fifths. The 

 volume of the stream, or rate of flow, in gallons per second is the 

 number of cubic feet of water passing in a given section of a stream 

 multiplied by the number of gallons in a cubic foot (7.48). This was 

 computed by multiplying the mean velocity (reduced to feet per 

 second) by the average width of the measured section of the stream, 

 by the average depth of the measured section, by 7.5. The technique 

 for measuring the velocity of the stream was checked with a special- 

 ized apparatus for measuring water currents, a pygmy current meter, 

 and found to be accurate enough to warrant continuing the cork 

 method rather than using the heavy and cumbersome meter. 



Since the width and depth of a water course influence its rate of 

 flow, it is easily understood that there is not necessarily a direct rela- 

 tionship between the velocity of a stream and its volume flow. A wide, 

 deep stream with a low velocity might have a lower volume than a 

 narrow, shallow stream with a very high velocity. Therefore, certain 

 species of black flies that prefer streams with a high rate of flow may 

 be found in narrow, shallow streams as well as in wide, deep ones, 

 depending primarily upon the velocity. Of course, those streams that 

 are wide and deep most commonly do have a greater rate of flow than 

 the narrow, shallow streams. Thus, when determining which species 

 potentially could breed in a particular water course, it is necessary to 

 consider its width and depth as well as its velocity (current speed). 

 The interrelation of factors affecting the breeding of Simulium species 

 is well illustrated in tables 28 and 29. 



It can be seen in table 29 that, although vS*. ochraceum prefers breed- 

 ing in streams with a rate of flow (volume) ranging from less than i 

 gallon per second to 20 gallons per second, it will also breed in streams 

 with a rate of flow up to 60 gallons per second, and occasionally in 

 streams with an even greater flow (up to 100 gallons per second). 

 In table 28, the optimum velocity for S. ochraceum is given as from 

 I to 10 inches per second, although it also breeds freely in streams 

 with a velocity up to 20 inches per second, and occasionally in streams 

 with a current speed up to 40 inches per second. 



