NO. I BLACK FLIES OF GUATEMALA — DALMAT 363 



The higher-volume streams in which ochraceum were found can 

 undoubtedly be accounted for by investigating the width and depth 

 of such breeding places. At times this species was found in streams 

 over 5 feet wide and over i foot deep (tables 26 and 27). In such 

 breeding places, although the velocity (current speed) may be within 

 the optimum range for ochraceum, the volume (rate of flow) will be 

 greater than in smaller streams. From observations made by the au- 

 thor, it appears that the current speed is a more important factor in 

 determining the presence of this species in a particular stream than 

 are the width, depth, or rate of flow. 



Although the optimum rate of flow for breeding of 5". metalHcum 

 is the same as for ochraceum (i-io gallons per second), its preference 

 is not nearly as marked as is that of ochraceum. Considering the 

 velocity, only 38.9 percent of the metalliciim pupae were collected 

 from streams with current speeds of i inch per second, while 63.4 

 percent of the ochraceum pupae were collected from streams within 

 this range. 



The optimum stream volume for ^. callidmn breeding also falls be- 

 tween I and 10 gallons per second, but this species is also commonly 

 found breeding in streams with much higher volumes. It is only 

 rarely found in streams with a volume flow less than i gallon per 

 second, habitats where 5. ochraceum abound. It is also less discrimi- 

 natory than either ochraceum or metalHcum in its choice of the current 

 speed of its breeding places, being found in streams the velocities of 

 which vary from i inch per second to that of a waterfall. 



Tables 28 and 29 demonstrate the specificity of certain species for 

 streams with comparatively narrow ranges of velocity and rate of flow 

 {aguirrei, aquamarensis, hurchi, carolinae, jacumhae, ochraceum, 

 pacheco-lunai, parrai, roblesi, and samboni) and the more universal 

 presence of other species in streams with a wide range of velocity and 

 rate of flow (acatenangoensis, callidum, do-umsi, delatorrei, earlei, 

 exiguum, johbhisi, metalHcum, pulverulentum, rubicundulum, smarti, 

 veracru^anum, and yepocapense. 



SUBSTRATUM AND STREAM TYPE IN RELATION TO SPECIES BREEDING 



More marked than the preference for streams with a particular 

 depth, width, current speed, volume flow, pH, or temperature, is the 

 specificity of the substratum used by the larvae and pupae of the 

 various species of black flies. Table 30 illustrates this point. Certain 

 species like SimuHum acatenangoensis, capricornis, carolinae, ethelae, 

 mathesoni, rubicundulum, tricornis, and yepocapense, Cnephia aguirrei, 



