368 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I25 



river passed through a narrow limestone gorge with banks 5 to 20 

 feet high. In general, Simidium spp. were scarce, the pupae of the 

 specimens described being found on sticks and dead roots in a few 

 existent rapids. The only other species found in the same stream were 

 6". {D.) mexicanum Bellardi and S. (D.) ardent Dalmat. 



Simulium (Dyarella) ardent Dalmat. — Found in the same stream 

 as S. (L.) trivittatum (see above). 



SEASONAL FLUCTUATION IN POPULATIONS 



In considering programs for the control of either the larvae or adults 

 of the Simuliidae, it is of utmost importance to know how many 

 generations are produced during the year and when the peaks of popu- 

 lation occur. With this information the frequency of treatment can 

 be determined and the budgets prepared for necessary labor and 

 insecticides. 



Prevost (1947) states: "We have gone beyond the hope of Fair- 

 child and Barreda (1946) that 'three or four treatments per year' 

 would give control of blackfly larvae for, with a single treatment of a 

 very low concentration of DDT at a critical time when only larvae 

 were present, we have eliminated blackfly larvae for a period whose 

 limit we have not yet reached, as those brooks treated a year and a 

 half ago have not yet been recontaminated." He also speaks of "the 

 failure of Fairchild and Barreda to achieve lasting results with the 

 same type of treatment" because "the treatment must be performed at 

 a time when only larvae are present in the brooks." 



From the remarks of Prevost it becomes apparent to the present 

 author that the life cycle and number of generations per year must 

 differ considerably between different species of black flies in different 

 parts of the world. Also, the streams to be treated must vary a good 

 deal in their physical character. In Guatemala, where Fairchild and 

 Barreda worked, it would be impossible to carry out a control program 

 at a time when larvae alone are present in the streams. Let us con- 

 sider only Simulium ochraceum, metallicum, and callidum, the three 

 principal anthropophilic species of black flies in Guatemala. Owing to 

 the moderate temperatures and rather heavy precipitation, the streams 

 in which these species breed run throughout the year and always con- 

 tain sufficient vegetation and other substances to furnish food and 

 substrata for the immature stages. Since the life cycle of these species 

 can be completed in approximately one month and since the duration 

 of each stage varies within rather broad limits (see section on life 

 history), it can be understood why eggs, larvae, and pupae can be 

 found in the same stream or in neighboring streams at any one time 



