NO. I 



BLACK FLIES OF GUATEMALA — DALMAT 



23 



out the year. Thus, insect development should be able to continue at 

 approximately an equal rate. However, onchocerciasis, as vi^ell as its 

 principal simuliid vectors, are not prevalent in all parts of this belt. 

 It may be that the amount and extreme seasonal fluctuation of rains 

 may play important roles as limiting factors in certain regions. Cer- 

 tainly, the size of the black-fly population in a particular area can be 



Map II. 



correlated with the seasonal variation in the rainfall. During the rainy 

 season, from May to October, the number of adult flies of the principal 

 anthropophilic species is definitely reduced, although numerous larvae 

 may be found. After heavy rain, it is invariably found that large 

 numbers of larvae have been washed downstream, many being killed 

 by the action of sand and stones. The rains may also destroy large 

 numbers of adults. However, a small adult population is always main- 

 tained. From the middle of October or the beginning of November, 

 when the dry season commences, through February the adult popula- 



