NO. I BLACK FLIES OF GUATEMALA — ^DALMAT 369 



throughout the year. It is true that at certain periods larvae may 

 predominate, but they are never present to the complete exclusion of 

 the other stages. Thus the type of control Prevost advocates would 

 be impossible in the onchocerciasis zones of Guatemala. 



The life cycle of 6". ochraceum, metalliciim, and callidum has already 

 been presented in the section on life history. The longevity of the adult 

 females was also mentioned in the same section and subsequently dis- 

 cussed more amply under "Flight Range and Longevity" in the section 

 on ecology. There now follows a discussion of the population trends 

 of these species throughout the year. 



To study the fluctuations in adult populations, collecting stations 

 were established at 27 fincas of the Yepocapa region. Each month, an 

 average of 135 collections of adult flies were made at these stations, 

 using human bait. Each collection period lasted 20 minutes, the first 

 10 minutes serving to attract flies to the subject, and the second 10 

 minutes being used to collect flies while biting. In studying the sea- 

 sonal variation in the larval and pupal populations, the number of 

 specimens of the particular species collected throughout the country 

 was determined. Each collection required approximately an hour's 

 search for the immature stages at a particular stream. From this, the 

 number of larvae or pupae per collection period was calculated. An 

 average of 7 collections per month were made of 5*. ochraceum larvae 

 and pupae, 125 of metallicum, and 90 of callidum. The data on the 

 seasonal variation of the larvae, pupae, and adults were combined on 

 a single graph for each of the three species. 



It can be seen on the graph (text fig. 11) that larvae, pupae, and 

 adults of vS". ochraceum are found throughout the year. However, 

 there are two peak periods of adult population, the higher one in Janu- 

 ary and the second in August, and also two peaks in larval production 

 which follow after those of the adults, the greater one being in April 

 and the less-pronounced one in October. The principal peak of the 

 adult population occurs during the middle of the dry season, at the 

 time of the coffee harvesting and processing (greatest finca activity) 

 when field workers are most exposed to the bites of black flies. The 

 secondary peak occurs during a regularly experienced dry spell in the 

 midst of the rainy season. The peaks of larval population occur either 

 before or after the heavy rains. As would be expected, when the adult 

 population is at its maximum, the larval population falls to its lowest 

 level. Since the pupal stage usually lasts only 3 to 5 days, the period 

 of its abundance should almost coincide with that of the adults which 

 emerge from them. This, in general, is borne out by the data given 

 in the graph, although the very small numbers of pupae involved do 

 not permit a very clear representation of this point. 



