NO. I BLACK FLIES OF GUATEMALA — ^DALMAT 327 



Flight Range and Longevity 



In discussing the epidemiological role of black flies in relation to 

 onchocerciasis the following three important problems must be con- 

 sidered : First, whether the flight range of the anthropophilic species 

 is sufficient to permit the introduction and subsequent establishment 

 of vector species into areas supposedly free of onchocerciasis. Second, 

 the length of life of the females should be known in order to correlate 

 it with the first point, thereby giving additional weight to the possi- 

 bility that the disease may extend beyond its present boundaries. 

 Third, it must be determined whether or not infection with the larval 

 stages of Onchocerca volvulus adversely affects the flight range and 

 longevity of the flies. If infection greatly reduces flight range and life 

 span, it may partially explain the restriction of the disease zone to its 

 presently known limited confines. From the standpoint of insecticide 

 control of the anthropophilic species of black flies, the first two men- 

 tioned considerations would indicate to what extent infiltration by the 

 flies from nontreated to treated areas might be expected. This in- 

 formation would serve in establishing the size of the area that must be 

 treated in order to achieve adequate control. 



Flight-range studies. — In 1949, Dalmat (1950c) reported the first 

 experimental studies on the flight range of black flies. They were 

 carried out in the Municipality of San Pedro Yepocapa, Chimalte- 

 nango, Guatemala, in the region of the Onchocerciasis Field Labora- 

 tory. In general, wild-caught flies were stained and set free, after 

 which collections were again made at fixed stations with the hope of 

 recapturing stained flies. Metallic dusts, fluorescent dyes, and radio- 

 active substances were ruled out as possible fly markers owing to the 

 difficulty of handling them under strenuous field conditions. In the 

 case of fluorescent dyes, the necessity of making initial field checks 

 to determine whether or not fluorescent particles occur naturally in 

 the region was considered an additional drawback. Instead, aniline 

 dyes and certain natural dyes were used as markers, after they had 

 been proved efficient in preliminary tests. Twenty collecting sta- 

 tions were established in all directions from 2 to 8 miles from the 

 release point of the stained flies. All captures were made from men 

 who served as bait. Records were kept of precipitation, relative hu- 

 midity, temperature, and winds. The only meteorological factor that 

 could probably have influenced the distance from the release point at 

 which stained flies were recaptured was the wind. When these ex- 

 periments were under way, the winds were irregular and changeable 

 during the hours of greatest fly activity. The finding of flies in all 



