The present work was designed to assess the relationship between the hourly O. volvulus microfilarial density (mfd) in patient's skin and the hourly microfilarial intake by S. ochraceum. From two experiments employing 14 patients, it is suggested that there may be a little elevation of mfd during the period from evening through midnight , but not pronounced. The microfilarial intake by the fly revealed two peaks, one in the morning and the other in the afternoon. Thus, the pattern of microfilarial intake did not correlate with the present diurnal pattern of mfd. The hourly intake was briefly discussed in relation to the biting activity and the parous rates of S. ochraceum. This report is the first case dealing with the hourly microfilarial intake by the black fly. Further such information would be required for a better understanding of the transmission mechanism of onchocerciasis in given endemic areas.

Summary: The biting activity of four zoophilic species of Simulium was examined in an area endemic for human onchocerciasis in Guatemala, using bovine baits. The surveys were performed at three sites within San Vicente Pacaya county (SVP) from daylight until nightfall of two days in January 1979. A total of 8,203 blackflies were collected from the baits throughout the collections. Of these flies, 6,531 (79.6%) wereSimuliummetallicum s. 1.; 977 (11.9%), S. pulverulentum; 654 (8.0%), S. callidum; and 41(0.5%),S.rubicundulum. Biting activity of S. metallicum s. 1. peaked between 0800 to 0900 hours and a second peak was noted between 1500 to 1600 hours. S. callidum also exhibited two peaks, one in the morning and the other, more pronounced, in the late afternoon, immediately before the night-fall. The remaining two species, S. pulverulentum and S. rubicundulum were mainly mid-morning attackers of cattle. Thus, the rate of species composition was presumed to differ within a day.