Studies on black-fly females using a net mounted on the roof of a motor vehicle showed the following: Simulium reptans and S. ornatum showed a diel rhythm of flight activity with a peak in early morning (sometimes omitted), low activity during the day and a peak in late evening that was of longer duration in 8. ornatum than in 8. reptans. 8. equinum showed a different diel rhythm with more prolonged activity from late morning to early evening, usually without peaks. Woodland adjoining the road traversed by the vehicle led to increased catch of S. reptans but had no effect on numbers of 8. equinum, suggesting the former species sought conditions associated with woodland. Spatial distribution of catches on roads up to 7 km distant from the river producing the flies showed that 8. reptans dispersed more vigorously than S. equinum. In both species large changes occurred in the spatial pattern within periods of one week. In S’, reptans and 8. ornatum activity peaks particularly that in the evening involved increased proportions of gravid females in flight, although this was less marked and more variable in S. reptans. The timing of activity by nulliparous and parous females was rather variable in S. reptans, and it is suggested that occasionally numbers of both groups in flight were less than expected at peak periods because high activity levels earlier in the day had depleted the resting reservoir of individuals available for flight.
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