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In an inland area of the Gambia the density of flying mosquitoes at successive distances from a pair of bait-calves or an equivalent source of carbon dioxide was measured with ramp-traps to determine the maximum range at which orientation towards the bait was initiated. Thirty-six traps were operated at intervals of 7·5 m up to a distance of 60 m from the bait, on 66 nights. Anopheles ziemanni Grünb., Mansonia uniformis (Theo.) and Aedes spp. were responding to the presence of the baitcalves from a distance of 22·5–30 m; with carbon dioxide as bait the same species showed a clear response at 15 m but, with the doubtful exception of M. uniformis , not at 22·5–30 m. M. africana (Theo.) responded to both calf and carbon dioxide baits at 15 m but not at more distant ranges. Culex thalassius Theo. and C. univittatus Theo. showed a response at 7·5 m but not at any greater distance. C. decens group showed no response to either bait even in the nearest traps (7·5 m). The parous rate in Mansonia spp. varied with distance from the bait. Similar variation in the proportion of females infested with mites was not detected.
Gillies et al. (Tue,) studied this question.