In Rhodesia, field studies were made of the initial attraction of G. morsitans morsitans Westw. and G. pallidipes Aust. to mobile and stationary baits, using flight traps which surrounded baits or which were placed in the densest part of the attracted swarm. With stationary baits, many flies were attracted by host odour and visual stimuli assisted final orientation. With mobile baits, many flies were attracted by visual stimuli alone; odour did not increase attraction. With both mobile and stationary baits, there was no indication that major hosts are much more effective than minor ones as initial attractants, although stationary men were exceptionally poor baits, for G. pallidipes especially. The sex and species compositions of catches from stationary baits other than men were representative roughly of the inactive population—70% females of both species, and a roughly 1:4 ratio of G. morsitans to G. pallidipes. The sex and species compositions of catches from all mobile baits were biased—40% female G. morsitans, 60% female G. pallidipes, and a roughly 2:1 species ratio. Nearly all flies attracted to stationary baits were hungry whereas 10–25% of both sexes visiting mobile baits had fed recently. The use of a variety of electrocuting devices showed that compact persistent responses and alighting reactions of attracted flies were evident more for males than for females, more for G. morsitans than for G. pallidipes, more near model animals with host odour than near odourless models, more near models than near men, and more with tenerals and hungry non-tenerals than with recently fed flies. Men with mobile baits depressed greatly the alighting reactions and with stationary baits men inhibited greatly the initial attractions. Both effects of men were greater with females than with males and greater with G. pallidipes than with G. morsitans. Men were recognised by their upright appearance and odour. Only desperately hungry flies probed men whereas less-hungry flies probed an ox with men. Food-seeking flies of differing nutritional state were not shown to distinguish between mobile and stationary baits. Although the results support the conventional view that mating and feeding functions in the response to hosts occupy distinct phases of the hunger cycle, it seems necessary to modify the conventional view by placing more emphasis on the role of mobile baits as food sources and by envisaging a definite mate-seeking response by mature females.
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