1. Male G. swynnertoni were placed in a cage in the middle of a grass enclosure (to curtail their sight) and their powers of smell tested by recording their reactions when a span of six oxen (with two men) passed to windward outside the enclosure. Reactions were obtained at the ranges of 110 and 180 ft. but failed at 300 ft.

Female G. swynnertoni reacted at 110 ft. but were not tested at greater ranges.

2. Men in groups of two and eight were substituted for the oxen, and human scent was found to be attractive, but in spite of this there was evidence that the oxen and not the men with them were the main attraction in the scent experiments with the oxen.

3. The scents of kamba (onion-scented bark rope), hessian and manure were tested at 110 ft. and were not attractive. There was evidence, however, that the exhaust of a lorry at 300 ft. was attractive.

4. The tsetses did not react to shouting, the trampling of oxen or the noise of a lorry.

5. These results are discussed and the experimental conditions compared with the conditions in the natural environment.

6. Wind of a strength from 1½ to 5½ m.p.h. outside the cage (i.e., from about ¾ to 2½ within it) successfully bore the scent of the oxen at a range of 110 ft. to the tsetses. Lower and higher velocities were not tested.

7. The powers of sight of male G. swynnertoni were tested by passing the span of oxen to leeward of the enclosure, from the circumference of which an arc had been cut. The direction of the wind ensured that the tsetses could receive no scent. A maximum tonal contrast was obtained (to human eyes) by using black oxen against a light background and carrying out the experiments in bright sunshine. Positive results were obtained up to and including a range of 450 ft., but at 600 ft. there was no reaction.

Female G. swynnertoni reacted at 110 ft. but were not tested at greater ranges.

8. There was evidence that as the tonal contrast between the oxen and their background became less (through a weak sun or the background in semi-shadow) so the distance at which the tsetses could see diminished. This was supported by an experiment at 110 ft. in which the effect of a dark-blue background was greatly to reduce the reactions.

9. A small dark screen at 110 ft. gave as good reactions as the span of oxen.

10. The results of the sight experiments are discussed and the experimental conditions compared with those in the natural environment.