The Makueni fly belt is chiefly infested with Glossina pallidipes Aust. and lies between 4,000 and 2,600 feet above sea level. The climate is hot and dry with two wet and two dry seasons, annually. The dry period during January and February is hot with bright sunlight while during the June to September dry period the temperatures are lower and with dull cloudy periods. The vegetation is Acacia-Commiphora dry grassland association with the Commiphora especially predominating at the lower altitudes in the east. The non-riverine deciduous bushland and thickets show two leafless periods in the year, corresponding with the dry seasons. A system of regular fly patrols has been in operation in this area since January 1949. The total flies caught show a regular seasonal increase during February-March each year and less regular and smaller increases during June-July and October-November. The same seasonal increases in the fly-populations are shown by the average number of old male flies caught per patrol.

Flies caught during 1951 were fewer than in 1950 in the settled areas and at Thwake camp, but at Kikumeni in the dense Commiphora-Acacia thickets the catches in 1951 were at least as numerous as in 1950.