A population of Glossina longipalpis Wied. was studied in southern Nigeria, in derived savannah of the Southern Guinea, zone type, between November 1953 and December 1956. Regular fly-rounds were carried out, using man as bait, and during the third year also using an ox as bait. Meteorological records were taken in the savannah woodland and within a forest island; the latter is cooler and more humid than the former.

The forest-island type of vegetation, with dense thicket under high shade, is the permanent habitat of the fly, although the inside of the forest apparently becomes too humid in the heavy rains, when the flies move out to the edge. Seasonal pools in the savannah, encircled with rather thicker vegetation, form temporary habitats during the rains and early dry season. Transition forest and woodland savannah, when in association with permanent habitats, are used as feeding grounds. During the dry season, the flies concentrate in and around the forest island, and evacuate the woodland savannah, which they recolonise in the early rains.

Fly concentration appears to be associated with saturation deficits (S.D.) reaching 7 mb. or over in the savannah, whereas dispersal is associated with an S.D. dropping to 6 mb. or below. When the mean monthly temperature is compared with the mean fly densities in that and the succeeding month, the value associated with the greatest density is about 77°F. The optimum saturation deficit, similarly judged, is 4–5 mb.; values below 2 or above 7 mb. appear to be relatively unfavourable. These observations suggest that laboratory cultures of G. longipalpis should be kept at 77°F. and 85 per cent, relative humidity.

Fly catches are at a minimum in the early rains (April). They then increase rapidly, level off in the mid-rains, and increase again rapidly to reach their peak in the early dry season (November). The decrease in catches during the dry season represents the adverse effect of high temperature and low humidity on the population, but the continuing decrease during the early rains is believed to represent the effect of fly dispersal, and not a further decrease in population.

In general, catches from the ox fly-round showed the same seasonal fluctuations in fly population as those from the man fly-round, although female flies constituted 39 per cent, of the total catch from the former and only 1·9 per cent, of the total from the latter. The catch is larger from an ox, which attracts more hungry flies (7·9 per cent, of all those caught, against 2·7 per cent, on man). The catches from the ox are higher, in relation to those from man, in the conventional feeding grounds, such as transition forest and woodland savannah, than in the permanent habitat. The proportion of hungry flies amongst non-teneral males caught from man is a maximum in the late dry season and early rains, after the grass fires; good visibility then facilitates hunting activity by man, and the consequent disturbance of the game may cause the fly to lose touch with its hosts. Conversely, flies are least hungry in the early dry season, by which time they have established intimate contact with the game, whose movement, like that of man, is limited by dense elephant grass, 10–15 ft. high.

Precipitin tests on 44 blood-meals, collected over a long period from gorged examples of G. longipalpis, suggest that bushbuck (Tragelaphus scriptus) provides the bulk of the meals, but that the red river hog (Potamochoerus porcus) is also of importance. Of 4,360 males of G. longipalpis dissected, 939 (21·5 per cent.) showed mature infections of trypanosomes (768 of Trypanosoma vivax group, 170 of T. congolense group, and one of T. brucei group, comprising 81·8, 18·1 and 0·1 per cent., respectively, of the infections).

Pupae of G. longipalpis were found under logs in the forest island, together with pupae of G. medicorum Aust., G. fusca (Wlk.) and G. palpalis (R.-D.). The pupae could be found throughout the dry season and early rains, but not during the heavy rains, when the soil is sticky.

It is suggested that since G. longipalpis becomes concentrated in islands and riverine strips of forest when the mean monthly saturation deficit exceeds 7 mb., It might be eradicated by partial clearing of such habitats, except towards the southern, humid limit of its range, where rapid regeneration of the vegetation would make maintenance too costly.