Climate, the vital Factor in the Ecology of Glossina

The Seasonal Factor. 1. The period of stress due to extreme climatic conditions is far shorter in the forest than in the meadow-pan. 2. G. tachinoides is primarily dependent upon riverine vegetation or residual forest, and has only a very limited wet season spread. G. submorsitans is primarily an open woodland fly with great powers of dispersal which are annually checked by adverse climate that enforces a temporary dependence upon the riverine forests. 3. G. tachinoides abounds within the stream-bed or residual forest ; G. submorsitans abounds on the forest fringe. 4. The dispersal of both species is closely associated with evaporation and saturation deficiency. A mean monthly evaporation of 20–28 cc. and a saturation deficiency of 5 to 8 millibars results in maximal dispersal ; dispersal yields to concentration as the evaporative power of the air increases above these zones. 5. The extent of the annual dispersal is indirectly governed by the duration of the wet season—the longer the season the more extensive the dispersal. 6. The population of G. submorsitans steadily increases throughout the rains as the evaporation falls, and becomes maximal towards the end when the evaporation is in the optimum zone (20—25 cc.) ; density steadily falls throughout the dry season as evaporation rises, and becomes minimal at the end when evaporation is maximal. G. tachinoides is believed to behave similarly, but the optimum zone of evaporation is probably about 6 to 10 cc.