HABITS OF TSETSE FLIES 493 



places and where there is great humidity. Glossina morsi- 

 tans (Fig. 237), the fly which is particularly well known to big- 

 game hunters in Africa and is the carrier of Rhodesian sleeping 

 sickness, is less dependent on water, and in fact prefers a rather 

 hot and fairly dry climate. It is confined to open brushy country 

 with scattered trees, where there is a moderate amount of shade 

 for cover. It is never found either in dense forest or in open 

 grass land. Most other species of tsetses resemble one of these 

 two species in choice of habitats, though few if any are as inde- 

 pendent of water as is G. morsitans. 



Tsetses are diurnal in habits, but the time of activity varies 

 with the species. G. palpalis is most active during the middle 

 part of the day on bright days; G. tachinoides, on the other hand, 

 is especially hungry on dull days and early in the morning; G. 

 morsitans is active in the morning and afternoon, but usually 

 disappears at midday; G. brevipalpis and G. longipennis bite 

 in the early morning from sunrise until about 8 a.m. and in the 

 afternoon from 4 p.m. until some time after dark. Both the last- 

 named species are attracted by lights at night, and enter lighted 

 railroad coaches passing through the " fly-belts." G. palpalis, 

 and probably other species, also, seldom rise more than a few feet 

 above the ground. 



It has been the universal experience of collectors of tsetse 

 flies that the males outnumber the females, often to the extent of 

 ten or more to one. Yet it is a remarkable fact that when bred 

 in the laboratory, males and females are obtained in equal pro- 

 portions. Many different explanations for these apparently 

 contradictory facts have been proposed, but the most probable 

 is the one recently brought out by Lamborn, based on his ob- 

 servations on G. morsitans in Nyasaland. Lamborn has ob- 

 served that copulation takes place after a rough capture, and 

 that, in captivity at least, females even in an advanced state of 

 gestation are not exempt from the attacks of the males, although 

 this often results in abortion. In nature, therefore, the preg- 

 nant females would necessarily have to hide to avoid the males, 

 and so would be less likely to be caught by a casual collector. 



Tsetses show marked preference for certain colors, being es- 

 pecially attracted to blacks or browns, and repelled by white. 

 The dark skin of negroes is selected in preference to pale skin to 

 such an extent that a white man is seldom troubled when ac- 



