FOOD OF THE FLY 85 



excess of fluid is exuded as a bead at the posterior end of 

 the body, so that the fly is enabled as it were to have a 

 larger feed of corpuscles than if it were compelled to retain 

 all the fluid part. After a full feed the abdomen becomes 

 as large as a ripe red-currant, and the red blood shows 

 through the distended skin ; the gorged insect flies heavily 

 away to a neighbouring leaf and rests awhile to digest 

 the meal ! 



On clear hot days, when a strong wind may be blowing, 

 palpalis is not so eager to bite ; nor on any day during 

 the hottest hours. But about 4 p.m. it is always ready 

 again, and I have been bitten quite in the dusk of the 

 late afternoon : at night, however, it is harmless, nor 

 does it come to light as the big East African species 

 brevipalpis is known to do. This habit of biting again 

 more freely in the evening is of interest, for it is at this 

 time that the Situtunga antelope, which is a source of 

 food, comes out from the dense forests and wanders 

 along the edge browsing on the bushes, where it meets 

 palpalis. 



The question of the food of Tse-tse flies is of the highest 

 importance, and was one of those set me by the Tropical 

 Diseases Committee of the Royal Society when I first 

 went out in 1910. In the case of morsitans certain ob- 

 servers, notably the late F. C. Selous, held that the blood 

 of buffaloes was essential to it, and that it died out or 

 migrated if none was available. Few observations had 

 been made on Glossina in a state of nature, but from 

 an examination of the blood taken from the stomachs 

 of wild flies the Commission in Uganda in 1910 ^ had 

 deduced that " the blood in the majority of the flies had 

 been obtained from birds or reptiles, and of these the 

 reptilian blood was twice as frequent as the blood of birds." 



It must however be confessed that the method employed 



^ Report XI of the Sleeping Sickness Commission, 1911, p. 112, 

 etc. 



