494 



OTHER BLOOD-SUCKING FLIES 



companied by natives. Black or dark clothes are preferred to 

 light ones; khaki color, however, appears to be particularly 

 attractive to them. Moving objects seem to attract the flies, 

 and they are said to follow launches when moving, though they 

 leave them alone when quiet. 



When biting, these flies spread apart their front legs, lower the 

 proboscis into the skin and begin to gorge. The abdomen of 



an unfed tsetse is very 

 flat . (F4gr282A) but after 

 30 or 40 seconds of feed- 

 ing it becomes distended 

 like a balloon, some- 

 times containing over 

 twice the weight of the 

 fly in blood (Fig. 232B). 

 The flies do not feed ex^ 

 clusively on blood, but 

 also suck plant juices 

 and show definite, selec- 



FIG. 232. Glossina morsitans before (A) and ^j ye ^ as ^ e f or various 

 after (B) feeding. X 4. (After Austen.) 



fluids presented to them 



under a membrane, according to experiments by Yorke and 

 Blacklock. Both warm- and cold-blooded animals are sucked, 

 but flies fed only on a cold-blooded animal (crocodile) never 

 produce offspring. It has been thought that perhaps water 

 fowl constitute an important article of diet for tsetses, but in 

 the case of Glossina morsitans, at least, birds' blood proved 

 rather indigestible for them, and often produced a clot in the 

 digestive tract, resulting in abortion in female flies. In the case 

 of such species as G. palpalis, however, bird blood may be more 

 easily digested, and the diurnal habits and close adherence to 

 the vicinity of water would argue in favor of subsistence on 

 water animals, in part at least. On the other hand, the habit of 

 many species of frequenting places where game animals come to 

 drink or browse and of feeding early in the morning and at even- 

 ing is apparently an adaptation to the habits of such hosts as 

 wild game animals. Examination of the stomach contents of 

 wild flies usually shows a preponderance of mammal blood, but 

 Carpenter, studying G. palpalis in Uganda, often found that a 

 large proportion of some collections of flies had fed on reptiles, 



