STRUCTURE AND LIFE-HISTORY OF CRITHIDIA MELOPHAGIA. 207 



Casual Infection. 



The method of cross-infection in many species of Crithidia 

 has not been demonstrated, but in the cases known the casual 

 or contaminative method seems to prevail. The post-flagellate 

 stages of Crithidia gerridis and C. tabani are known, 

 and the cysts of these parasites are shed in the faeces of the 

 insectan hosts. The crithidian cysts are swallowed by new 

 hosts when they feed on material accidentally contaminated 

 by the faeces of their neighbours. The cysts then develop 

 in the alimentary tracts of the new hosts. Me lop hag us 

 ovinus also becomes infected with its Crithidia by the 

 casual inethod. 



When studying C. melophagia I have noticed that the 

 faeces of Melophagus ovinus are voided near spots on the 

 sheep from which blood has recently been sucked (particularly 

 is this the case at times of extrusion of puparia) ; that the 

 faeces contain crithidian, post-flagellate cysts, and sometimes 

 active flagellates; and that other Melophagus, feeding at 

 the same spot, have thrust their proboscides into the semi- 

 fluid faeces to reach the blood of the sheep. Ingestion of 

 cysts under such circumstances is easy. The ingestion of 

 faeces has been seen particularly well when a number of keds 

 have been kept confined to a small area of the sheep's body. 



At shearing a slight injury was caused to one sheep, and 

 the keds seemed to collect round the small bleeding patch. 

 Their habits were carefully observed then, and were similar 

 to those described above. I do not agree with Swingle that 

 casual infection is only a remote possibility ; to my mind it is 

 a certainty. 



A modified contaminative cross- infection is rendered pos- 

 sible by the cannibalistic habit of Melophagus ovinus. 

 The keds have been seen to attack one another, the point of 

 seizure invariably being at the end of the abdomen near the 

 anus. When a ked so attacked has been freed from its 

 aggressor and then dissected, I have found that the abdominal 

 cavity was almost empty, the viscera having been devoured 



