190 ANNIK POliTER. 



Introduction. 



Thk part played by insects as agents in the transmission of 

 the pathogenic organisms of sleeping sickness and other pro- 

 tozoal diseases gives great importance to the investigation of 

 the parasites found within them. It is necessary for any- 

 one seeking developmental stages of pathogenic flagellate 

 Protozoa to have also a first-hand working knowledge of the 

 possible flagellates that may be purely parasites of the insect 

 involved, for certain stages of insect flagellates may resemble 

 possible developmental ]ihases of such organisms as Trypano- 

 somes. Much useful information regarding stages of flagel- 

 lates can be gained from the study of such a parasite as 

 Crithidia melophagia (Flu), occurring in the alimentary 

 tract, ovaries, and ova of the sheep-" ked," Melophagus 

 ovinus. This insect, Avhich is blood-sucking, is also known 

 as the sheep-" tick " or sheep-" louse." It belongs really to 

 the Diptera (Hippoboscidse), possessing extremely reduced 

 wings. 



Crithidia melophagia (Flu) was recorded by E. PfeifFer 

 in 1905, but not named by him. The parasite is of peculiar 

 interest, for I am able to bring forward evidence of a double 

 mode of infection, both hereditary and casual. Swingle (1909) 

 studied the flag-ellate staores and brieflv described infection in 

 the egg of Melophagus. Flu (1908) found parasites in the 

 gut, ovaries, and larva, but was not clear as to the mode of 

 infection (see p. 211). 



Owing to conditions of environment it was impossible to 

 conduct the whole of this investigation in a large city. Con- 

 S(»quently the work has entailed travelling, and I have to 

 thank many friends in agricultural centres for their kindly 

 help. 



Material and jMethods. 



Many specimens of Melophagus ovinus were examined 

 during a long period of investig-ation, but owing to the 

 effective operation of the dip laws in England there was 



