STRUCTURE AND LIFE-HISTORY OF CRITHIDIA MELOPHAGIA. 191 



much difficulty in obtaining the ^' keds." ^ Indeed, it seems 

 probable that the sheep-ked may soon become almost extinct 

 in England. Those obtained came in very small numbers from 

 many localities in the south of England, namely, Sussex, 

 Hampshire, Kent, Middlesex, and Gloucestershire. I also 

 received a number of keds from different parts of Scotland, 

 but these never contained the Crithidia. 



Many of the Melophagus, however, were infected by a 

 fungus (see Appendix II). Where fungus was present 

 Crithidia very rai*ely occurred. I shall show later, from 

 experimental evidence, that this fungus was fatal to the 

 Crithidia (p. 210). 



Unlike Swingle (1909), who found that practically every 

 Melophagus he examined in Nebraska was infected with 

 Crithidia, I found that this Avas very far from being the 

 case. Much depended on the locality from which the Melo- 

 phagus was obtained. The more heavily infected individuals 

 came from the southern districts of England. Often entire 

 stocks of keds from one locality proved to be uninfected. 

 Again, it was impossible to keep keds alive more than three 

 days after their removal from the sheep. 



Both young and adult Melophagus and many puparia in 

 all stages of development were carefully examined. Raising 

 puparia naturally upon a sheep was tried, but was not an easy 

 matter, and as one could not be sure of having infected keds, 

 there was always a percentage of uninfected puparia. 



Eor observations of the living organism two methods of pro- 

 cedure were followed. The alimentary canal was isolated and 

 divided into oesophageal, crop, stomach, intestinal and rectal 

 portions, which were separated one from another. These 

 were either teased with needles, mounted in 0*75 per cent. 

 salt solution, and covered, the cover-slip being carefully 

 vaselined, or the contents of the isolated portions of the gut 

 were expelled by gentle pressure, and these only were 

 examined, being mounted as before. Alkaline methylene 



' In this paper I shall frequently use the term '' keel " to denote 

 Melophagus ovinus. 



