CLASSIFICATION 345 



form {Leishmania) or the trypanosome form {Trypanosoma). The latter, 

 again, can be subdivided into those forms, such as T. leivisi, which are 

 carried by fleas; T. cruzi, conveyed by reduviid bugs; T. melophagiutn, 

 transmitted by the sheep ked; and possibly T. theileri and its allies, the 

 invertebrate hosts of which are probably tabanid flies and the trypano- 

 somes of land reptiles, including crocodiles, in which development in the 

 invertebrate leads to the formation of metacyclic trypanosomes in the 

 hind-gut, and passage of these in the faeces (development in the posterior 

 station); or into those like T. gambiense, T. vivax, and T. congolense, and 

 the trypanosomes of some cold-blooded vertebrates, the development of 

 which in the invertebrate results in the formation of metacyclic trypano- 

 somes in the region of the proboscis and their inoculation during the biting 

 act (development in the anterior station). The trypanosomes which 

 develop in the anterior station, a term first proposed by Duke (1913), can 

 further be grouped into those developing in biting flies (the pathogenic 

 trypanosomes of mammals) and those which develop in leeches (the try- 

 panosomes of aquatic reptiles, amphibia, and fish). The trypanosomes 

 of birds are difficult to place, for some have claimed that development 

 takes place in the mosquito, and that they are inoculated at the time the 

 mosquito bites. On the other hand, it seems very probable that the true 

 host of the bird trypanosomes will be found amongst the ectoparasites 

 which infest the young in the nest, and it is possible that infection may be 

 contaminative, as in T. lewisi. For this reason, in the scheme of classifica- 

 tion given below, the trypanosomes of birds have been placed in both 

 groups with a note of interrogation. Similarly, the trypanosomes of 

 land reptiles, including crocodiles, are placed in both groups, for it is not 

 definitely known whether the development is in the anterior or posterior 

 station, though the latter is probable in the case of the trypanosomes of 

 the crocodile and the monitor which develop in tsetse. flies. 



The trypanosomes which develop in biting flies in the anterior station 

 include the pathogenic forms of tropical Africa, which are conveyed by 

 species of Glossina (tsetse flies), and possibly the pathogenic forms of the 

 T. evansi type, including similar forms in many parts of the world, which 

 are conveyed by Tabanidse and their allies. It has not been actually demon- 

 strated that T. evansi develops in the anterior station in tabanid flies, 

 though its similarity to T. hrucei renders this not improbable. A develop- 

 ment in the posterior station is, however, possible. 



As regards those which develop in tsetse flies, it will be shown below 

 that three types of development occur, as pointed out by Duke (1913) and 

 Bruce (1914). In one type the process commences in the stomach, but 

 the infection spreads forwards to the proboscis and ultimately to the 

 salivary glands, in which infective metacyclic trypanosomes are produced. 



