170 H. M. WOODCOCK. 



In the ease both of Culexand An oph eles tlie number of individuals 

 inheriting the parasites appears to be limited. In contra-distinction to 

 Rhipicephalus, where this is, apparently, the only mode in which infec- 

 tion is transmitted, Schaudinn thinks that the infection of gnats by this means 

 is not common in nature. It probably occurs chiefly in the autumn, when 

 the Trypanosomes penetrate into the young eggs, there to pass the winter 

 in a quiescent condition. 



Thank'; to the ehiborate and painstaking investigation of 

 Schaiulinn, we are thus enabled to form a very good idea 

 of the manner in which the Trypanosomes are transmitted 

 from the gnat to the owl, and vice versa. There can be 

 little or no doubt that the process is of an essentially similar 

 nature in the relations of Mammalian Trypanosomes to other 

 biting Insects. In fact, the elucidation of the many factors 

 upon which infection and re-infection are dependent, and of 

 tlie adaptive modifications of the parasite to the biology of 

 the Insect, goes far towards explaining why many previous 

 investigators, unsuccessful in ascertaining anything definite 

 of the parasites in various '^ carrying " insects, have con- 

 cluded that these are not true hosts. 



These views are borne out by the recent work of Prowazek (1. c), who 

 finds that the behaviour of T. lewisi in Ha^matopinus and its passage 

 through the body of the Insect agrees in the main with that above described 

 for the Avian Trypanosomes in Oulex. Such differences as there are stand 

 inclose relation, on the one hand, to the somewhat different mode of feed- 

 ing and of absorption of nutriment in the louse, and on the other hand to 

 the fact that this parasite appears to be more resistant to " external " 

 influences. 



B. Relation of Trypanosomes in general to their 

 Vertebrate hosts. 



Once an entrance into the blood is effected the parasites 

 pass rapidly into the general circulation, and are thus carried 

 to all parts of the body. In considering the distribution and 

 numerical abundance or otherwise, of the Trypanosomes in 

 any given individual, it is necessary to bear in mind whether 

 they are in a tolerant host or in an unaccustomed one. 

 Dealing with the former case first, the general trend of 



