CHAPTER I 



INTRODUCTION 



Although biting-flies had long been suspected of being 

 responsible for the spread of various diseases, it was not until 

 1877 that any direct proof was brought forward in support of 

 this hypothesis. In that year Manson, working in China, 

 discovered that the minute worm, Filaria bancrofti, present 

 in the blood of a large percentage of the natives that he 

 examined, underwent a development inside the body of 

 the common grey-legged mosquito, Culex fatigans. Although 

 these observations were incomplete and the exact mode of 

 transmission remained undiscovered until more than twenty 

 years later, yet they were of the highest importance, since 

 Hanson's work laid the foundation for all subsequent inves- 

 tigations on the part played by biting-flies as carriers of disease. 



With the exception of these few observations on the de- 

 velopment of Filaria in the mosquito, practically the whole 

 of our knowledge of the transmission of disease by insects, 

 has been acquired within the last twenty years. Thus Bruce, 

 in 1895, discovered the cause of Nagana, Trypanosoma brucei, 

 and its transmission by the tsetse-fly, whilst two years later 

 Ross, by his brilliant researches on the development of Proteo- 

 soma and the manner in which it is spread by the mosquito, 

 placed the methods of eradicating malaria on a scientific basis. 

 The rapid progress in this subject may be appreciated by the 

 fact that, with the exception of the specific descriptions of 

 certain insects and the above-mentioned work on Filaria, the 

 present volume is entirely concerned with discoveries of the 

 last twenty years. Within this period biting-flies have been 



