CHAPTER I 



INTRODUCTION 



ENTOMOLOGY as a branch of preventive medicine is a study of recent 

 growth, dating only from the two discoveries which initiated the modern 

 science of parasitology, viz., the demonstration by Manson of the r61e 

 played by certain mosquitoes in the transmission of filaria, and of the 

 mode of transmission of malaria by Ross. The latter discovery has 

 proved of infinite value to the human race : the two, rapidly succeeding 

 one another, placed the relations of the blood-sucking arthropods to 

 the vertebrates on w r hich they feed, in an entirely new light, and opened 

 up a vista of possibilities undreamed of before. Insects which had 

 previously been regarded as pests, detested on account of the immediate 

 trouble and annoyance they cause, now became suspected as carriers 

 of disease. 



Under the stimulus of Laveran's discovery of the parasite of malaria, 

 and the proof of its causal relationship to the disease, the study of 

 blood parasites was already progressing rapidly. Once the way was 

 pointed out, and the conception of a complex life cycle involving a 

 change of host accepted, our knowledge increased by leaps and bounds ; 

 every parasite present in the blood of a vertebrate was suspected of 

 passing one phase of its existence in the body of an arthropod, and 

 with each succeeding discovery there came a demand for further know- 

 ledge concerning the life history, bionomics, and structure of the 

 suspected carrier. Entomology has thus become a science accessory, 

 in some of its branches, to protozoology, a knowledge of the insect and 

 of the conditions governing its existence being an evident necessity for 

 the proper study of the parasites which it harbours. 



It was soon recognized, however, that it is not only through their 

 capacity to act as the intermediate hosts of protozoal parasites inimi- 

 cal to man, that insects become of medical and economic importance. 

 In many diseases, such as plague and cholera, while there is no sug- 

 gestion that the insect is an essential factor in the continued existence 

 of the causal organism, it is beyond doubt that it may be the means 



