2 MEDICAL ENTOMOLOGY 



of transmission from man to man, directly or indirectly. The r61e of 

 house flies in the propagation of cholera, typhoid, and dysentery is now 

 well established, and practical sanitarians take it into account in devising 

 measures for their prevention and elimination. 



From the nature of the case, medical entomology is a specialized 

 branch of the study of insects, dealing with them only in so far as they are 

 concerned in the mode of infection in various diseases, or are in them- 

 selves noxious. An expert knowledge of individual forms is required, 

 rather than a general knowledge of the class. Moreover, the attention of 

 the observer has to be directed on certain well-defined lines, which in 

 many respects deviate from those on which the study of entomology is 

 ordinarily carried out. The important facts to be ascertained are the rela- 

 tion of insects to man and domestic animals, their food, method and 

 time of feeding, and their life histories ; their relation to the disease in 

 question, such as their distribution and its coincidence or otherwise with 

 the area over which the disease occurs, and their seasonal prevalence ; 

 their structure and physiology, with special reference to those internal 

 organs which will constitute the environment of the parasite after it is 

 ingested. Such matters as the relations of groups of insects to one 

 another, their phylogeny, classification, etc., fall more properly within 

 the domain of the systematic entomologist, who studies the class as a 

 whole, as a branch of general zoology. At the same time some know- 

 ledge of systematic work is a practical necessity, in order to be able to 

 identify a suspected or incriminated species, and to follow intelligently 

 the general literature on the subject. 



Although the medical and sanitary aspects of entomology are thus 

 limited, it must be borne in mind that the study of any organism or group 

 of organisms is materially aided by the study of allied forms, the analogy 

 with which provides working hypotheses. Some acquaintance with the 

 whole class, as regards their life histories, conditions of existence, struc- 

 ture, etc., is therefore essential, as a foundation on which to build up an 

 expert knowledge of the groups of practical importance. Familiarity 

 with what occurs in non-disease carrying forms has led on many occasions 

 to the disclosure of important facts regarding true carriers. Special 

 attention in this connection is due to those arthropods which harbour 

 'natural' non-pathogenic parasites, for both the parasite and its host are 

 in many cases closely related to others which are concerned in the etiology 

 of disease. The student will do well to remember that Ross's work on 

 proteosoma in sparrows preceded his demonstration of the life cycle of 

 the parasite of human malaria. 



