MANKIND AND THE INSECT 329 



been suggested as a possible " reservoir host." All these 

 and many other points connected with the life-histories 

 of trypanosomes and tse-tse flies are being investigated. 

 These studies are not merely of scientific and human 

 interest, but of vast commercial importance in connection 

 with the development of tropical Africa. 



The plague bacillus, which afflicts rats and certain 

 other mammals, is transported by fleas, especially the 

 particular kind of rat-flea known as Lcemopsylla cheopis, 

 to man ; while numerous other insects (not to mention 

 the spider-like ticks and mites) are either known or 

 believed to be disseminators of disease. Indeed, all 

 blood-sucking Arthropods may justly be dreaded as pos- 

 sible go-betweens, liable to convey infection from one 

 animal to another. Moreover, certain insects carry about 

 the microbes of disease in a purely accidental or 

 mechanical manner. For instance, the common house-fly 

 (Musca domestica) breeds in stable manure and refuse of 

 various kinds. It also feeds largely upon filth ; and its 

 feet and proboscis become covered with microbes of all 

 descriptions. This may be demonstrated by allowing a 

 fly to walk over a specially prepared and sterilised plate 

 of gelatine such as is used in laboratories for the experi- 

 mental cultivation of bacteria and moulds. In twenty-four 

 hours every footstep of the insect on the gelatine is 

 marked by a large and varied crop of microbes. In like 

 manner the excreta of the fly may be shown to contain 

 infective agents. Thus, when a fly crawls over our food, 

 or falls into a vessel of milk, these substances are inevit- 

 ably contaminated ; and it is known that flies which have 

 access to the necessary infective material carry about 

 with them the microbes of such diseases as infantile 

 diarrhoea and typhoid fever. 



In view of their manifold delinquencies insects are 



