OF WASHINGTON, VOLUME XVII, 1915 65 



Lepidoptera and Coleoptera may be involved in special ways 

 as has been suggested previously. 



Various relations are found to exist between disease organisms 

 and the insect host, among them the mechanical and special or 

 obligatory relations. 



It was supposed for sometime that the transmission of disease- 

 causing organisms in which insects are the special intermediate 

 hosts could only occur when the infection was derived and trans- 

 mitted by the same stage. It appeared, for instance, that the 

 persistence of disease organisms from the larval to the adult 

 stage of the house fly would be impossible on account of the 

 processes of histolysis and histogenesis in the pupal stage. Of 

 course, in the best known examples of hereditary transmission of 

 disease organisms, as by ticks, there is no such apparent barrier 

 to the development of the parasite. Recent observations by 

 Graham-Smith 1 and others have shown, however, that certain 

 pathogenic organisms may persist through the pupal stage of the 

 house fly so that we may have hereditary transmission by insects 

 with complete as well as with incomplete metamorphosis. At 

 any rate, the investigations have been carried far enough to 

 indicate that spore-bearing bacilli like the Bacillus of anthrax can 

 easily be carried through in this way. There is doubt as to 

 whether non-spore-bearing bacteria will survive, but it is possi- 

 ble that they may do so in some cases. 



The list of animals in which insect-borne diseases may occur 

 is undoubtedly incomplete but it includes man, rodents, horses, 

 cattle, dogs and birds. In fact, there does not seem to be any 

 restriction on the list of hosts that may become infected. 



In the modes of infection certain striking diversities are to be 

 noted. For instance, the sucking of blood and its regurgitation, 

 the contamination of food, and possibly the secretion of specific 

 toxins. 



The geography of insect-borne diseases may also be mentioned 

 here. Although the majority of such diseases known are endemic 

 in tropical and sub-tropical regions we have such noteworthy 

 exceptions as typhus fever which may occur everywhere, and 

 spotted fever in the northern part of the United States as well 

 as such other widespread diseases as tuberculosis and pneumonia 

 in which the function of the insect is altogether mechanical. 



Such multifarious divergencies in the conditions and modes of 

 transmission, in the functions of the vectors, and in the nature 

 of the causal organisms involved lead us to enquire whether there 

 is no end to the possibilities of insect connection with diseases, 

 and must every disease the etiology of which is not known be 



1 Graham-Smith, G. S. Flies in relation to disease, 1913, 186. 



