2 : M^DIC&L ENTOMOLOGY 



indirect infection, third, through internal parasitism, fourth, 

 through external parasitism, and lastly, through insect venoms. 

 The same species may fall as legitimately into two divisions, as, 

 for example, the Texas fever tick, which if not infected with the 

 causative organisms of the fever need only be considered as an 

 external parasite, but when the causative fever organisms are 

 present in the tick would relate it also to direct infection. 



Direct infection. Direct infection under ordinary conditions 

 can only be produced by an insect or arachnid possessing pierc- 

 ing mouthparts, and here no special order or larger group can 

 well be referred to, inasmuch as closely related insects may have 

 very different mouth structures. The common housefly and the 

 stable fly, for example, belong to the same family, therefore are 

 closely related, yet have very different mouthparts; though both 

 are suctorial, the former is unable to pierce the skin, whereas the 

 latter can do so with ease. 



By direct infection is meant the introduction of a pathogenic 

 organism, whether bacterial or protozoan, into the circulation 

 of a higher animal. The Anopheles mosquito is thus related to 

 this manner of transmission, since this insect introduces the 

 malaria parasite directly into the blood stream of man. The 

 same is true of the Stegomyia mosquito and yellow fever, the 

 Glossina flies and sleeping sickness, the horseflies and anthrax, 

 the Texas fever tick and Texas fever, etc. Direct infectors are 

 usually temporary ectoparasites (intermittent parasites), per- 

 mitting transfer of activity from animal to animal. 



While the conditions just discussed are to be referred to direct 

 infection, there is still a possibility for an insect with mandibu- 

 late mouthparts or with haustellate mouthparts of non-piercing 

 form to infect an animal as directly as one possessing piercing 

 mouthparts. Thus the housefly may, by means of mouthparts 

 and foot structures, transmit gangrene from an animal thus 

 affected to an animal undergoing surgical operation, or suffering 

 from an open wound. Pustular matter might be transmitted 

 in an equally direct manner. 



