15 



Only the other day, whilst I was examining a tuberculous gland, a 

 fly settled on it and then flew off and went into the window ! 



Not only do flies carry infection in this way, but eggs of worms 

 and pathogenic bacteria have been found in their digestive apparatus 

 and in their faeces. 



Biting flies may, of course, carry diseases as in the first group, but 

 they also infect by thrusting the germs into the blood of the person 

 or animal they may bite ; both anthrax and septic diseases may be 

 caused in this way. Not only flies, but also fleas, bed- bugs, and lice 

 can infect by biting, thus directly inoculating the blood of the new 

 host. In this way it is thought that bed-bugs may carry tuberculosis, 

 anthrax, cerebro-spinal meningitis, etc., fleas may carry plague, and 

 lice carry skin diseases. 



The insects which carry the immature parasite, and which only 

 become infective after this parasite has developed in them, are, 

 however, the most serious carriers of disease. It is in this way 

 probably that the tse-tse fly may carry the germ of sleeping sickness. 

 These flies haunt scrub and bushes at the margin of lakes and rivers 

 in certain localities, and suck the blood of various animals, especially 

 of the crocodile. The tse-tse flies are better known as carriers of a 

 horse and cattle disease, such animals being destroyed if they go 

 into the fly area. 



The most deadly scourges in the insect world are the mosquitoes. 

 An Anopheles sucks the blood of a person suffering from malaria ; 

 the parasite of this disease develops in the mosquito and is then 

 carried to a fresh individual, who is thus infected. The tens of 

 thousands of cases of malaria abroad are due to these insects. 

 Another mosquito, the Stegomyia, is the carrier of yellow fever, 

 although in this instance the germ has not yet been found ; neverthe- 

 less it is proved that killing this insect and poisoning its breeding- 

 places diminishes the fever to the vanishing point ; and Panama, 

 formerly one of the most deadly regions of the earth, is now 

 rendered comparatively healthy by the wholesale destruction of 

 these insects. 



Although the ticks are only insects in the loose popular sense, it 

 may be legitimately noted here that they certainly carry parasites, 

 and are believed to cause plague, blackwater fever, spotted fever of 

 the Rocky Mountains, and typhus in some cases. These carry also 

 a parasite that causes cattle disease. 



You will see from this short summary how very important insects 

 may be as agents in the development of disease, and what an 

 important field for study is opened up for entomologists. 



