CHAPTER LXXIII 

 INSECTS AS SPREADERS OF DISEASE 



437. Insects eat. About one half of the different kinds of 

 animals known to man belong in the class insects. This class 

 of animals is spread over most of the earth's surface, and 

 many of the species live in water (although all are air breathers). 

 In every main division of the class are to be found species that 

 are closely related to human welfare in one way or another. 



Most insects are known to us chiefly as eaters ; and they 

 eat either materials that are of use to us, or they prey upon 

 plants or animals that are of use to us. 



Like other animals, man is exposed to the attack of insects 

 in search of food. Many species of fleas and lice, of bedbug 

 and horsefly, of midges, black flies, and mosquitoes, have 

 made themselves obnoxious to man by sucking his blood, by 

 causing more or less serious irritations of the skin, and, as 

 we have discovered only in recent times, by infecting him 

 with microbes capable of causing more serious injury. 



438. Insects move about. As carriers of disease, insects are 

 related to us in two different ways. The first is illustrated by 

 the common house fly, which has been shown to carry various 

 bacteria, protozoa, and the eggs of parasitic worms on its legs 

 and proboscis, and to leave these germs where they have a 

 good chance of entering the body of some human being. 



Experiments have shown that the number and kinds of bacteria 

 clinging to the feet of flies depend altogether upon the kinds of places 

 in which the flies live. Flies caught in dirty streets showed more than 

 those caught in clean streets ; those caught in a pigsty showed more 

 than those caught in the open, and flies caught while feeding in a 

 swill barrel showed many millions of bacteria. 



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