CHAPTER VIII 

 THE HOUSE FLY AND DISEASE 



No animal has been subjected to a more careful study within 

 recent years than the house fly. The reason for this is that the 

 house fly distributes the germs of various diseases, thus causing 

 the death of thousands of human beings every year. As we 

 shall see later (Chaps. IX and X), the house fly is not the 

 only guilty insect, but its abundance makes it especially im- 

 portant. 



Disease Germs. — The disease germs that are carried by flies 

 and other insects must be described before it is possible to dis- 

 cuss properly their method of transmission. These germs are 

 either plants called Bacteria or animals called Protozoa. (See 

 Chapter XXV.) In either case they are exceedingly small, so 

 minute in fact that many of them can only be seen with the 

 highest magnifications of the compound microscope and some, 

 like the yellow-fever germ, have never been seen. 



Bacteria. — The bacteria (Fig. 48) are of various shapes and 

 sizes, being as a rule spheres (micrococci), straight rods (bacilli) ; 

 or bent rods (spirilla). They range in size from ^niou to about 

 j-^5- of an inch in length. Some bacteria are able to move, but 

 many must be carried from place to place. Bacteria increase 

 in number by reproduction, just as do the grasshopper and other 

 living things. The body of the bacterium divides in the middle 

 into two, a process known as binary fission. Then each part 

 grows rapidly and divides again. In some cases bacteria become 

 full-grown and divide every half hour. How many offspring 

 would be produced in twenty-four hours by such a bacterium 



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