CHAPTER IV 



HOW INSECTS CAUSE OR CARRY 

 DISEASE 



T has been estimated that there are 

 about four thousand species or kinds of 

 Protozoans, about twenty-five thou- 

 sand species of Mollusks, about ten 

 thousand species of birds, about three thousand 

 five hundred species of mammals, and from two 

 hundred thousand to one million species of insects, 

 or from two to five times as many kinds of insects 

 as all other animals combined. 



Not only do the insects preponderate in number 

 of species, but the number of individuals belonging 

 to many of the species is absolutely beyond our 

 comprehension. Try to count the number of little 

 green aphis on a single infested rose-bush, or on a 

 cabbage plant ; guess at the number of mosquitoes 

 issuing each day from a good breeding-pond ; es- 

 timate the number of scale insects on a single 

 square inch of a tree badly infested with San Jose 



scale; then try to think how many more bushes or 



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