CHAPTER XXXI 



INSECTS IN RELATION TO DISEASE 



Questions. 1. How can insects cause disease? 2. What kinds of dis- 

 eases are caused by insects ? 3. What kinds of insects are most danger- 

 ous? 4. How can we tell that diseases are caused by insects and not 

 merely by the conditions in which they live — for example, hot weather 

 or bad night air ? 5. How can we best keep insects from causing dis- 

 ease ? 6. Is it right to destroy flies and other animals ? 



241. Insects eat. Insects concern us chiefly as eaters. They 

 may eat materials that are of value to us, or they may prey 

 upon plants and animals that are of use to us ; and, like most 

 other animals, we are ourselves exposed to the attack of insects 

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

 and horseflies, 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 dis- 

 covered in recent times, by infecting him with microbes capable 

 of causing more serious injury. 



242. 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, lately called the typhoid fly. This animal 

 has been shown to carry various bacteria, protozoa, and the eggs 

 of parasitic worms on its legs and proboscis, and it leaves these 

 germs where they have a good chance of entering the body of 

 some human being. The female lays her eggs in horse manure ; 

 but where there is no horse manure, she will use cow, sheep, pig, 

 or chicken manure, or decaying fruit, fish, meat, or vegetables 

 (ordinary garbage, for example), or any mass of decaying or- 

 ganic matter. The adult fly will visit, for feeding, not only 

 such materials but all kinds of perfectly good food that may 

 be exposed in groceries, meat shops, kitchens, dining rooms, 



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