CHAPTER XXI 

 INSECTS AND DISEASE 



BENEFiaAL AND HARMFUL INSECTS. — Many insects, 

 such as the silkworm moth, honey bee, lac and cochineal 

 bugs, as well as several species which prey upon insect 

 pests, are of great benefit to man. The great majority, 

 amounting to perhaps 300,000 named species, confer no 

 special benefit and work no harm. A considerable num- 

 ber, such as bees and wasps and certain species of ants 

 and true bugs, may produce injury ranging from trifling 

 to serious by injecting poison into the body of another 

 animal by means of stings or piercing mouth parts. And 

 finally there are mosquitoes, certain flies and fleas, which 

 in true parasitic fashion subsist upon the blood of an 

 uncomfortable though not seriously threatened host. 



Such was about the nature and extent of our informa- 

 tion along this line prior to 1880. Since that date a 

 large amount of work has been done with parasitic in- 

 sects, and it is now known that while they of themselves 

 are rarely more than annoying pests they are very im- 

 portant agents in the distribution of some of the most 

 serious scourges afflicting man and many of the lower 

 animals. 



How Insects Transmit Disease. — In a few in- 

 stances only are diseases actually produced by insects. 

 The burrowing flea of tropical countries, for example, 

 tunnels its way beneath the human skin, and with its 

 progeny may destroy a certain amount of tissue. Certain 

 flies in their larval or young stages may also occasionally 

 infest the human body and destroy the cells. To this 

 extent these insects do interfere with the proper func- 



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