LIFE ON THE EARTH 



that is not injured in some way by insects. The strug- 

 gle between man and insects is intense, and it never 

 stops. The insects keep everlastingly at it, and so 

 must we. In this topic we shall learn about some of the 

 most dangerous enemies that man must combat. 



Why is the house fly a deadly enemy of man? Less 

 than a hundred years ago people in general thought 

 that flies were harmless, and some persons advocated 

 protecting them. Today we know that house flies dis- 

 tribute disease germs that cause the death of many 

 human beings each year. 



The house fly carries disease germs in several ways : 



1. If you examine a house fly (preferably with a 



hand lens) you will notice that its feet and leg~, 



>=cr~>y \ / 



FIG. 351. HOUSE FLIES AND DISEASE 



are very well adapted for carrying bacteria. The feet 

 are sticky and the legs have many hairs which take 

 up bacteria (see Fig. 350). Flies become soiled with 

 the filth in which they live, and some of this filth is 

 left upon any object the fly touches. More than 

 6,000,000 bacteria have been found on the body of a 

 fly from a filthy place. The average number of bac- 

 teria on flies is about 1,500,000. 



2. Flies regurgitate some of their food. After a 

 fly has eaten heartily from a garbage can or a swill 

 barrel, it may enter the house and regurgitate some 

 of the contents of its stomach on our food. If bac- 

 teria are present they will be taken into our digestive 

 system and may produce disease. Solid substances 

 such as sugar crystals must be dissolved before the 

 fly can swallow them. When a fly eats sugar it pours 

 out saliva which dissolves some of the sugar crystal. 



Then it draws the dissolved sugar into its stomach. 

 If the saliva contains bacteria, some of them may be 

 left on the food. 



3. Flies deposit waste matter called fly specks on 

 food. Bacteria may pass completely through the di- 

 gestive system of a fly without being killed. If a fly 

 has been feeding in a filthy place, the fly specks may 

 contain bacteria capable of producing disease. 



Germ diseases are carried by house flies. Typhoid 

 fever is perhaps the most common disease spread by 

 flies. It is for this reason that the house fly is now 

 often spoken of as the typhoid fly. Typhoid bacteria 

 lodge principally in the intestines of man. The germs 

 are taken into the body with foods, especially with 

 liquids such as milk and water. The germs are dis- 

 charged from the body in excretory matter and in 

 sputum. Flies may carry the typhoid bacteria from 

 human wastes and sputum to food. Therefore it is 

 of utmost importance that excretions and sputum 

 from typhoid patients be treated in such a way that 

 all germs are destroyed. Sometimes people seem to 

 recover from typhoid fever and yet continue to ex- 

 crete typhoid bacteria. Such persons are known as 

 "carriers." It has been definitely established that the 

 presence of unrecognized "carriers" and a large num- 

 ber of flies with access to food supplies has been the 

 cause of typhoid fever epidemics in certain communi- 

 ties. 



Flies carry tuberculosis germs and they help spread 

 the disease. Flies alight on sputum and take some 

 of it into their bodies. If bacteria are present they 

 may remain alive in the intestines of the fly for days 

 and be deposited with waste matter. The sputum from 

 tuberculosis patients should always be destroyed in 

 order that flies- may not get to it. Flies are known to 

 spread other diseases. Many children suffer and a 

 large number die each year from summer diarrhea. 

 The number of cases of this disease varies according 

 to the number and activity of house flies. 



What is the life history of the house fly? In order 

 that we may know how to control and exterminate 

 flies, it is necessary that we know where and how flies 

 breed and develop. About ninety-five per cent of all 

 house fly eggs are laid in horse manure. The eggs 

 may also be deposited in various kinds of garbage, 

 other animal excreta, and decaying vegetable matter. 

 The development of the automobile as a means of 

 transportation has caused a decrease in the number of 

 flies in many communities. Why? 



Flies begin to breed in early summer and continue 

 multiplying until fall. The female fly lays about five 

 hundred eggs. Within twenty-four hours an egg de- 

 velops into a larva, commonly called a maggot (Fig. 

 352). The fly lives as a maggot on the filth surround- 

 ing it for five days and then forms a pupa. The pupa 



