360 THE WONDER OF LIFE 



The carrying is twofold, external and internal. After 

 it has been feeding on highly infective substances, the fly 

 must have many germs about its legs and mouth-parts and 

 body, and it may readily implant these in human food, 

 But its food- canal is also charged with concentrated 

 infective material, which may be dropped on food, on 

 dishes anywhere. Professor Nuttall remarks that " in 

 potential possibilities the droppings of one fly may, in 

 certain circumstances, weigh in the balance as against 

 buckets of water or of milk '. 



Dr. Gordon Hewitt cites some important experiments 

 made by Giissow, who allowed a house-fly (Musca domestica), 

 caught in the room of a house, to walk over a culture plate 

 of agar-agar. He obtained thirty colonies comprising 

 six species of bacteria and six colonies comprising four 

 species of fungi. From another, caught in the open, he 

 obtained forty-six colonies comprising eight species of 

 bacteria and seven colonies comprising four species of 

 fungi. " The tracks of a house-fly caught in a household 

 dustbin yielded 116 colonies of bacteria comprising eleven 

 species, and including such species as Bacillus coli, B. 

 lactis acidi, and Sarcina ventriculi, and ten colonies com- 

 prising six species of fungi'. A very important fact, 

 proved by Faichne, is that if the maggot stage be de- 

 veloped in infected typhoid material, then the fly has also 

 typhoid bacilli in its alimentary canal. 



It might seem to the uninitiated a sad waste of time to 

 inquire into the House Fly's flying capacities. But it is 

 a very important practical question, for the range of flight 

 determines the fly's range of mischief. Dr. Hindle finds that 

 house-flies tend to travel either against or across the wind. 

 This direction may be directly determined by the action of 



