i9i3" I 9 I 4-] The Common House Fly. 123 



and on the early fine warm days they are tempted out to 

 begin the duty of propagation. A warm day in winter will 

 even tempt some to break their long sleep, but in many cases 

 this must end disastrously. 



The connection of flies with the carriage of disease is no new 

 idea. As long ago as the middle of the seventeenth century, 

 Sydenham stated that if flies were abundant in the summer 

 the autumn would be unhealthy. In the Old Testament 

 Scriptures there are many regulations as to the conduct of the 

 camps of the children of Israel during their march through 

 the wilderness, and references to flies in this connection. In 

 Exodus viii. 24, "And there came a grievous swarm of flies 

 into the house of Pharaoh, and into his servants' houses, and 

 into all the land of Egypt : the land was corrupted by reason 

 of the swarm of flies," and amongst the plagues which fol- 

 lowed were the murrain of cattle and the death of all the 

 first-born of Egypt. In 1871, Lord Avebury (Sir John 

 Lubbock) wrote, " Far from looking upon flies as dipterous 

 angels dancing attendance on Hygeia, I regard them rather in 

 the light of winged sponges, spreading hither and thither to 

 carry out the foul behests of contagion." We do not speak 

 here of the biting flies — e.g., mosquitoes, gnats, tsetse flies, 

 &c. — for in them the protozoa undergo a certain developmental 

 change in their life-cycle, and their bites may only convey the 

 particular disease if the cycle has been complete in this re- 

 spect. In the house fly, however, infection can only be carried 

 by the legs, body, or proboscis of the fly, or in food vomited or 

 excreted by them. We have seen how thickly clothed the 

 body and legs of the fly are with hairs or setse. No doubt 

 the fly is a cleanly insect, and in spite of the loathsomeness 

 of its food, it does its best to keep itself free from contamina- 

 tion, and we see it constantly brushing its legs, body, and 

 head to free them from accumulations. Unfortunately it 

 may alight on food or on the lips of people, and so in this 

 way direct infection is brought about. If a fly be made to 

 walk over the surface of a culture, and then over sterile 

 culture medium, numerous colonies develop. Thus the non- 

 sporing organism, B. prodigiosus, may remain alive and active 

 on the legs of the fly for seven days, while in the crop and 

 intestines it may continue active up to seventeen days. 



