202 DISPERSAL OF PARASITIC EGGS 



presence of even a small amount of moisture, other conditions 

 being suitable, the eggs of most parasitic worms will remain 

 alive for a great length of time. Thus it is stated by Davaine 

 that the eggs of Trichuris tricJiinriis may remain alive for as 

 long as five years. With regard to other conditions, tempera- 

 ture is probably the most important. A fairly high temperature 

 (80 — 90" F.) hastens development, but temperature much above 

 that will destroy the eggs in many cases. Continued exposure 

 to cold is also fatal, although freezing, if not too prolonged, is 

 not necessarily fatal. Little information, however, is available 

 on this point except in regard to a {q.\m species. 



TJie feeding of flies and their larvce in relation to the 

 ingestion of eggs. 



" It is a matter of common observation that fresh and moist 

 faeces attract flies much more readily than old and dried fsces. 

 Flies feed on warm fresh fseces with considerable avidity, and 

 they will do so even although they have been previously feeding 

 on other material. To flies which have not fed for some time 

 the presence of fresh human faeces acts as an immediate source of 

 attraction, and in some of my experiments the eagerness with 

 which they attacked it was most striking. When the portion of 

 faeces was so small that all the flies could not find standing 

 room upon it or around it, they struggled together and pushed 

 each other aside, and more than once I have seen them so 

 closely packed together that each fly could find room for only 

 the tip of its proboscis, the flies on top practically 'standing on 

 their heads,' supported by the bodies of those around. Their 

 behaviour towards older faeces, however, is very diff"erent. When 

 the material has become cold it does not attract flies nearly so 

 readily. So long as it remains moist it continues to attract and 

 does so quite as much as moist bread, although very much less 

 so than moist sugar. When it has become dry it possesses little 

 or no attraction, but this is increased when it is moistened again, 

 it is evident therefore that the presence of moisture pla\-s an 

 important part in a fly's attitude towards fitces as an article 

 of food. 



