78 THE HOUSE FLY DISEASE CARRIER 



complete change of form, acquiring hard, brown, pro- 

 tective coverings into which all of the legs can be drawn 

 in repose. In this hard shell, or hypopus state, it may 

 remain for many months without food. 



In the majority of cases, however, where a given 

 cheese is completely destroyed, all of the young and 

 old mites perish, and only those of middle age, which 

 are ready to take on the hypopus condition, survive. 

 These fortunate survivors, possessing their souls in 

 patience, retire into their shells and fast and wait, and, 

 as everything comes to him who waits, some lucky day 

 a house fly comes that way and the little mite clings 

 to it and is carried away to some spot where another 

 cheese or food in some other form is at hand. 



SPIDERS AS FLY ENEMIES 



In spite of the well-remembered poem beginning 

 "'Will you walk into my parlor?' said the spider to 

 the fly," it is a curious psychological fact that the writer 

 had practically completed the writing of this chapter 

 on the natural enemies of the house fly before he dis- 

 covered that he had forgotten to say anything about 

 spiders. That was not because he is getting old and 

 forgetful, but because in the rooms which he has had 

 the good fortune habitually to frequent during later 

 years he has rarely seen a spider. Although, if given 

 the opportunity, they would kill an unlimited number 

 of flies, they are not permitted to build their webs and 

 increase in localities where the flies are the greatest 

 nuisances; that is to say, in houses, shops, and hos- 



