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- 
