NATURAL ENEMIES 67 



before the host is dead, and increase greatly in number 

 with the appearance of the conidiophores. 



This will suffice perhaps for a general account of the 

 development of these curious parasitic fungi. Empusa 

 muscce Cohn., one of the most abundant of them, at- 

 tacks the house fly, and also certain other large flies, 

 such as the blow flies and many flower flies. It was 

 first described by DeGeer in 1782, and has since been 

 carefully studied by many observers. It is almost as 

 universal as the house fly itself, and is the only Em- 

 pusa known south of the Equator. As a rule, accord- 

 ing to Thaxter, the species is found about houses, usu- 

 ally within them, and occurs in great abundance from 

 late June until late in the autumn. It seems altogether 

 likely that the majority of the deaths of flies in the late 

 autumn are caused by this species. In England, ac- 

 cording to Hewitt, it is found from about the begin- 

 ning of July to the end of October, usually indoors. 

 In Washington the epidemic ceases in December. 



It is not yet known how this fungus lasts over from 

 one year to another. Mycologists have never grown it 

 in artificial cultures, and there is evidently much yet 

 to be learned about many important points in its life 

 history. Much experimental work has been done with 

 the fungus diseases of other injurious insects, particu- 

 larly with those of forms injuring cultivated crops, 

 but no striking large-scale results of value have 

 been obtained. It is possible that something practical 

 can be gained from a close and prolonged study of this 

 disease of the house fly, and it is interesting to note 



