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 
niiisca 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 
