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374 C. GORDON HEWITT. 



late autumn of one year and the summer of the next, is filled. 

 A number of suggestions have been made, many of which 

 cannot be accepted ; for example, Brefeld believes that the 

 Empusa is continued over the winter in warmer regions, 

 migrating northwards with the flies on the return of summer! 

 In the case of Entomophthora calliphora, Griard believes 

 that the cycle is completed by the corpses of the blow-flies 

 falling to the ground, when the spores might germinate in the 

 spring and give rise to conidia which infect the larvae. Olive 

 (1906) studied the species of Empusa which attacks a species 

 of Sciara (Diptera) and found the larvae infected. He 

 accordingly thinks that the disease may be carried over the 

 winter by those individuals which breed during that period in 

 stables and other favourable places. As I have shown, 

 M. domestic a, under such favourable conditions as warmth 

 and supply of suitable larval food, is able to breed during the 

 winter months, although it is not a normal occurrence so far 

 as I have been able to discover. If, then, these winter-pro- 

 duced larvae could become infected they might assist in 

 carrying over the fungus from one year to the next, and thus 

 carry on the infectjori to the early summer broods of flies. 

 This suggestion and the possible occurrence of aresting-spore 

 stage appears to me to be the probable means by which the 

 disease may be carried over from one " fly-season " to the next. 

 E. muscae, besides occurring in M. domestic a, has been 

 found on several species of Syrphidae, upon which it usually 

 occurs out-of-doors, as I have already mentioned. In addi- 

 tion to these Thaxter records its occurrence in Lu cilia 

 caesar and Calliphora vomitoria. 



VI. TRUE PARASITES. 

 1. Flagellata. Herpetomonas muscae-domesticae 



Burnett. 



This flagellate has been known as a parasite of the ali- 

 mentary tract of M. domestica for many years. Stein 

 (1878) figures a flagellate which he calls Cercomonas 

 muscae-domestica, and identifies it with the Bodo muscae- 



