D. L. MACKINNON 
259 
majority are passed out with the faeces, but a few half-eucysted forms 
may be found attached to the disintegrating gut-epithelium during the 
metamorphosis. Of a number of flies I examined on emerging from 
their pupal cases and before they had fed, only one was found to contain 
a few half-encysted flagellates in the hind-gut. 
I think it likely therefore that the flies reinfect themselves when 
they begin to feed. The infection of the adult fly , is probably not to any 
extent directly continuous with that of the larva , but is freshly acquired. 
The cycle in the fly follows much the same course as in the larva. 
Pre-flagellate stages are infrequent, but when they occur are found in 
the mid-gut along with the adult flagellates. Rounding up stages occur 
in the intestine. By far the most common condition was that in which 
the rectum was full of enormous numbers of encysting and encysted 
forms. The cysts were found in the faeces. All the stages were never 
seen at one time in one host. 
It will be seen from the above that the mode of infection and the 
behaviour of the parasite in the adult flies of Scatophaga lutaria, 
Neuroctena anilis, and Homalomyia agree with Patton’s account of 
H. muscae-domesticae in Musca domestica. That author, however, while 
stating that infection is probably not hereditary, gives no information 
regarding larval infection, and one is left to conclude that only the adult 
flies harbour the parasite. Prowazek (1904) found H. muscae-domesticae 
in the ovaries of Musca domestica , and records larval infection. 
I have examined a considerable number of larvae of Musca domestica 
and have never found a trace of infection—a great contrast to the 
condition of things in Scatophaga and Homalomyia, where the larvae are 
seldom quite free from the parasite. 
Description of Parasite. 
The morphology of Herpetomonas muscae-domesticae has received 
much attention from many skilled observers. The flagellates forming 
the subject of the present paper agree closely with the type-species, but 
in looking through a large material, I have noted certain points that 
seem to me worth recording. 
A. Stained material. 
Patton has divided the life-cycle of herpetomonads and allied forms 
into three periods—(1) pre-flagellate, (2) flagellate, and (3) post- 
flagellate. I propose to follow this arrangement. 
