256 
Herpetomonads 
Fallen, and Homalomyia, sp. (probably H. corvina, Verrall 1 ). The 
material was sufficiently abundant to admit of my working out the 
life-cycle, and I found that the herpetomonads of these three flies were 
at each stage of their development morphologically indistinguishable 
from one another and from H. muscae-domesticae, so far as I have been 
able to study that species. 
I regard Musca domestica and other non-biting flies frequenting 
similar feeding-grounds as subject to infection by a common flagellate. 
In view of the known polymorphism and great plasticity of herpeto¬ 
monads, it seems more logical to refrain here from the multiplication of 
species, and to regard slight deviations from the muscae-domesticae type 
as no more than might be looked for in response to the slightly different 
environment 2 . One is tempted to doubt whether forms such as Her- 
petomonas lesnei, Leger, Herpetomonas sarcophagae , Prowazek, and 
Herpetomonas ( Leptomonas) drosophilae, Chatton and Alilaire should 
really rank as distinct species 3 , and whether Herpetomonas ( Leptomonas) 
mesnili, Roubaud, and Herpetomonas ( Leptomonas) mirabilis, Roubaud, 
are not slight variations of one peculiar form. 
In studying the herpetomonads of flies it is necessary to keep in 
mind that one rarely finds more than one stage well represented in the 
gut of an individual insect. It follows therefore that without careful 
study of a sufficiently large number both of the adult flies and of their 
larvae, the account is too incomplete to be of much value except as a 
record of distribution. 
So far as I am aware, Patton (1909) is the only observer who has 
succeeded in ti’acing out the life-cycle of a herpetomonad from a non¬ 
biting fly. I am glad to be able to substantiate his account from what 
I have seen in the three species of dung-flies that I examined. A rich 
material has enabled me to treat certain points in fuller detail. 
Material and Methods. 
The flies were caught during the summer months near a pond in the 
neighbourhood of Aberdeen, where they were usually found feeding on 
1 I am much obliged to Mr Percy Grimshaw for kindly identifying these flies. 
2 Alexieff (1909) makes similar observations on the variability of the flagellate parasites 
of amphibians. 
3 Chatton and Alilaire would of course object that their flagellate had but one flagellum 
and is therefore not a Herpetomonas in their sense of the name. I shall return to this 
point presently. 
