0, Strickland 
231 
The fact that I’ectal cysts are found in the ‘ crop ’ of the fly indicates 
that the parasite is transmitted from fly to fly by contamination of its 
pabulum. 
Hereditary transmission does not take place. Several larvae from 
eggs laid by infected flies were dissected, but parasites were never 
found. Neither do these larvae become infected if they feed on 
contaminated meat, this being in contrast to Miss Mackiunon’s observa¬ 
tions (1910) on the life history of Herpetomoncis from dung-flies. 
In the case of H. luciliae, therefore, I conclude that infection is 
usually produced in the imago by its feeding on pabulum that has been 
contaminated with cysts, derived from other infected flies. 
Biology. 
Movement. The parasites that occur in the ‘crop’ and hindgut, each 
possessing only a short flagellum, have little power of progression. The 
flagellum waves feebly to-and-fro, but without causing much movement. 
Those with a long flagellum, as in the midgut, can only with difficulty 
be kept within the field of vision, the flagellum being generally directed 
forward under these circumstances. The ‘cysts’ are quite inert, floating 
about in the fluid in which they are examined. 
Growth and division. At various stages in the life-history of the 
parasite, the relative rapidity of its growth and its change of structure 
can be estimated by noting the comparative numbers of the parasite at 
that stage. In the ‘crop’ the cysts and the oval forms without a 
flagellum are the rarest, so it must be assumed that these forms are the 
most rapid in their growth. In the midgut the parasite does not remain 
long without division; while in the hindgut and rectum the cysts and 
the small oval forms with a flagellum are found in enormous numbers, 
and consequently represent the slowest stage of growth. 
It seems therefore that rapidity of gr-owth and change of structure 
is most rapid in the upper part of each subdivision of the gut, while at 
the lower end it is slowest and there is not much change of structure in 
the parasite. Division only occurs while the parasite is in the 
midgut. 
Individual variability. The study of stained specimens of the 
parasite shows that it is very variable in structure, particularly in the 
forms in the midgut, which we may regard as at the most plastic 
stage. 
