258 Herpetomonads 
I place great importance on the study of the living organism. 
I tried neutral red and methylene blue as intra vitam stains, but did 
not find them helpful: with good lighting, all the essential structures 
are quite clear on the unstained flagellate. 
Life-cycle of Parasite and Mode of Infection of Host. 
The parasites were always found in the alimentary canal of the fly. 
Careful search failed to show any infection of the ova, nor were flagellates 
ever found in or near the ovaries. This points to the infection being 
“ casual ”— i.e. by accidental ingestion of the cysts with the food. The 
high percentage of infection is scarcely surprising, considering the 
feeding habits. The flies crowd thickly over the surface of the dejecta, 
abandoning it only when startled, and returning as soon as possible to 
continue their meal : their food must quickly get thoroughly con¬ 
taminated with herpetomonad cysts from their droppings. In this 
connection it is interesting to note that the Neuroctena, which is the 
most heavily infected, is also the most sluggish, and rarely flies to any 
distance from its feeding-grounds : if not actually feeding, it may usually 
be found in the close vicinity on the leaves of low-growing shrubs. The 
flies examined were mostly caught in the months of June, July and 
September. As they were frequently seen in coitil upon their feeding 
grounds, I thought it not impossible that their larvae would also harbour 
the parasite. This proved to be the case. In the end of September the 
patches of excrement on which the flies had been caught were found to 
be a moving mass of infected dipterous larvae, mainly of Scatophagci 
lutaria and Homalomyia, sp. As was to be expected, the infection in 
the larvae, remaining as they do on their feeding-grounds throughout, 
was both more frequent and very much more intense than in the adult 
insects. As Patton has suggested, the degree of infection probably 
depends directly on the number of cysts ingested. Examination of 
larvae of all sizes showed me that infection may occur at any period of 
larval life, even very young larvae being richly infected. It is difficult 
to find the stages showing formation of the adult flagellate from the 
ingested cysts. When such stages occur they are found in the upper 
end of the mid-gut. Enormous multiplication of the flagellate takes 
place throughout the length of the mid-gut, so that in places the dark- 
brown gut contents are almost replaced by a seething mass of parasites. 
When the larva stops feeding previous to pupation, the flagellates begin 
to round up and collect in the hind-gut, where they encyst. The 
