Insects and Parasitic Diseases. 45- 



lengthy period of viability of tlie embryo, the rehitive scarcity in 

 horses' stoina-chs of the adult stages of H. microstoma as compared 

 witli H. miiscae and the apparent seasonal occiiri-ence of the former 

 species, from my observations, confined to a period from September 

 to January. 



Unlike the adult Musca domestica flies, which are to be found 

 here more or less plentifully throughout the year, Stomoxys flies are- 

 extremely scarce during the winter and spring. These fliesy. 

 generally ajDpear in numbers during the latter part of December, 

 becomes inci-easingly numerous in January, February and March, 

 and gradually disappear until very few remain in June. Why none- 

 of this species. (H. microstoma.) was found in the animals post- 

 mortemed during late January to early March is not easy of 

 explanation, since the fly host has been numerous, and the season 

 very mild, and the horses examined came from various locations. 



This investigation shows that the life-history of H. microstoma is 

 somewhat similar to that of H. muscae, although the principal 

 intermediary host is a different species of fly. Briefly summarised, 

 the life-history is as follows : — The embryos passed out in the faeces^ 

 from the horse are taken up by the larvae of Stomoxys calcitrans 

 which have oviposited on the faeces. The faeces remain infective 

 in this respect up to at least fifteen days, the fly-larvae being known 

 to react to infection Avhen two days up to -nine days old, and pos- 

 sibly earlier and later. After apparently undergoing a slight 

 development in the faeces (Fig. 19), the embryo of H. microstoma 

 (Fig. 20) enters the larva of Stomoxys calcitrans (or rarely and pos- 

 sibly accidentally the larva of Musca domestica), where it continues 

 to develop through the stages shown in Figs. 21-23. 



Development continues in the pupa through the stages shown in 

 Figs. 24-30; and in the adult fly, as seen in Figs. 31-33, in which 

 condition it is ready to develop in the stomach of the horse, where 

 it reaches maturity. Doubtless, infection of th© alimentary canal 

 of the horse is brought about at least in part by ingestion of living 

 and dead infected flies. 



Whether a living Stomoxys infected with H. microstoma is able- 

 to infect the definitive host with this parasite by means of direct 

 inoculation into the skin yet remains to be proved. So far as my 

 experiments go it has not done so, though this may have been due 

 to a clogging of the proboscis by the fifteen and twenty parasites 

 present in it in the two cases specially examined, preventing it 

 from properly piercing the skin, over-infection tlnis defeating the 

 object of infection of the intermediary. 



