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of Musca domestica and Stomoxys calcitrans, the former predominating^ 

 and many of these are doubtless ingested. Attempted experimental 

 infection of S. calcitrans with H. rmiscae gave negative results. Other 

 theories of the infection of the horse, by ingestion of the parasite in 

 water or moist material or by ingestion after its escape from the fly 

 whilst feeding upon the mucous membrane of the horse's mouth, have 

 no fovmdation in fact. 



H. microstoma in the adult stage has been known since 1866 as a 

 parasite occurring in the stomach of the horse, but nothing has been 

 known definitely of its life-history. Experiments have shown that 

 S. calcitrans is the principal intermediary host of H. microstoma and 

 that M. domestica only occasionally, possibly only accidentally, acts 

 in this capacity. Fresh horse-faeces are not the usual breeding place 

 of S. calcitrans, which frequently, if not generally, breeds in decaying 

 grass, straw and similar matter, and in loose soil conta ^ninatecl by 

 stable drainage, while the larvae and pupae are commonly found in 

 the older portions of manure heaps. 



The embryos of H. microstoma passed out in the faeces from the 

 horse are taken up by the larvae of >S. calcitrans that have oviposited 

 on the faeces, which remain infective in this respect up to at least 15 

 days, the fly larvae being known to become infected when 2-9 days 

 old, and possibly earlier and later. Development occurs to a slight 

 extent in the faeces and continues in the larval, pupal and adult 

 stages of S. calcitrans, which last, either living or dead, is ingested by 

 the horse and thus infects the alimentary canal. 



It is yet unproved whether a living S. calcitrans infected with 

 H. microstoma can infect the definitive host Ijy means of direct inocu- 

 lation into the skin. In two experiments the negative results obtained 

 may have been due to a clogging of the proboscis by the 15 or 20 

 parasites present in it, over-infection thus preventing it from properly 

 piercing the skin. 



H. megastoma, which has been recorded as the causative agent of 

 tumours in the stomach of the horse, has been shown by experiments 

 to have M. domestica as its intermediary host, all available evidence 

 being against S. calcitrans acting, even accidentally, in such capacity. 

 The larvae are infected by embryos passed out in the faeces from the 

 horse which remain infective up to 15 days after leaving the rectum. 

 The fly larvae are known to react to infection when from 3-5 days 

 old, and possibly earlier and later. Development continues in the fly 

 in all its stages and is completed in the stomach of the horse, where 

 the larva finds its way into a nodule already formed, or else penetrates 

 the lumen of glands, there setting up the irritation which results in 

 the formation of a new tumour. The adult parasites occur naturally 

 only in tumours, from which they escape after the death of the host, 

 being rarely found on the external surface of the tumour or adjacent 

 membrane. The adults are known to occur also in the splenic 

 abscesses that have become more common of recent years, causing 

 considerable increase of mortality in horses in certain seasons and 

 certain districts in south-eastern Australia. No evidence is forth- 

 coming to show that the larvae of this and the preceding species can 

 enter the fly larvae by penetration or in any way other than by 

 ingestion. 



No flies have ever been met with in the stomach of the horse in the 



