113 



There seems to be little doubt that the larva present in 

 the lesions belongs to the genus Habronema. 



In tracing the evolution of these tumours from the earliest 

 recognizable lesion, and taking into consideration their situa- 

 tion in or just beneath the skin or external mucous mem- 

 branes, it seems reasonable to assume that the larvae are 

 introduced from without and are not carried to the surface 

 from within. A point to be decided is whether larvae of 

 Habronema can enter the submucosa of the external mucous 

 membranes or the subcutaneous tissues, and so make their way 

 to the alimentary canal, or not. If the larvae are capable of 

 doing this, as larvae of Ankylostoma are, then it is remarkable 

 that they should so often be held up in the submucous or 

 subcutaneous tissues. There is nothing to suggest that this 

 is a common or even probable mode of invasion. 



Experimental observations have shown that, although 

 larvae are capable of migrating in the subcutaneous tissues 

 for some little distance from the point of entry, this migration 

 is very limited, and the larvae are soon surrounded by leuco- 

 cytes. It would appear, therefore, that the presence of the 

 larvae in the submucosa of the conjunctiva, and of the 

 urethra or the subcutaneous tissues, is an accidental 

 phenomenon. 



All the larvae found in the tumours have presented the 

 same appearances, and must be regarded as being of the same 

 stage of development. There is nothing to suggest that the 

 larvae have passed through any developmental stages in the 

 tissues of the horse. 



As the larvae found in the granulomata are in the same 

 stage of development as those found in the head and proboscis 

 of adult flies, it would appear that flies are in some way respon- 

 sible for the production of the lesions. This is also suggested 

 by the fact that the granulomata only occur at that time of 

 the year when flies are present in abundance. 



Observations on the life-histories of the three species of 

 Habronema have shown that H. muscae develops through its 

 larval stages in Musca domestica, but it is not capable of such 

 development in Stomoxys calcitrans, at least under experi- 

 mental conditions; H. megastoma has the same life-history as 

 H. muscae; H. microstoma develops through its larval stages 

 in Stomoxys calcitrans, and shows, sometimes at least, an 

 aberrant development in Musca domestica. 



Harvey Johnston (1912) has recorded the finding of a 

 larva somewhat resembling that of H. muscae in Musca 

 vetustissima in Queensland, so it seems possible that these 

 species may be capable of developing through their larval 

 stages in other flies, particularly Muscids. Nothing, however, 



