Insects and Parasitic Diseases. 63 



It is interesting to note that in Musca domestica, even when very 

 lieavily infected with H. niuscae, I have never been able to find any 

 Jarvae in the proboscis proper, i.e., not fuitlier forward than the 

 rostrum. On the other hand, as already stated, the proboscis of 

 ^tomoxys calcitrans most frequently harbours the " Sixth Stage " 

 larvae of H. microstoma. 



Whether thei'e is any tendency for the larvae to gravitate towards 

 the head of the fly-host I cannot positively say, except that in the 

 ■case of M. domestica the majority are usually in the abdomen, 

 whereas in Stomxys at least half the total parasites found ai"e in the 

 head. 



Met hod of hifectioii of the Horse. 



In view of the present knowledge the tAvo obvious possibilities are 

 ^1) ingestion of the infected fly, or of free larvae which have 

 escaped from the fly-host into the food or drink of the hoise, or of 

 larvae which have escaped from the fly-host on the lips or spot 

 .accessible to the lipa or tongue of the horse, and (2) by penetration 

 •of, or inoculation into, the skin of the horse. 



It is somewhat curious that in all these experiments I have never 

 •once come across any flies in the stomach of the horse, although 

 1 have no doubt that they are ingested and so reach the stomach. I 

 have therefore no positive evidence as to whether the larva of the 

 worm is passively libj^rated by the digestion of the fly, or whether it 

 -escapes prior to such digestion. I am strongly inclined, however, 

 to think that tiie former is the case. 



As to the possible infection of the horse by way of the skin, tlie 

 -evidence is quite insufficient. It has been shown very clearly (Figs, 

 34 and 35) that the ** sixth stage" or final larval stage of H. 

 "dnicrostoma finds its way very freely into tlie piercing proboscis of 

 the adult Stomoxys calcitrans, but whether under any circum- 

 stances this ** sixth stage" larva of H. microstoma can be trans- 

 «iitted to the skin or blood-stream of the equine host is not known — 

 the only experiments to this end so far attempted by the writer 

 having failed apparently from too heavy infection of the fly-host, 

 so choking the proboscis. 



As shown by Des<:-azeMux, Rivolta and others (Railliet's Report, 

 1915), and by Bull (1916), and Lewis and Seddon (in press) in Aus- 

 tralia, the larvae of some member of the Superfamily Spiruroidea 

 (characterised by the presence of a spinous tipped tail) are un- 

 Kioubtedly associated with the formation of lesions in the skin known 

 ;as '* Cutaneous habronemiasis," and ** Habronemic conjunc- 



