942 Transactions of the Society. 



of the disease. Dr. Grassi says it would be imprudent to suppose 

 that the ingestion in the fly caused the death of the organism, for 

 it does not suffice to kill the germs of mildews and schizomycetes, 

 and that flies eat more than their gastric juice can destroy. He 

 points out that they are likely to carry about the various organisms 

 by their feet and proboscis. He remarks on the difficulty to arrive 

 at positive proofs, on account of the numerous causes of error, and 

 concludes by declaring that the agency of the earth, air, and water 

 does not suffice for the diffusion of disease. Finally, he proposes 

 that flies should be exterminated by trying to give them the malady 

 in the spring of which they often die in the autumn. 



These remarks stand in strong contrast with those of the late 

 Frank Buckland, who in speaking of the acclimatization of rats and 

 bluebottle flies without the aid of human agency, says, " when we 

 come to consider the matter philosophically, rats and bluebottle 

 flies are, in reality, among the most useful of created things to the 

 human race. True it is, indeed, that we cannot eat them ; but 

 everything in this world was not made to be eaten, and these 

 despised creatures really do great service to us by getting rid of 

 decaying substances which would otherwise breed fever." * 



My friend Mr. G. F. Dowdeswell has reminded me that M. 

 Davaine found that flies by feeding off infected blood could convey 

 the infection ; while Dr. Manson, of China, has shown that mos- 

 quitoes are carriers of the ova of the Filaria sanguinis hominis, 

 and that it is possible the dreadful tsetse fly of Central Africa may 

 transmit infection to the animals it attacks. 



Grassi seeing the many difficulties in establishing his views, 

 intended to further prosecute his studies. How far I may have by 

 patience and trouble contended against these difficulties, the results 

 of the following experiments will determine :— ' 



Having examined very many slides prepared from the natural 

 excreta of sundry insects, Eristalis tenax, of the family of the 

 Syrphidae, was selected as one to experiment with (as this fly was 

 found to support captivity fairly well, and 1 had discovered no 

 curved bacillus in the dejections), and the common blowfly, Musca 

 vomitoria, as the other. There was another reason that prompted 

 me to choose Eristalis ; part of its early career is passed in sewers 

 or dirty waters, and it struck me that, although changed into a gay 

 or flower-haunting fly, it might still have some predilection for im- 

 pure food, or other than pollen and honey ; also it was abundant in 

 the garden during a part of July and August, in the sunny hours of 

 the day. 



For the culture material I am greatly indebted to the kindness 

 of my friend, Dr. E. Klein, F.K.S., who purposely inoculated four 

 of his sterihzed tubes for me, two in gelatin and two in agar- agar, 



* ' Life of Frank Buckland,' by Mr. G. C. Bompas. 4th ed., 1885, p. 130. 



