CARRIAGE BY HOUSE-FLIES 151 



(3) How do the Cysts escape from the Fly /v\We may say at once 

 that we have not yet been able to demonstrate that flies regurgitate 

 through the proboscis cysts they have previously ingested. In the 

 faeces passed by the flies soon after feeding the cysts are readily 

 found.* Within twenty to thirty minutes of feeding on human faeces, ' 

 the flies begin to deposit droplets of liquid faeces and in these it is 

 easy to detect the unaltered and living cysts. If the fly has con- 

 tinuous access to faeces it will feed every few minutes and as often ^ 

 evacuate its intestinal contents. The amount of material passed 

 through the gut of a single fly in this way must be considerable and 

 within a few hours many thousands of cysts must have followed this 

 course. The cysts, however, may remain in the gut of the fly for 

 some hours and be deposited later, as the following experiment 

 demonstrates. A batch of flies was fed upon faeces containing cysts 

 of E. histolytica. After they had fed, the faeces were removed and 

 the flies left without food for sixteen hours. They were then given 

 sugar and water upon which they fed greedily. Shortly after this, they 

 passed droplets of faeces, and in these typical unaltered cysts were 

 found. It is thus evident that flies which have ingested cysts will 

 retain them for considerably periods only to deposit them later upon 

 anything which appeals to their varied tastes. The experiments 

 described were conducted with small numbers of flies, yet there was 

 no difficulty in recovering the infective amoebic cysts in the 

 numerous droplets of faeces they passed. When one reflects on the 

 myriads of flies which swarm about the latrines or faeces deposited 

 in the open in hot countries, one can only be surprised that amoebic 

 dysentery is not more widespread than is actually the case. In 

 these countries faeces, especially when liquid, are devoured and 

 transported in toto by these insects only to be deposited broad- 

 cast in millions of cyst-infected faecal droplets upon all kinds of 

 human food, which appears to occupy as an article of diet only a 

 second place in the estimation of these dipterous pests-V^ 



It might be urged that cysts could be transported by the adher- 

 ence of moist faeces to the legs, proboscis and body of the fly. 

 Observations on this question were made by Kuenen and Swellen- 

 grebel, who, like Nicol in his work on the passage of worm ova 

 through flies, came to the conclusion that these insects when fouled 

 by fseces did not move far till they had perfectly cleaned themselves. 



* \Ve explained our results and methods to Captain J. G. Thompson, R.A.M.C., 

 who |bsequently was able to repeat and confirm some of our observations on the 

 escape of cysts from the fly. 



