12 



catch two or three common house flies, kill them by squeezing 

 them gently without breaking the tough skin in which 

 the body of the fly is enveloped, and rub them softly 

 once or twice over a patch of diseased skin. Drop the 

 flies into the tumbler, and they will float at the top on 

 account of the large quantity of air which their bodies 

 contain. 



If the bodies of the flies are examined at this time with 

 a good magnifying glass, nothing will be seen but a few 

 threads representing the hyphae of the saprolegnia, which 

 have been entangled by their legs and bodies. 



By way of a check experiment it is advisable to put two 

 or three flies caught at the same time, and treated in the 

 same way, into a tumbler of the same water, but taking care 

 that they are not brought into contact with the diseased skin. 

 In this tumbler, covered as before, the bodies of the flies may 

 remain for weeks, and gradually decay without showing any 

 trace of fungus. Sometimes, however, Empusa, and some- 

 times, Penicillium or Mucor may make their appearance. In 

 the first case, the germs of the Empusa were certainly in 

 the body of the fly when it was killed. In the second, they 

 were very likely on it, but may have been imported from 

 without. In no instance that has ever come under my 

 observation, has Saprolegnia made its appearance in flies 

 treated in this way. This is singular, considering that it is 

 often stated that Saprolegnia appears regularly on flies thus 

 treated ; but I imagine that this is the case only when pond 

 or river water is used. 



The phenomena presented by the flies which have 

 been brought into contact with the diseased salmon skin 

 are very different. In about four-and-twenty or, at most, 

 six-and-thirty hours, that part of the body of the flies 

 which is in contact with the water presents a more or less 



