DISTRIBUTION OF BACTERIA IO7 



constantly produced infection in syrup up to the 29th day, 

 though the degree of infection was small after the 14th day. 

 Milk was infected to a smaller extent, and only occasional 

 colonies were cultivated from it after the 8th day. 



It is of course likely that the table shows a smaller degree 

 of infection than really existed since only a small proportion of 

 the milk was cultivated on each occasion. 



Observations on the habits of bloiv-Jiies and their bearing 

 on feeding experiments^. 



Blow-flies spend a large proportion of their time, when in 

 captivity, in cleaning their limbs and proboscides in the following 

 manner : 



The wings are usually cleaned with the posterior pair of 

 legs, which are passed over and under each wing. Often two 

 legs are simultaneously applied to a wing which is drawn 

 between them. Subsequently the legs which have been used 

 are rubbed together. The posterior legs are also used for 

 cleaning the abdomen, and, in so doing, are passed over the 

 anus. Probably they are frequently infected in this way and, 

 later, infect the wings. For cleaning the proboscis and head 

 the anterior pair of legs is made use of. During this process 

 the proboscis is usually extended to its fullest extent, and the 

 legs are passed along it from its proximal to its distal extremity. 

 Occasionally the oral discs are applied to some part of the leg, 

 probably in order to remove some irritating material. Flies 

 have been seen to clean their proboscides immediately after 

 feeding, and to leave infected material on their legs. 



The efforts made to cleanse their limbs are often very 

 prolonged when flies have been allowed to feed upon very sticky 

 materials, such as concentrated syrup, and some observations 

 which have been made appear to indicate that, under such 

 circumstances, gross re-infection of the limbs may occur. On 

 one occasion a fly sucked at some semi-solid syrup mixed with 

 carmine for a long time and moved away from the food 

 with a large mass adhering to its proboscis. This mass was 

 1 See also Chapter xx. 



