80 



THE HABITS AND BIONOMICS OF THE HOUSE-FLY 



been allowed to feed naturally on coloured jam have been found 

 with jam still in their crops on the second day after feeding. 



After feeding the fly usually retires to a quiet spot. It 

 invariably cleans its head and proboscis, as is also the case with 

 Stomoxys. Very frequently it regurgitates its food from the crop 

 in the form of large drops of fluid which may equal in diameter the 

 depth from front to back of the head (fig. 34). This regurgitation 

 of "vomit" drops conveys, as Graham-Smith, who has also observed 

 and recorded the fact states, the impression that the flies "have 

 distended their crops to an uncomfortable degree and that some of 

 the food is regurgitated to relieve the distension." While this is 

 conceivable, it is not unlikely that the regurgitation of the food 

 may be primarily concerned in the digestion, as I have observed it 

 to take place when there had been no unusual distension of the 



Fig. 34. M. domestica in the act of regurgitating food, x 4i. 



crop. One can understand that the regurgitation would enable 

 the food to become mixed with an additional amount of salivary 

 fluid which would further facilitate the digestion on the absorption 

 of the drop. The drops are slowly extruded and then withdrawn. 

 One fly which I had under observation alternately and regularly 

 regurgitated and absorbed a drop of fluid eight times, each regurgi- 

 tation and absorption lasting one and a half minutes. In some 

 cases these " vomit " drops are deposited upon the surface on which 

 the fly is resting and they may be easily recognised as light- 

 coloured opaque spots (see fig. 35). Their colour will, of course, 

 vary according to the nature of the flies' last meal. Graham- 

 Smith found that flies fed on coloured syrup often regurgitated 

 coloured fluid 24 or more hours later, though fed in the interval 

 on uncoloured syi'up. In his infection experiments he observed 

 that " when infected food has been given, the infecting organisms. 



