HABITS 87 



" Flies fed on coloured syrup often regurgitate coloured fluid 

 24 or more hours later, though fed in the interval on plain 

 syrup. When infected food has been given the infecting 

 organisms are usually found in great numbers in these ' spots/ 

 and moreover, as will be shown later, fluid regurgitated from the 

 crop is used to dissolve or moisten sugar and other similar dry 

 food materials. The importance of the habit cannot therefore 

 be overestimated. 



" The term 'vomit' will be used to differentiate the stains left 

 by these drops from faecal deposits and proboscis-marks made 

 in half dried material. 



" If a fly, which has been fed on coloured syrup, is killed with 

 chloroform and pressure made with the forceps on the thorax 

 some of the syrup may exude from the proboscis. Further 

 pressure on the thorax or abdomen causes the proboscis to be 

 protruded, and occasionally a large quantity of fluid may be 

 exuded from it. Usually, however, though the proboscis appears 

 to be distended with fluid, very little is exuded. 



" Possibly some mechanism exists, near the tip of the proboscis, 

 for preventing the expression of the fluid. If, however, the tip 

 of the proboscis is cut off, or the head removed, the contents 

 of the crop can easily be expressed from the cut end of the 

 proboscis or the oesophagus, even up to five or six hours after 

 feeding^" 



(2) Semi-fluid material. 



" At various times flies were allowed to feed on milk which 

 had been spread on glass in a thin layer and allowed to partially 

 dry, and on other materials of similar consistency. The flies 

 walked over the areas covered by the dry milk and frequently 

 applied their proboscides to them. In all cases the application 

 was of some duration and fluid was often deposited by the fly 

 on the area it was sucking. After each application an oval 

 depression was made in the surface, in many cases showing 

 most beautifully an imprint of the end of the proboscis, or an 

 oval area was completely denuded. Plate XIV, fig. 2, shows 



^ Severe pressure on the sides of the head may cause turbid red-coloured fluid to 

 be exuded. This seems to be derived from the eyes. 



