FEEDING PROCESS 



77 



In flies which have never previously fed or have not fed for some 

 time, I believe the solid food is liquefied solely by the action of flie 

 salivary secretion; this would specially apply to flies which have 

 recently emerged from the pupal case or finiii hibernation. In 

 feeding on dried films of food material such as sugar, milk, etc., the 

 fly generally sucks clean the surface to which the oral lobes are 

 applied, leaving a heart-shaped area and the whole of a small area 

 is rapidly cleaned up in this manner (see fig. 32). In some cases 

 only the impressions of the pseudo- 

 tracheal channels are left (fig. 33). 

 In the case of fluids the resulting 

 fluid is sucked up into the oral pit 

 through the pseudo-tracheal channels 

 and by the action of the powerfid 

 phar3mgeal pump, the action of which 

 may be seen when the feeding fly is 

 observed from the fi"ont\ it is sucked 

 up into the oesophagusthrough whicli 

 it runs into the crop. If the food is 

 coloured with nigrosin or carmine 

 the filling of the crop can be readily 

 observed. The slightly concave, ven- 

 tral surface of the abdomen of the 

 hungry fly rapidly becomes convex 

 in the anterior region where the crop lies. The filling of the crop 

 takes place very rapidly in the case of liquid food ; sometimes it is 

 gorged in less than a minute, as Graham-Smith also observed. If 

 feeding continues the food begins to flow directly into the ven- 

 triculus, and the abdomen, as a whole, becomes rather distended. 

 Such gorges, however, frequently prove fatal, as I find that there 

 was gi'eater fatality among abnormally gorged flies than among 

 those which fed, as one might say, rationally. As a rule flies do 

 not appear to cease feeding until they are gorged. Their meals 

 are fi'equently disturbed and here, I believe, we see the natural 

 function of the crop as a food reservoir. Food can be taken into 



Fig. 32. Proboscis marks of ^1/. 

 domestica allowed to feed upon 

 film of sweetened Indian ink. 

 Enlarged. 



1 I have found that the action of the pharyngeal pump can be studied most 

 advantageously in Stomo.rys when the same is allowed to feed on the back of the 

 hand and watched through the Zeiss binocular microscope. 



