76 THE HABITS AND BIONOMICS OF THE HOUSE-FLY 



which keep open the pseudo-tracheal channels. In the house-fly 

 these interbifid spaces, as Graham-Smith aptly termed them, 

 measure from '004 to "003 mm. in diameter, indicating how im- 

 possible it would be for particles of food to be absorbed with the 

 food* in the normal manner and pass along the pseudo-tracheal 

 channels to the mouth. The only possible manner in which solid 

 particles such as the eggs of tapeworms could gain access to the 

 pharynx would be by direct entrance into the mouth. In experi- 

 menting with Indian ink I found that through the sucking action 

 of the oral lobes the solid particles were heaped up in a slight 

 ridge in the channel between the lobes and remained between the 

 lobes while the fly continued feeding. Graham-Smith observed 

 the same process in feeding the blow-fly with pollen grains. It is 

 not difficult to understand how, when feeding on solid food which 

 is being liquefied or on liquid food containing minute particles, 

 such particles which were not too large might be sucked up into 

 the oral pit and into the mouth. This undoubtedly happens in the 

 case of the absorption of the tapeworm eggs. Ordinarily, however, 

 the entrance of food into the oral pit is prevented by the pre- 

 stomial teeth and the close apposition of the inner edges of the 

 cushion-like oral lobes. 



In order to feed on dry substances such as sugar, dried specks 

 of milk or sputum, etc., the fly has first to liquefy the substance. 

 This is accomplished by the secretion of what I termed the lingual 

 salivary glands. This is poured down the labium-hypopharynx 

 into the oral pit and on to the surface of the solid substance, both 

 directly and through the pseudo-tracheal channels. This process 

 can be observed by inverting a film of dried up sugar solution on 

 a slide over a glass capsule containing a fly, the process being 

 watched through a Zeiss binocular microscope. The solid matter 

 is soon liquefied by the rapid sucker-like movements of the oral 

 lobes moistened by the salivary secretion. Frequently the lique- 

 faction is aided or brought about by means of the regurgitated 

 food when the fly has fed previously. Graham-Smith found 

 carmine stains in the proboscis marks of flies feeding on semi-fluid 

 material as long as 22 hours after they had been fed on carmine 

 coloured food, indicating that some of the previous meal had been 

 ixpparently regurgitated, thereby assisting the salivary secretion. 



