n6 The Common House Fly. [Sess. 



while the lower or true haustellum is expanded at the tip 

 into two lobes, called the oral lobes. On their under surface 

 they bear transverse chitinous bars, called the tracheal (pseudo- 

 trachere). These hard ridges enable the fly to rasp solid food 

 — e.g., sugar. The orifice of the haustellum is situated be- 

 tween these lobes. There are two pairs of salivary glands — 

 the lingual are of great length, extending far back into the 

 abdomen, and are greatly convoluted near the stomach. 



Liquid food is sucked into the pseudo-trachese of the 

 labellum and so to the mouth, and in the case of solid food 

 the contents of the crop and salivary glands are forced out 

 and over the food in order to moisten it. These pseudo- 

 trachere are incomplete membranous tubes, embedded more 

 or less completely in the oral surface of the labellum. 

 Through the long fissure, fluid is sucked up and passes 

 into a gutter, which opens between the prestomal teeth, w T hich 

 are ten in number. When at rest the oral surfaces of the 

 labellaj or oral lobes are opposed to each other, but they are 

 spread out when the fly feeds, so as to form an oral disc. 

 When the teeth are used to scrape some solid substance the 

 oral lobes are more widely opened out. When a fly feeds on 

 a fluid or semi-fluid substance, it merely applies these oral 

 lobes to the surface and sucks up the fluid. When, however, 

 the food is solid, the fly moistens it by ejecting saliva and 

 then strong sucking movements are made. When sucking 

 dried sputum, the fly puts out a large quantity of saliva and 

 moistens a relatively large area, then it sucks up as much 

 as possible and moves on to a new area. You must have 

 noticed that flies prefer the margin of the milk-jug to the milk 

 itself — in other words, the sweet dried milk is more attractive 

 to it than the more or less watery solution, and the process of 

 absorption of the dry milk is the same as in the case of the 

 sputum. One may see a fly constantly applying its proboscis 

 to the surface over which it is walking, doubtless with the 

 object of obtaining nourishment, and the imprints of their oral 

 lobes can be made out by the microscope. 



The alimentary system is much simpler than in the case 

 of other flies — as, for example, the blow fly. There is first a 

 narrow pharynx, supplied with strong muscles, which is con- 

 tinued as a wider oesophagus or gullet, a proventriculus or 



