52 THE INTERNAL ANATOMY OF THE HOUSE-FLY 



shortly divides into two tubules, which are very long and con- 

 voluted and internately bound up with the fat-body, which 

 occupies the space not taken up by the intestine and other organs. 

 The malpighian tubes are excretory in function. The distal 

 intestine runs into the rectum, from which it is separated by a 

 cone-shaped dilatation, the rectal valve (Fig. 19, I). The rectum 

 finally opens at the amis. 



In connection with the alimentary system mention must be 

 made of the two pairs of salivary glands, the lingual and the 

 labial. The lingual salivary glands are of great length, and 

 consist of blind-ended tubes. From the blind ends, which are 

 situated near the posterior part of the abdomen the glands pass 

 forwards into the thorax, and run along the ventriculus, when 

 they are much convoluted. Thence they run forwards along the 

 sides of the oesophagus, losing their glandular structure, and 

 becoming ducts in the neck region. Finally in the neck they 

 unite below the cesophagus and the single duct runs direct 

 to the end of the hypostome, where it opens (Fig. 19, N). The 

 labial salivary glands lie at the base of the oral lobes of the 

 proboscis and their ducts open into the oral pits. 



In order to explain the process of feeding it is necessary 

 to give in the succeeding chapters a detailed account of the 

 structure and function of the proboscis and of the functions of 

 the crop and proventriculus. 



CHAPTER V 



THE STRUCTURE AND FUNCTION OF THE PROBOSCIS 



In the course of a long series of experiments carried out by 

 the writer (Graham-Smith, 1910 — ii) on the distribution of 

 bacteria by non-biting flies {Musca doniestica and C. erythro- 

 cephala) it became evident that such flies are able to filter off 

 and reject the larger particles contained in the fluids on which 

 they feed. This fact seems to have escaped the notice of most 



